DEVELOPMENT OF A MONITOR TO QUANTIFY LEAD-BASED AEROSOLS
ROBERT WILLIAM VANDERPOOL
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
This dissertation is dedicated to the memory of my parents.
I would first like to thank Ken Reed for his assistance in machining
several pieces of the experimental apparatus. His expertise is most
The useful discussions with John Applewhite concerning the capabilities
and limitations of the polarograph are appreciated.
I would also like to acknowledge the assistance of Tom Peters during the
field evaluation of the prototype monitor. The successful demonstration of the
instrument's capabilities would not have been possible without his effort.
I also wish to thank the members of my doctoral committee for their time
and effort required to serve in this capacity. Each member, in his own way, has
served as a source of inspiration during the course of my studies at the
University of Florida. I am particularly appreciative of Dr. Lundgren for his
guidance, encouragement, and confidence. He has provided me with
opportunities for laboratory, teaching, and field experience that contributed
greatly to my professional development.
Finally, I would like to thank my wife, P.J., for her patience and support
during the last three years.
TABLE OF CONTENTS
LIST OF TABLES .................................
LIST OF FIGURES ................................
KEY TO SYMBOLS ...............................
1 INTRODUCTION .............................
Description of Battery Production Process ...........
Instrument Design Criteria .......................
Summary of Design Approach ....................
2 REVIEW OF UTERATURE ......................
3 DESIGN AND CALIBRATION OF PARTICLE COLLECTION
Design Theory ...............................
Prediction of Impactor Performance ................
Calibration of the Particle Collection Section ..........
4 DESIGN OF AIR SAMPLING SYSTEM ..............
Description of Air Sampling System ................
Prediction of Particle Sampling and Transport
. .. =..
. o. ,
5 DESCRIPTION OF LEAD ANALYSIS SYSTEM .................
Design Criteria ................ ............. ........
Description of Polarographic Analysis ...........................
Analysis of Field Bulk Samples .................. ... .......
Description of Modifications to the Polarograph ...............
6 DESCRIPTION OF FLUID HANDLING SYSTEM ...............
7 PROGRAM DESCRIPTION AND OPERATION .................
8 FIELD EVALUATION OF PROTOTYPE INSTRUMENT ...........
Experimental Methods .................................
Experimental Results ...................................
9 SUMMARY AND RECOMMENDATIONS .....................
A IMPACTOR DESIGN DRAWINGS .......................... 155
B IMPACTOR CALIBRATION RESULTS ...................... 160
C CONFIGURATION OF KIETHLEY 575 CHANNELS ............. 162
D LISTING OF PROGRAM ................................ 165
E OPERATING MANUAL FOR THE LEAD-IN-AIR MONITOR ........ 185
REFERENCE LIST ........................................ 223
BIOGRAPHICAL SKETCH .................................. 225
LIST OF TABLES
Predicted Operating Characteristics of Teflon
Description of Module 1 Air Flow Components ......
Description of Module 2 Air Flow Components ......
Module 2 Electrical Component Listing and
Module 2 Electrical Connector Pin Descriptions .....
Polarograph Response Using Copper Internal
Comparison Tests Between Polarograph and GFAA ..
Component Listing and Description of RUN/STOP
RUN/STOP Board Connector Pin Functions ........
Component Listing and Description of PCM3 Board ..
Description of Various PCM3 Channels ...........
Component Listing and Description of Decade
Resistance Board ..........................
Decade Resistance Board Connector Pin Functions ..
Component Listing and Description of Model 264A ...
Model 264A Connector Pin Descriptions ..........
Component Listing and Description of Digital
Board ......... .........................
Digital Logic Board Connector Pin Descriptions .....
Listing and Description of Liquid Handling
Listing and Description of Nitrogen Handling
Handling Components ......................
Module 1 Electrical Component Listing and
Module 1 Electrical Connector Pin Descriptions .....
Analysis of Laboratory Lead Standards ...........
Analysis of Field Control Lead Standards ..........
Results of Collocated Sampling Performed at Plate
Offbearing Operation .......................
Results of Unattended Sampling Performed at Plate
Offbearing Operation .......................
Results of Collocated Sampling Performed at Paste
Drying Operation ..........................
Results of Unattended Sampling Performed at Paste
Drying Operation ..........................
Results of Collocated Sampling Performed at Plate
Stacking Operation .........................
UST OF FIGURES
Fow chart of lead-acid battery production
Summary of prototype lead-in-air monitor design
approach .. ...........................
Measured particle size distributions at a
battery plant .............................
Schematic of single-stage impactor used for
particle collection ............... ..........
Schematic of generation system used for production
of calibration aerosols greater than 0.5 micrometers
aerodynamic diameter ......................
Scanning electron photomicrograph of ammonium
fluorescein calibration aerosols produced by VOAG
Schematic of generation system used for production
of calibration aerosols less than 0.5 micrometers
aerodynamic diameter ......................
Measured particle size collection efficiency curves
for single stage impactor ....................
Schematic of Module 1 air flow system ...........
Schematic of Module 2 air flow system ...........
Wiring diagram of Module 2 electrical components ...
Measured performance of Module 2 mass flow
Theoretical particle transport efficiency through the
sampling probe's horizontal section .............
Theoretical particle transport efficiency through the
probe's 900 bend ..........................
Polarographic response to 40 ppm concentrations of
lead, cadmium, and copper in 0.1 N HNO3 .........
Typical polarographic response as a function of lead
Polarographic response to 0-200 ppb lead
concentration using 200 ppb internal copper standard
Polarographic response to 0-8000 ppb lead
concentration using 200 ppb internal copper standard
Schematic of printed circuit board which provides
run/stop functions .........................
Wiring diagram of Keithley PCM3 power control module
Schematic of printed circuit board which provides
decade resistance ranging functions ............
Schematic of Model 264A showing recommended
locations for auxiliary circuit boards ..............
Schematic of printed circuit board which provides
logical switching of stir, purge, and drop
dispense/dislodge functions ...................
Schematic of Module 1 liquid handling system ......
Detail drawing of flow-through cell designed for the
Schematic of Module 1 nitrogen handling system ....
Wiring diagram of Module 1 electrical components ...
Photograph of lead-in-air monitor during tests
conducted at plate offbearing process ............
Photograph of Module 1 inlet section showing position
of collocated filter holder relative to Module 1
KEY TO SYMBOLS
C particle concentration
C, slip correction factor
D particle diffusion coefficient
Dp0 impactor cutpoint
D, tube diameter
g gravitational constant
L tube length
M gas molecular weight
n number of impaction jets
P fractional penetration
P, stack pressure
Q volumetric flowrate
R bubble radius
STK particle Stokes number
T gas temperature
V gas velocity
Vb bubble velocity
V, gas velocity pressure
V, particle settling velocity
W jet width
ad coefficient of diffusional deposition
a, coefficient of inertial deposition
a, coefficient of gravitational deposition
p, gas density
p gas dynamic viscosity
T particle relaxation time
Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
DEVELOPMENT OF A MONITOR TO QUANTIFY LEAD-BASED AEROSOLS
Robert William Vanderpool
Chairperson: Dale A. Lundgren
Major Department: Environmental Engineering Sciences
Manufacture of lead-acid batteries results in the advertent formation of
gas-borne particles containing lead. Since the adverse health effects of
significant lead exposure are well documented, the battery industry is required
to undergo periodic monitoring to demonstrate compliance with existing lead-in-
A prototype instrument has been developed to quantify lead-based
particulates on a near real-time basis. Aerosol samples can be obtained from
high velocity, flowing airstreams as well as from static airstreams. The
instrument thus has potential as an emissions monitor, a recirculation air
monitor, or a monitor of workplace environments. The prototype instrument
contains an air sampling and transport system, a particle collection section, and
a lead analysis section. Proper sequencing of the system's components and
archival of the experimental test results are provided by an IBM-compatible
computer interfaced with a data acquisition and control system. The prototype
monitor may be operated either in attended or automated sampling and
analysis mode. The details of the instrument design will be presented along
with results of its laboratory and field evaluation.
Industrial manufacturing processes often result in the formation of gas-
borne particles (aerosols) containing hazardous metals such as lead, cadmium,
and copper. Since the adverse environmental and occupational health effects
of these substances are well documented, periodic air monitoring of
manufacturing processes involving their use is required to demonstrate
compliance with existing federal and local regulations.
The lead acid battery industry, which accounts for 72% of all domestic
lead usage (Environmental Protection Agency, 1986), is a potential source of
significant lead-based aerosol production. Monitoring of lead emissions in the
battery industry typically requires a separate aerosol sampling and analysis
technique. First, a representative aerosol sample is obtained from the process
using approved sampling techniques. This procedure typically involves aerosol
collection through a high efficiency filter to remove particles from the airstream.
The collected aerosol sample must then be extracted from the collection
substrate and analyzed using an appropriate analytical technique. However, the
time delay between sample collection and analysis is typically on the order of
days to weeks. Moreover, improper handling and extraction of the collected
sample can result in significant errors in estimation of the total lead
concentration. Positive sample bias is a particular problem during
measurement of trace lead levels common during evaluation of occupational or
recirculation air monitoring. Finally, specially approved equipment, sampling
and analysis techniques, and trained personnel are necessary to achieve
accurate results. The need existed, therefore, for an instrument capable of
providing both aerosol sampling and analysis of lead-based particles on a near
The advantages of an automated air sampling and analysis instrument for
lead measurement tests are obvious. Such a device could be used both for
occupational and emission measurements and would significantly reduce the
cost of each test. Such an instrument would report test results within minutes
of the air sampling. Finally, by proper design and construction of the
instrument, the sources of measurement error inherent to the current
methodology could be significantly reduced. Compliance with existing emission
and occupational regulations could be routinely demonstrated. This report
details the design, construction, calibration, and field evaluation of such an
instrument designed for use by the lead-acid battery industry.
Description of Battery Production Process
The design criteria for the lead-in-air monitor was based on careful review
of the various operations involved in the manufacture of lead-acid batteries. Of
particular importance was consideration of the concentration, size distribution,
and chemical composition of the lead-based aerosols produced at each of the
process points. Figure 1-1 is a flow chart of the various operations involved in
the battery production. Not shown in the figure are various paths involving the
reclaim and reuse of lead scrap which occurs at several of the process points.
Battery manufacture first begins with the production of lead monoxide
powder by grinding solid lead metal in the presence of air at elevated
temperatures. Lead oxide paste is then produced by combining the lead oxide
powder with water and dilute sulfuric acid along with small amounts of
Battery plate grids are cast from molten lead-based alloys which consist
primarily of lead but also contain various trace metals to improve the plate's
mechanical properties. Following a specified curing time, the grids are
transported to the pasting operation where they are pasted and flash dried prior
to being off-loaded from the assembly line. The pasted plates undergo another
curing process before use. Plate stacking then occurs which involves alternate
stacking of plates with paper separators. The stacked plates are then loaded
into plastic cases and the appropriate cells connected together during the cast-
on-strap operation. The case cover is then installed, cells filled with sulfuric
C5 0 | 5 M
acid, and the batteries electrically charged prior to the packaging and shipping
The characteristics of the lead-based aerosol produced at each step of
the production process depends primarily on the composition of the material
handled and the mechanism by which the aerosol is produced. The aerosol
produced during the grid casting operation tends to be relatively small in size
and may contain only 5% to 20% lead by mass. Lead mass concentrations of
aerosol produced at the grid casting operation typically meet allowable emission
limits and are therefore ducted to the environment without the need for any
Conversely, production and handling of lead oxide powder, paste and
pasted plates tends to form dispersion-type aerosols relatively large in diameter
and contain 95% to 99% lead by mass. Aerosols from these processes also
tend to be of much higher lead mass concentrations and are therefore ducted
to control equipment (typically baghouses) prior to the airstream's exhaust to
With the exception of the acid fill, charging and packaging operations, all
the steps in the battery production process are sources of elevated lead
exposure to the battery plant workers. These processes, therefore, all possess
ventilation systems designed to remove the majority of the lead-based aerosols
and transport them to the particulate control equipment. Since these airstreams
are ultimately exhausted to the environment, an equivalent supply of makeup air
is required to maintain a flow balance. Battery plants in colder climates have
incorporated recirculation air systems to reduce heating costs associated with
heating the makeup air to plant temperature. This recirculation air is typically
obtained from grid pasting, drying, plate offbearing, cast-on-strap, and plate
stacking operations and is cleaned through a system of two high efficiency
particulate air filters (HEPA) in series with one another. This clean recirculation
air is then supplied to various process points as a source of makeup air.
The Occupational Safety and Health Administration has approved use of
this type of recirculation air system but requires continuous monitoring of the
recirculation air to ensure that the airborne lead content not exceed the
regulated 50 micrograms/m3 permissible exposure limit concentration.
Experience has shown that if the double HEPA filter system is installed and
maintained correctly, concentrations much lower than 5 micrograms/m3 can be
It is primarily for the purpose of monitoring recirculation air quality that
the lead-in-air monitor was developed. Other potential applications to the
battery industry will be discussed.
Instrument Design Criteria
The development of a lead-in-air monitor must be based on the sampling
and analysis requirements specific to the lead-acid battery industry. First, the
instrument must be capable of obtaining representative aerosol samples from
both flowing airstreams (stack or duct sampling) as well as from static
airstreams (room sampling). Particle size distribution measurements have
shown that efficient particle collection must take place over a fairly wide range
of particle sizes.
Accurate quantitation of the particulate lead content is also essential.
The analytical technique chosen must be specific for elemental lead and
possess minimum physical and chemical interference from other particulate
and gaseous components. Analytical accuracy and repeatability must exist over
the measurement range of interest. The analytical technique must possess low
blank and background response for the lead measurement.
Sampling should be performed over a 10 to 20 minute time period.
Instantaneous measurements are neither technologically feasible nor particularly
desirable because variations in lead handling processes and activities result in
considerable concentration variability with time. Time averaged measurements,
therefore, are more appropriate indicators of average process emissions. In
conjunction with a 15 minute analysis time, a 15 minute air sampling period
would allow a complete sampling and analysis cycle to be repeated
approximately every 30 minutes.
The instrument must also be capable of quantifying lead levels over a
wide range of concentrations. These concentrations range from recirculation
system alarm levels of 10 micrograms/m3 to levels of greater than
1000 micrograms/m3 for emissions from paste mixing facilities and other three-
Instrument reliability and ease of use was also a design consideration.
Cleaning, maintenance, and external calibration of the instrument should be
minimized. The working instrument must be at least semi-automated, fairly
portable, and operate on standard 110 VAC, 60 hz power sources.
Finally, it was desired that the instrument's development and construction
costs be kept to a minimum. Once a final instrument design had been
completed and its capability demonstrated, it was desired that duplication costs
be limited to approximately $10,000.
Summary of Design Approach
Development of an automated sampling instrument for the specific
quantitation of lead involved careful consideration of the design criteria outlined
in the previous section. Ideally, the developed instrument would satisfy all of
the listed criteria although certain design and operational tradeoffs could have
been made if necessary. Of primary concern, however, was the instrument's
ability to obtain representative particulate samples from particle-laden airstreams
and accurately quantify lead levels over a reasonable concentration range.
Design of the instrument can be conveniently divided into three separate
sections: the air sampling system, the particulate sampling system, and the lead
analysis system. The sampling system must efficiently capture particles over a
range of sizes and effectively deliver them to the lead analysis section.
Representative sampling in flowing airstreams requires that isokinetic conditions
be achieved (nozzle velocity equal to freestream velocity) through proper
combination of nozzle inlet area and sampling flowrate. It was envisioned that
the sampling system contain several nozzles of varying inlet areas in
conjunction with a 60 to 90 liter per minute (Ipm) capacity sampling pump
whose sampling rate could be controlled and monitored by the user. The range
of operating flowrates was selected to be 10 to 30 Ipm. Following consideration
of available particle collection techniques (filtration, electrostatics, etc.), inertial
impaction was chosen as the mechanism to be employed in the instrument's
particle collection section. Subsequent calibration of the impactor showed that
it would collect 95% to 100% of the lead-based particles present in the in-plant
processes of interest.
Design of the instrument's analytical section required careful
consideration of its specific operating requirements. Of primary concern was
the ability to accurately quantify collected lead in amounts which were expected
at the various in-plant operations. Considering a 15 minute sampling time and
a nominal 15 Ipm sampling flowrate, approximately 0.23 m3 of air would be
sampled. For expected concentrations ranging from 10 micrograms/m3 to
2000 micrograms/m3 at the various in-plant processes, lead deposits of
2.3 micrograms to 460 micrograms would result. If these deposits were placed
in liquid solution (50 ml, for example), lead concentrations of 46 micrograms per
liter (ppb) to 9 milligrams per liter (ppm) by mass would be expected.
A wide variety of analytical techniques was considered during design of
the instrument's analytical section. Most of these techniques were eliminated
from consideration either due to their high cost, level of complexity, lead
detection capability, or failure to lend themselves to convenient automation.
Ultimately, an electrochemical technique was chosen for the quantitation of lead
in liquid solution. A commercially available polarograph was purchased and
adapted for automation.
A schematic of the completed instrument is shown in Figure 1-2. Air flow
through the system's particle collection section is provided by a 90 Ipm capacity
air pump with flow controlled and monitored using a commercially available
mass flow controller. Deposition of the lead-based particles takes place
primarily by inertial impaction into 50 ml of 0.5M nitric acid which is supplied
from a 5 liter capacity reservoir. Following a sufficient sampling time, an aliquot
of the solution is transferred to a flow-through cell in the polarograph for lead
analysis. The lead-in-air concentration can then be calculated from the
sampling time, sampling flowrate, solution volume, and the measured liquid lead
Automated control of the system's components takes place using a data
acquisition and control system (Kiethley Instruments, Inc., Model 575) interfaced
with an IBM-compatible computer. Computer control not only allows for proper
sequencing of the various components but also allows for data reduction and
automatic storage and retrieval of the test results. The Keithley 575 data
acquisition and control system receives necessary power from the computer
and provides both digital and analog input/output capabilities. A Keithley Model
AMM1 analog measurement module provides eight separate analog input
channels with a range of 10V, an input rate of 50 khz, and 12 bit
measurement resolution. The Kiethley 575 contains two analog output channels
with similar capabilities. At power up, the Model 575 self-calibrates the analog
channels with an accuracy of 2%.
Digital control is provided by 32 digital input/output channels with signals
which are transistor-to-transistor (TTL) compatible. A separate 16 channel
module (Kiethley, Model PCM3) provides external AC or DC power control
using TTL activated solid-state relays.
In the 575 system, the user can access the various module channels
using Kiethley KDAC500/I software linked with a GW-BASIC programming
environment. Use of programming logic rather than hard-wired logic allowed
for design flexibility during development of the prototype system.
Design of the proposed lead-in-air monitor was based on a number of
important criteria. The instrument must capture airborne particles in a
representative manner and transfer them effectively to the instrument's lead
analysis section. The analytical technique chosen must be specific for
elemental lead and yield a response that is linearly proportional to mass
concentration. Analytical accuracy and repeatability must exist over the
measurement range of interest and possess minimal interference from the
collection substrate and other particle components.
During the last two decades, considerable progress has been made
towards development of direct reading instruments for aerosol measurement.
These devices include electrical aerosol analyzers, condensation nuclei
counters, aerodynamic particle sizers, optical sizing and counting devices, and
piezoelectric and beta gauge based devices. Note that each of these
techniques measure a different property of airborne particles and thus do not
provide comparable information. Of these devices, only piezoelectric and beta
gauge devices provide direct reading of collected aerosol mass. None of these
techniques provide information in terms of the chemical composition of the
measured aerosol. Typically, devices such as filters, impactors, or electrostatic
collectors are merely used to capture an aerosol for subsequent analysis by a
Conversely, there exist a wide variety of analytical techniques available
for chemical quantitative analysis. Typically, these techniques require that the
aerosol sample be previously collected in a separate step and manually
transferred to the instrument's analytical section. Some sample pretreatment or
extraction is often required to obtain reliable test results.
Rather limited success has been achieved in integrating these separate
aerosol sampling and analysis techniques in a single, direct reading instrument.
Many of these analytical techniques required for trace level analysis are fairly
sophisticated and do not adapt well to the development of inexpensive, portable
instruments. These sophisticated techniques often require daily calibration and
frequent maintenance to ensure accurate test results. The need for automated
sample transfer and preparation also complicates the instrument design.
Several tape samplers are available which provide combined aerosol
sampling with mass determination. In these automated devices, air flow is
normally directed through a porous tape which acts as the collection medium
for the airborne particles. This technique thus provides an integrated sample
with results time-weighted for the sampling period. In the GCA Model RDM301
sampler, particle matter is deposited by impaction onto a mylar film coated with
an adhesive (Uoy, 1983). At the end of the sampling period, the collected
material is quantified by beta attenuation. For low energy beta radiation, the
attenuation is a function of mass density of the absorber and is largely
independent of chemical composition or the particle's physical properties.
Spagnola (1989) developed an automated sampling instrument based on this
analytical principle. The instrument uses electromechanical devices for
automatic positioning of membrane filters in the sample holder. At the end of
the sampling period, the filter is analyzed and returned to its original position in
the sample holder. Application of beta attenuation has also been applied to a
dichotomous sampler developed by Macius and Husar (1976).
A paper tape sampler was also modified to provide lead analysis by x-ray
fluorescence (Smith et al., 1986). The system samples air at 0 to 10 Ipm and
irradiates the collected aerosol mass with a 10 mCi cadmium 109 radioisotope.
A Nal scintillator in conjunction with a multi-channel analyzer provides counts
versus x-ray energy for the resulting x-rays. The count corresponding to the
x-ray energy characteristic of lead provides a measure of the collected lead
mass. The authors note that the presence of arsenic and zinc positively
interfere with the lead measurement.
Automated mass monitors have also been developed based on the
concept of piezoelectric sensing of changes in aerosol mass. The TSI Inc.
Model 5000 deposits particles onto a piezoelectric crystal by electrostatic
precipitation. The rate of change of the crystal's output frequency is directly
proportional to the deposited mass concentration. Recently, this mass sensing
technique was applied to the development of an instrument which uses a low-
frequency tapered element coupled to a filter collector (Wang, 1985).
Successful measurement of aerosol composition "in-situ" has been limited
and has generally not evolved beyond the laboratory. Arnold and Folan (1987)
devised an instrument which levitates a single airborne particle within an
electrostatic field and irradiates the particle with an infrared beam. The
instrument optically detects changes in particle size caused by evaporation of
the particle due to the heat induced by the infrared absorption. The measured
rate of change is proportional to aerosol mass. The instrument is capable of
detecting picogram quantities of material for both organic and inorganic
functional groups such as sulfate and carbonate (Allen and Palen, 1989).
The majority of analytical techniques which have been adapted for trace
aerosol chemical analysis are available only in an "off-line" mode and do not
adapt well to field use. Martinsson and Hansson (1988) have used particle
induced x-ray emission (PIXE) for the multielement analysis of aerosol
previously deposited onto thin aluminum foils. Deposits were stepwise heated
to several hundred degrees centigrade by applying an electric current through
the sample. An ion beam impinged on the sample during the process. Low
atomic weight element information (such as oxygen and carbon) was obtained
by particle elastic scattering analysis while elements with atomic numbers
greater than 14 were determined by PIXE analysis. The chemical composition
of the sample was inferred from noted vaporization temperatures and
stoichiometry. Laboratory tests found the technique useful for quantitation of
ammonium chloride, ammonium nitrate, and ammonium sulfate.
Xhoffer et al. (1989) used a transmission electron microscope to analyze
laboratory generated aerosols deposited onto carbon coated electron
microscope grids. The analytical technique of electron energy loss
spectroscopy was used to observe the change in kinetic energy as the electron
passes through the sample. This technique is generally useful only for low
atomic weight elements. The authors also noted that high background signals
can be associated with the substrate and that rapid deterioration of
environmental samples can occur. The technique was also found to possess
limited quantitative application since the particle's absorption of the incident
electron beam was a function of particle size.
Electron spectroscopy for chemical analysis (ESCA) was investigated by
Muller et al. (1987) as a means of characterizing sub-micrometer sodium sulfate
aerosol collected on various filter media. This technique used incident x-rays to
excite photoemission of core level electrons from the sample. The measured
energy of the emitted photoelectrons is characteristic of the emitting element's
oxidation state. ESCA provides only surface analysis and is thus of limited
quantitative analysis. ESCA signals can also be difficult to interpret
quantitatively. Incorporating a similar surface analytical technique, Fletcher
(1983) used a laser microprobe to vaporize and ionize a collected sample for
analysis by a time-of-flight mass spectrometer. Reported limitations of this
technique were similar to those experienced by Muller et al. using ESCA
Neutron activation analysis (NAA) has been employed to provide
elemental information of collected aerosol samples. In this technique, the
sample is irradiated with neutrons to produce radionuclides. The activities of
the radionuclide products are then measured and related to the quantities of the
elements present in the sample. Podzimek and Wojnar (1990) used NAA to
quantify masses of laboratory generated titanium oxide particles deposited on
polyester filters. In a related study, Ondov et al. (1990) used NAA to quantify
45 elements in rural sub-micrometer ambient aerosols collected by micro-orifice
impaction. Although this technique was found to be generally useful, the
substrate blank precluded analysis of several elements of interest.
X-ray powder diffraction analysis of collected aerosol deposits was
reported by Tanninen et al. (1985). Powder diffraction allows for direct
determination of crystalline compounds. The authors used the diffraction
technique to analyze fumes generated by metal arc welding of steel. The main
analytical difficulty encountered was low instrument output intensity due to the
small particles analyzed in conjunction with the small quantity of material
Both conventional flame and graphite furnace atomic absorption
spectroscopy have been widely used for the analysis of collected aerosol
samples. Davidson (1979) used graphite furnace techniques to characterize
size fractionated aerosol samples obtained from three national parks. Attempts
to incorporate atomic absorption analysis in an automated monitor have met
with limited success (Smith et al., 1986).
DESIGN AND CALIBRATION OF THE PARTICLE COLLECTION SECTION
Accurate quantitation of lead-in-air concentrations requires efficient
capture of lead-based particles. If an aerosol was only collected with 10%
efficiency, the resulting lead analysis would obviously greatly underestimate the
actual lead-in-air concentration. This would, in turn, result in an underestimation
of the lead exposure hazard. This section describes the design and calibration
of a device capable of sampling and capturing lead-based particles with the
necessary high efficiency to ensure accurate test results.
There currently exist a variety of aerosol samplers and collectors which
offer a basis for the collector design. These devices include inertial devices
(impactors and cyclones), electrostatic precipitators, thermal precipitators,
diffusion devices, and filtration devices. Each of these devices relies on a
particular property of airborne particles to remove the particles from the
airstream in which they are suspended.
Fundamental to the design of any particulate collection section is
understanding the nature of the aerosol to be measured. In predicting
individual particle properties (settling velocity, diffusional behavior, etc) the most
important parameter is that of particle size. Since industrial aerosols are always
a collection of particles of varying sizes, it is understanding of the aerosol's size
distribution which is of importance.
Figure 3-1 presents particle size distributions measured at a battery plant
in the southeastern United States. The specific sampling locations represent
sources of aerosols which supply recirculation air. It is believed, therefore, this
size distribution can be assumed to be fairly representative of aerosols which
exist in recirculation air systems. Plotted in the figure for the various process
points are cumulative lead mass distributions versus aerodynamic particle
diameter as measured using a University of Washington cascade impactor. The
aerodynamic diameter is defined as the diameter of a spherical particle of unit
density which has the same settling velocity as the particle under consideration.
Use of an equivalent aerodynamic diameter accounts for the particle's size,
shape, and density.
Inspection of Figure 3-1 reveals that these measured size distributions
are fairly large with half of the lead mass associated with particles greater than
10 micrometers aerodynamic diameter. These results agree well with similar
measurements by Hodgkins et al. (1990) from two different battery
manufacturing plants. Note that only 1% to 5% of the lead mass is associated
with particles less than one micrometer aerodynamic diameter. A device
capable of collecting all particles greater than one micrometer, therefore, would
have an overall mass collection efficiency on this aerosol of 95% to 99%.
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Based on a review of these distributions, both thermal and diffusion type
collectors were eliminated as candidates for the collector design. Although
these type collectors can capture submicron aerosols with high efficiency, their
capture efficiencies for particles greater than one micrometer are quite low
Filtration probably represents the most common form of aerosol
collection in air sampling. Unlike thermal and diffusion type devices, samplers
which rely on filtration mechanisms possess high collection efficiencies over a
wide range of particle sizes. Aerosols encountered in the battery industry could
be expected to be removed by filtration with virtually 100% efficiency. Despite
these advantages, however, filtration was eliminated from design consideration
primarily for logistic reasons. Due to high particle-substrate attractive forces,
particles collected on high efficiency filters are extremely difficult to remove
efficiently even if wet chemical techniques are employed. Wetting of either
fibrous or porous membrane filters dramatically increases its pressure drop
making the filter difficult to reuse. The alternative to filter reuse is to replace it
after each sampling test. Automated replacement of the filter would have
involved design and cost complications which were considered to be
Electrostatic collection was also given consideration during the
instrument's design phase. In these devices, collection relies on the
electrostatic forces which can be exerted on charged particles in an applied
electric field. Similar to filtration samplers, electrostatic samplers possess high
collection efficiencies over a wide range of particle sizes. Use of electrostatic
collection would have involved use of a point-to-plane precipitator for deposition
of airborne particles to a liquid surface similar to the design approach adopted
during the design of the LEAP sampler (Andersen Samplers, Inc.).
This design approach was initially investigated during the instrument
development but was ultimately discarded for two primary reasons. First,
preliminary laboratory tests with a design prototype showed that the flow of
electrical current through the electrolyte resulted in the inadvertent plating out of
previously collected lead ions from solution. Loss of collected lead in solution
would have resulted in underestimation of the total lead-in-air concentration and
thus an underestimation of the lead exposure hazard. Use of electrostatics was
also eliminated due its greater complexity and anticipated maintenance
requirements in the corrosive nitric acid atmosphere of the collection region.
Ultimately, the use of inertial impaction was adopted for the final
instrument design. Inertial impactors have been widely used for aerosol
collection and sizing and can efficiently capture particles greater than one
micrometer diameter. Inertial impactors are relatively simple in construction,
contain no moving parts or electrical components, and require little
Impactors rely on the high inertial forces which can be established on
particles in a high velocity airstream. In a typical impactor stage, particle-laden
gas is accelerated through one or more nozzles and directed toward a solid
surface which forces the airstream to change its flow direction. Due to their
inertia, however, particles will be unable to exactly follow their respective
streamlines and will deviate from the airstream by a distance which is a function
of the particle's aerodynamic diameter and the jet velocity. Particles of sufficient
inertia will deviate a sufficient distance to strike the collection surface and be
removed from the airstream. Conventional impactor stages with high jet
velocities can remove particles of 0.3 micrometers aerodynamic diameter and
Impactor performance is typically described by its Dp, cutpoint which is
the particle size collected with 50% efficiency. Particles larger than the cutpoint
diameter are collected with much higher efficiency. Extensive theoretical and
practical impactor research has shown that the cutpoint can be predicted with
some confidence by
D r=| 9 r I n W3 rS()
Pso Vi 9 i4 pp Q (3-1)
where C, is the Cunningham's slip correction factor determined from the
particle size and the fluid properties, p is the gas dynamic viscosity, n is the
number of jets, W is the jet width, p is the particle density, and Q is the
volumetric flowrate. The Stokes value (STK) can be predicted from knowledge
of the jet Reynolds number, ratio of jet width to jet to plate distance, and ratio of
jet throat length to jet width.
Based on these accepted design criteria, an inertial collector was
designed and constructed for use as the instrument collection section.
Figure 3-2 is a schematic of this device showing key components. Complete
detail drawings of each of the unit's components are presented in Appendix A.
In most inertial impactors, the impaction jet is directed onto a preweighed
stationary surface previously coated with a viscous, nonvolatile grease to
minimize particle bounce and losses due to blowoff. Following the sampling
run, the impaction surface is reweighed to quantify the mass of the collected
aerosol deposit. If chemical analysis is to be performed, the aerosol must be
extracted from the grease in an efficient and representative manner.
The collector developed for the lead-in-air monitor does not impact
particles onto a solid surface but into 50 ml of 0.5 M HNO3. Use of the liquid
impaction substrate eliminates the need for greased substrates and provides for
direct transfer of the particles from the gaseous phase to the liquid phase. High
velocity impaction also provides for the liquid turbulence necessary for efficient
particle dissolution so that aqueous lead ions are formed. High liquid
turbulence also tends to homogenize the liquid sample so that only an aliquot of
the sample need be analyzed for accurate lead determination.
/f y Yle
/ f /
/ / / /
^ / /
01 0 0
Figure 3-2. Schematic of single-stage impactor used for particle collection.
During the sampling run, particle laden air enters the impactor inlet at a
flowrate of 10 to 30 Ipm (depending on the sampling situation) and is
accelerated through a single stage impactor. As shown in Appendix A, the
impaction stage contains 20 circular jets of 0.026 inch diameter. Following its
impingement into the liquid, the airstream then enters an expansion section
which reduces the upward air velocity and thus reduces inadvertent liquid carry
over. Use of a small plug of glass fiber filter at the air outlet has shown to
further minimize the liquid carry over. At the conclusion of the air sampling
period, a liquid outlet provides for transfer of the solution to the instrument's
lead analysis section.
Prediction of Impactor Performance
Inspection of Equation 3-1 allows for the generalized prediction of
impactor performance as a function of sampling air flowrate and the critical
impactor dimensions. Table 3-1 presents a summary of these calculations
assuming standard temperature and pressure conditions for air. Also presented
are actual measured stage pressure drops as a function of the sampling
flowrate. Calculated cutpoints at flowrates of 10, 20, and 30 Ipm are 0.92, 0.63,
and 0.42 micrometers, respectively.
It should be emphasized that these calculations are based primarily on
theoretical and experimental evaluations of impaction onto a solid surface.
However, immersion of an impaction stage and impingement of an airstream
o '3 (
1r CJ 3
0. I CM
QE 00) 0
0 ,- COJ
It 9 c N
CMl lqt I-
into a liquid represents an entirely different flow situation. While impaction is
certainly the most dominant collection mechanism involved, the entire process
may more correctly be described as a bubbling phenomenon.
Unlike the extensive studies related to conventional inertial impaction,
there has been relatively limited research devoted to the absorption of aerosols
by bubbling. As a simplified model, some researchers have considered particle
deposition in air bubbles which freely rise through a stationary liquid. As a
further simplification, these studies only consider the motion of bubbles less
than 0.2 cm diameter which have been found to be approximately spherical in
shape. Larger bubbles become flattened as they rise through the liquid and
pulsations in the bubble shape can occur.
Deposition of particles with a buoyantly rising bubble results primarily due
to the relative motion of the bubble with respect to the liquid. Due to the shear
force which exists at the liquid-gas interface, a gas circulation within the bubble
is established and maintained. As a result of this circulation pattern, large
particles may be removed by inertial impaction to the liquid-gas interface.
Gravitational settling of large particles can also occur as well as removal of
submicron particles by molecular diffusion.
The first analytical and experimental treatment of this particle absorption
phenomenon within a freely rising bubble was performed by Davies and Taylor
(1950). This theory was extended by Fuchs (1964), who developed coefficients
of absorption (deposition efficiency per unit cm of bubble rise) for the separate
absorption mechanisms of inertial impaction, sedimentation, and diffusion.
Later, related experimental studies by Melandrie and Prodi (1971) confirmed the
validity of this approach.
The coefficient of absorption due to inertial deposition, a,, is related to
the bubbles upward velocity, Vb, the particle's relaxation time, T, and the radius
of the bubble, R, by
a 9 = 2 (3-2)
Because particle relaxation time increases with the square of the particle
diameter, this inertial mechanism is dominant for large particles.
Similarly, the absorption coefficient due to particle sedimentation, a,, is
a 4R Vb (3-3)
For most bubble sizes of interest, inertial deposition is an order of magnitude
greater than that of sedimentation.
Finally, diffusional deposition efficiency per unit path length can be
calculated by accounting for the particle's diffusion coefficient, D
a = 1.8 VD
Vd-b VR (3-4)
Large bubbles (R > 1 cm) rising in a liquid tend to flatten and pulsate
such that their rate of rise is difficult to predict. However, smaller bubbles can
be assumed to be spherical and their upward velocity can be predicted by a
empirical relationship developed by Melandrie and Prodi (1971)
Vb = 3.05 v (3-5)
where the dimensions of Vb and R are m/sec and m, respectively.
For a known bubble size and particle diameter, the overall change in
particle concentration (C) per unit length of bubble rise can be calculated by
= ( as + a, + ad ) C (3-6)
For example, Jonas and Schulz (1991) used this relationship to calculate an
overall collection efficiency of 98.8% of one micrometer radius iron oxide
particles within 0.5 cm radius bubbles buoyantly rising through one meter of
Inspection of the absorption equation reveals that for a given particle
size, decreasing the bubble radius results in an increase in absorption
efficiency. The equation also indicates that for a given bubble size, the
dominant collection mechanism depends strongly on particle size. For large
particles (greater than 0.3 micrometers radius) which possess large relaxation
times, inertial and sedimentation forces dominate while diffusive absorption is
negligible. Conversely, particles less than 0.05 micrometers radius are removed
almost entirely by molecular diffusion. Thus, regardless of bubble diameter,
there exists an absorption minimum which exists for particles of approximately
0.2 to 0.4 micrometer diameter. The actual magnitude of the minimum depends
primarily on bubble radius.
Use of these theoretical considerations to exactly predict the
performance of the lead-in-air monitor's impactor proved to be difficult. This
simplified approach assumes bubbles of known size rising through a stable
liquid of constant height. Experimental verification of the theory by other
researchers has been limited and performed at relatively low flow rates using
uniform sized bubbles. However, the dynamics within the teflon impactor
represent a vastly different flow situation. Temporary replacement of the
impactor's opaque housing with a glass enclosure of similar dimensions allowed
for the direct observation of the bubbling phenomenon. This observation
revealed the limitations of the simplified theory to this particular collector. First,
it was evident that the bubbles produced were not uniform in size but existed
over a wide range of sizes. To apply the absorption theory to a distribution of
bubble sizes, the distribution itself must be known and the collection efficiency
equation integrated over the bubble size range. At these high flowrates, it was
also evident that the bubbles themselves experienced considerable oscillation
and occasional breakup due to the high liquid turbulence. Finally, it was
observed that the liquid itself underwent considerable turbulence which changes
the nature of the liquid-bubble interface and results in uncertainties in predicting
the path rise of the individual bubbles.
Due to this lack of reliable design theory, no exact predictions of the
impactor's actual performance were made. Preliminary review of conventional
impactor design theory and bubbling theory were used only as a basis for the
impactor design. For accurate evaluation of the impactor's actual performance,
extensive experimental efficiency tests were conducted with the impactor in the
Calibration of the Particle Collection Section
In evaluating the performance of the particle collector, it was understood
that the collection efficiency would depend on the unit's critical dimensions,
liquid solution volume, particle diameter, and the sampling flowrate. Since the
impactor's dimensions and the solution volume were fixed by design, the
particle size and sampling flowrate were normally the only two parameters
varied during the collection efficiency tests.
The particle collection characteristics of the impactor were evaluated in
the laboratory using monodisperse calibration aerosols of solid, spherical
ammonium fluorescein. These calibration aerosols were produced by two
separate generation systems depending upon the size of the particle required.
For the generation of calibration aerosols 0.5 micrometers aerodynamic
diameter and larger, a Thermal Systems Inc. Model 3050 vibrating orifice
aerosol generator (VOAG) was used. A schematic of the complete generation
and sampling system is presented in Figure 3-3. The VOAG operates on the
principle of the controlled mechanical breakup of a liquid jet into uniform
droplets of predictable size. Periodic breakup of the liquid jet was achieved by
application of an AC signal at a known frequency to a piezoelectric crystal
which housed the liquid orifice. Since the liquid consisted of a known quantity
of a non-volatile solute dissolved in a volatile solvent, the generated droplets
dried to form uniform particles of predictable size. For the calibration of the
impactor, the liquid solution was prepared by dissolving a fluorescein powder in
aqueous ammonia. After their production, the droplets dried to form spherical
particles of solid ammonium fluorescein. The particles were then mixed with
clean dilution air and charge neutralized by exposure to a "Kr radioactive
source. The particles were then mixed with additional dilution air and sampled
by the impactor for the calibration tests. An optical microscope was used to
verify the size and quality of the generated particles collected on a fibrous or
membrane filter before each test series. Figure 3-4 is a scanning electron
photomicrograph of a monodisperse aerosol generated by this technique.
For calculating the particle's aerodynamic diameter from its physical
diameter, an ammonium fluorescein particle density of 1.35 g/cm3 was
assumed. This value was based on density measurements by Stober and
Flachsbart (1973), John and Wall (1983), Vanderpool and Rubow (1988), and
Chen and Crow (1986).
Figure 3-4. Scanning electron photomicrograph of ammonium fluorescein
calibration aerosols produced by VOAG.
Calibration aerosols less than 0.5 micrometers aerodynamic diameter
were generated using a Thermal Systems Inc. Model 3071 electrostatic
classifier. Figure 3-5 is a schematic of this generation setup. Polydisperse
droplets were first generated by air nebulization of fluorescein solutions through
a Collison nebulizer at an operating pressure of 20 psig. The polydisperse
droplets were then dried in a diffusion drier to form a polydisperse aerosol of
solid ammonium fluorescein particles. This aerosol was then passed through a
single stage impactor to reduce the large particle concentration. The pressure
drop across the impactor stage was monitored to verify the magnitude and
stability of the aerosol flowrate. Monodisperse particles of the desired size were
then produced by electrical classification within the classifier. Lastly, the
monodisperse aerosol was diluted to the necessary sampling flowrate using
clean, dry dilution air and sampled by the impactor.
The air exiting the impactor was passed through a filter holder housing a
47 mm diameter glass fiber filter which was used to collect any airborne
particles not collected by the impactor. Flowrate through the sampling system
was provided by a 90 Ipm capacity vacuum pump fitted with a primary valve
and a recirculation air valve. The pressure drop across a calibrated orifice was
used to adjust and monitor the sampling flowrate. A calibrated dry gas meter
measured the total volume of the gas sampled during each sampling run.
The collection efficiency of the impactor was defined as that mass
fraction of particles exiting the impaction jet which was collected in the liquid
solution. The total mass exiting the.jet was estimated by summing the mass
measured in the solution with that mass measured on the 47 mm after-filter.
Collected aerosol mass deposits were quantified fluorometrically using a
Turner Model 112 fluorometer calibrated for fluorescein concentrations ranging
from one part per billion to 14,000 parts per billion. For the collection efficiency
tests, the impactor's liquid solution consisted of 50 ml of 0.1N NH4OH. At the
conclusion of the sampling run, this solution was analyzed directly. To quantify
the mass of aerosol collected on the after-filter, the filter was placed in a 100 ml
polypropylene container with 50 ml of 0.1N NH4OH and the container placed in
an ultrasonic bath for approximately 20 minutes. Previous analytical tests have
shown that virtually 100% extraction efficiency can be achieved using this
Run times for the calibration tests varied depending upon the amount of
aerosol mass collected. For the large particle calibration tests, only 10 minute
run times were necessary. For the smallest particles generated, however, three
hour run times were necessary to achieve sufficient mass deposits for reliable
The performance of the impactor was measured as a function of both
particle size and sampling flowrate. Thirteen discrete particle sizes were used
ranging from 0.05 micrometers to 20 micrometers aerodynamic diameter. For
each particle size generated, collection efficiency tests were performed at
flowrates of 10, 20, and 30 Ipm. Each test series combination of particle size
and flowrate consisted of three separate collection efficiency tests performed
under identical test conditions.
Results of the collection efficiency tests are plotted in Figure 3-6.
Individual test results are tabulated in Appendix B. As expected, the impactor's
collection efficiency varies as a function of both aerodynamic particle size and
sampling flowrate. The right hand side of the collection efficiency curves is of
particular importance. Independent of flowrate, the shape of the curves
resemble that of conventional inertial impactors with impaction efficiency
increasing monotonically with increasing particle size. Clearly, inertial impaction
is the predominant collection mechanism for particles in this size range. In
accordance with impaction theory, the collection efficiency also increased with
increasing flowrate. Measured cutpoints in this region at 10, 20, and 30 Ipm
were 0.90, 0.53, and 0.46 micrometers aerodynamic diameter, respectively.
These measured values compare somewhat favorably with those predicted by
standard impactor theory (0.92, 0.63, and 0.42, micrometers).
The overall shape of the complete curves is quite similar to that predicted
by the simplified bubbling theory. Impaction is clearly dominant for large
particles while molecular diffusion accounts for deposition of small submicron
particles. Note also that an absorption minimum occurs in the range of 0.2 to
0.36 micrometers aerodynamic diameter which is approximately the size region
predicted by bubbling theory. The position and magnitude of the absorption
minimum was also noted to be a function of sampling flowrate. Below
I I I I I I I II
OI C 00
4 0 0
I I I I I I I I o o
O O o o o o ( '0 N
(%) AOua!90013 uo!01alo00
0.2 micrometers diameter, molecular diffusion dominates the particle absorption
and efficiency actually decreases with increasing flowrate. This effect is
believed to result as a consequence of the gas' lower residence time within the
liquid column at higher flowrates.
In terms of quantitative chemical analysis, it is the overall mass efficiency
of the collection device on the aerosol which is of importance. Recall from the
measured size distributions in Figure 3-1, that 95% to 99% of the lead-based
mass is associated with particles greater than one micrometer aerodynamic
diameter. Since the impactor's collection efficiency on these particles is high,
integration of the collection efficiency curve over the measured size distribution
predicts that the impactor should collect this aerosol with over 95% mass
Particle losses through the impactor's inlet section were also quantified
during the calibration tests. Following each test series, the impactor was
disassembled and the inlet section carefully rinsed with a known volume of
0.1N NH4OH. Particle mass deposits were quantified by fluorometric analysis of
the collected rinse solution. In general, test results showed that inlet losses
represent only a small fraction of the sampled aerosol mass. Submicron
aerosol deposits were normally below the limit of quantitation of the fluorometric
technique. Large particle losses were found to be more significant with losses
of 2.2%, 3.2%, and 4.0% measured for particles of 6, 10, and 20 micrometers
aerodynamic diameter, respectively. Subsequent evaluation of the impactor
during its actual field use supported these test results.
Based on the calibration tests, it was concluded that the impactor's
design met the criteria outlined for the overall instrument design. For the
aerosols of interest in the battery industry, the impactor is capable of capturing
the aerosols with high collection efficiency. Use of a liquid solution as the
impaction substrate eliminates particle bounce and substrate overloading which
can exist for solid impaction substrates. Turbulence within the fluid during
collection provides for effective dissolution of the lead-based particles as well as
providing for a well mixed, homogeneous solution. Unlike solid impaction
substrates, the liquid is easily replaced after each sampling test and is
compatible with the chosen lead analysis technique. Lastly, the impactor can
be easily constructed, is physically compatible with the corrosive nitric acid, and
requires little periodic maintenance.
DESIGN OF AIR SAMPLING SYSTEM
Accurate quantitation of lead-in-air aerosols requires representative
collection and efficient transport of the lead-based particles to the instrument's
lead analysis section. Since it is infeasible to capture and analyze an entire
aerosol, a fraction of the aerosol must be extracted which reflects the aerosol's
particle size distribution, concentration, and chemical composition. Because
sampling flowrates may vary from one test to another, the designed
instrument's sampling flowrate must be controllable and accurately measured.
Description of Air Sampling System
Figure 4-1 is a schematic of the air sampling components associated
with Module 1. A description of these components is given in Table 4-1. In
addition to the components shown, Module 1 contains the system's nitric acid
reservoir, liquid pumps, and lead analysis system. Referring to Figure 4-1, the
vertical orientation of the impactor is compatible with its liquid contents and
minimizes particle transport losses from the Module 1 inlet to the impaction
stage. The Module 1 sampling system consists of a set of sharp-edged
To Module 2
Figure 4-1. Schematic of Module 1 air flow system. Component descriptions
are provided in Table 4-1.
Description of Module 1 Air Flow Components
Figure 4-1 Code Description
1 3/16, 1/4, or 3/8 in. Diameter SS Nozzle
2 SS Female Connector, 1 in. Tube 1 in. NPT(F)
3 SS Rexible Hose, Teflon Lined
Cajon, Inc. Model SS-16BHT-36
4 SS Bulkhead Union, 1 in. Tube 1 in. Tube
5 SS 3/8 in. O.D. Tube, 4.25 in.
6 SS Male Connector, 1 in. Tube 1 in. NPT(M)
7 SS Male Elbow, 3/8 in. Tube 1/2 NPT(M)
8 SS 3/8 in. O.D. Tube, 3 7/8 in.
9 SS Union Elbow, 3/8 in. Tube
10 SS 3/8 in. O.D. Tube, 7 7/8 in.
11 SS Bulkhead Union, 3/8 in. Tube
12 3/8 in. Polypropylene Tubing, 6 ft.
nozzles, sampling probe, and impactor in addition to their associated tubing
and compression fittings.
Sampling of indoor air (non-ducted airstreams) is used to evaluate
occupational lead exposures. For indoor air measurements, the sampling
nozzles and sampling probe are not needed. Instead, a sharp-edged nozzle
has been fabricated from a one inch diameter stainless steel tube and attaches
to the top of Module 1. Its vertical orientation allows for direct collection and
transfer of airborne particles to the collector with minimal transport losses.
Unlike indoor air sampling, the sampling of ducted airstreams such as
recirculation airstreams requires sampling from a horizontal duct with high
associated air velocities. As will be discussed, accurate particle sampling from
a flowing airstream requires that a thin-walled nozzle be oriented directly into
the airstream. In addition, the sampling flowrate must be controlled such that
the nozzle inlet velocity equals that of the freestream velocity at the point of
sampling. Depending upon the measured freestream velocity, a nozzle of 3/16,
1/4, or 3/8 inch diameter can be used.
Sampling from horizontal ducts requires that the sampled aerosol must
undergo a 90 degree change of direction prior to entering the instrument's
vertical collection section. The sampling system contains a 39 inch long flexible
probe which is used for this purpose. The probe consists of a teflon inner
lining surrounding by a stainless steel braid. The teflon lining is compatible with
elevated temperatures up to 400 OF and is resistant to corrosion. The probe's
0.88 inch inner diameter matches that of the impactor inlet tube thus minimizing
particle transition losses due to air expansion or contraction.
Air flow through Module 1 is provided by connection to the vacuum
source located in Module 2. As shown in Figure 4-2, Module 2 contains an air
filter, mass flow controller, and cooling fan. Component descriptions are
presented in Table 4-2. The cartridge filter contains both activated charcoal and
fibrous filters and serves to protect the mass flow controller and sampling pump
from any particles remaining in the airstream. A high capacity vacuum pump
with teflon coated wetted surfaces provides the driving force for air flow through
the sampling system. As shown in Figure 4-2, this pump has been fitted with a
recirculation valve to reduce the load to the pump.
The sampling flowrate in the lead-in-air monitor is controlled using a
surplus Tylan Model FC-262 mass flow controller which has been adapted for
use in the system. The FC-262 is a 0-50 slpm capacity unit with flowrate
controlled by an internal solenoid valve. Selection of the flowrate is normally
provided by manual adjustment of a potentiometer to supply a 0 to 5 VDC
command signal to the flow controller. Feedback circuitry within the flow
controller continually adjusts the valve setting so that the actual air flow equals
the desired flowrate selected by the user. A measure of the actual flowrate is
provided by a corresponding 0 to 5 VDC output signal which is normally read
by the user from a digital panel meter. For adaptation to the prototype monitor,
the potentiometer input circuit and output display were removed and the
From Module 1
1 19 //
2 7 13
Filter 4 15 P Pump
5 7 13 1
Figure 4-2. Schematic of Module 2 air flow system. Component descriptions
are provided in Table 4-2.
Description of Module 2 Air Flow Components
Figure 4-2 Code Description
1 SS Bulkhead Union, 1/2 in. Tube 1/2 in.
2 SS 1/2 in. Tube, 2 3/16 in.
3 SS Female Connector, 1/2 Tube 3/8 NPT(F)
4 Disposable Air Filter
Koby Inc., Model "Junior King"
5 SS Female Connector, 3/8 in. Tube 3/8 NPT(F)
6 3/8 in. Polypropylene Tube, 6 5/16 in.
7 SS Elbow, 3/8 in. Tube 3/8 in. Tube
8 SS 3/8 in. Tube, 1 3/4 in.
9 Mass Flow Controller, 0-50 slpm
Tylan Corp., Model FC262
10 SS 3/8 in. Tube, 2 in.
11 3/8 in. Polypropylene Tube, 5 3/8 in.
12 SS Male Tee Run, 3/8 in. Tube 3/8 NPT(M)
13 SS 3/8 in. Tube, 2 5/8 in.
Figure 4-2 Code Description
14 SS 3/8 in. Tube, 2 9/16 in.
15 Valve, Swagelok Corp., Model B1RS6
16 Vacuum/Pressure Pump, 1/4 hp, teflon lined
3.2 cfm, 4A at 0 psi
Thomas Industries, Model 727CM39-TFE
17 3/8 in. Polypropylene Tube, 2 1/4 in.
18 SS Bulkhead Female Connector, 3/8 in. Tube
to 3/8 in. NPT(F)
19 Muffler, Koby Inc., Model M-1-N
command signal is now supplied by an analog output channel from the data
acquisition and control system. The flow controller's output signal is read by a
corresponding analog input channel on the data acquisition and control system.
Figure 4-3 is a complete wiring diagram of the Module 2 circuitry.
Component descriptions, functions, and commercial sources are listed in Table
4-3. Table 4-4 provides connector pin descriptions. A standard 120 VAC, 60
hz source supplies power to the mass flow controller power supply, vacuum
pump, and cooling fan. Power to the flow controller is supplied by a 15 VDC,
200 mA power supply. The power supply is protected using a 0.5 amp fuse.
The sampling pump is internally protected against current overload and thus
requires no additional fusing. On/off control of the air pump and the cooling
fan is provided by use of a solid state relay capable of handling load currents
up to 25 amps. The relay itself is activated through application of a 3-32 VDC
signal which is compatible with standard TTL digital command signals. All
Module 2 input and output signals are communicated by a standard 9 pin
ribbon cable to the Kiethley 575 data acquisition and control system.
Prior to its use, the Tylan mass flow controller had been repaired and
recalibrated by the manufacturer. Its condition and calibration were then
verified in the laboratory. At a variety of flowrates and upstream and
downstream pressures, the actual mass flowrate was measured as a function of
selected mass flowrate. For these tests, a National Bureau of Standards
traceable Gillian Model D-80028 bubble flow meter was used as a flow
Module 2 Electrical Comoonent Ustina and Descriotion
Solid State Relay
Fan Filter Screen(2)
3 Contact, Double Row, 20A
Cinch, Inc. type 3-141
SSR Series, 240 VAC/25A
3-32 VDC input
Potter and Brumfield
250/115 VAC, 10/15A
Feller, Inc. type 6100-31
115 VAC, 0.15A
34 cfm at 0 in H20 S.P.
Comair/Rotron type SV2A1
Anodized, for 3.5 in fan
Fan-S/Qualtek type 06325-B
115 VAC, 1/4 hp, teflon lined
3.2 cfm, 4A at 0 psi
Thomas Industries type 727CM39-TFE
Dual.Voltage (-15/+ 15 VDC), 200 mA
Sola, Inc. type 85-15-2120
1 1/4 X 1/4 in, 0.5A slow-blow
SPC Technology type F-500SS-0.5
SPC Technology type FB1PS
9 pin D-Sub, crimp style plug
SPC Technology type DEC-9P
22 position, card-edge connector
Module 2 Electrical Connector Pin Descriptions
Pump, fan on/off
LL-A MFC Command Signal
LL-3 MFC Output Signal
"High", 0-5 VDC
LL-2 MFC Output Signal
"Low", 0-5 VDC
KK-5 MFC Output Signal
"Low", 0-5 VDC
KK-4 MFC Output Signal
"High", 0-5 VDC
Power +15 VDC Supply
KK-3 MFC Command Signal
-15 VDC Supply
calibration standard. For each operating pressure and flowrate range, six
separate measurements were made. As shown in Figure 4-4, the correlation
between the actual mass flowrate and the desired flowrate was generally good.
In the 10-30 slpm flowrate range of interest, the agreement was within 2%.
Note that conversion from actual to standard flowrate must account for the
fluid's temperature and pressure at the point of sampling. Conversion from
standard temperature as defined by the mass flow controller's manufacturer
(0 C) to standard temperature as defined by occupational regulations (25 OC)
must also be taken into account. In the prototype lead-in-air monitor, these
conversions take place automatically through the instrument's software.
Prediction of Particle Sampling and Transport Efficiency
Accurate measurement of lead-in-air concentrations requires that lead-
based particles be sampled, transported, collected, and analyzed efficiently.
Design of an efficient particle sampling and transport system depends primarily
on the particle size distribution expected, the fluid properties at the point of
sampling (such as temperature and velocity), and the air sampling flowrate
required by the instrument's particle collection section. Efficient design,
calibration, and field testing of a separate sampling system was beyond the
scope of this project. As a preliminary approach, a particle sampling and
transport system was adapted from EPA Reference Method 12, "Determination
of Inorganic Lead Emissions from Stationary Sources." In the forthcoming
15 20 25 30 35
Figure 4-4. Measured performance of Module 2 mass flow controller.
sections, the sampling and transport efficiencies of this design will be predicted
based on published theoretical and empirical studies. Alternatives to this
sampling and transport system will be discussed.
Sampling from Still Air
Evaluation of occupational exposures to lead-in-air concentrations
represents sampling from an airstream at zero or low velocity. Because
particles possess higher stopping distances at higher velocities, sampling and
transport losses during still air sampling can be negligible when compared to
sampling from ducted, high velocity airstreams. There exist, however, two
types of still air sampling biases which can result particularly with regard to the
large particle distributions characteristic of the battery industry.
Efficient still air sampling requires use of a vertically oriented sampling
tube. When the inlet air velocity is low, a sampling bias can result due to high
settling velocities characteristic of particles with large aerodynamic diameters.
At low sampling velocities, this results in an overestimation of particle
concentrations. An additional bias can result due to the effect of particle inertia
on particle trajectory. For sampling in still air, curved streamlines result as air is
accelerated towards the inlet nozzle. Due to their inertia, particles will be unable
to follow their respective streamlines and will be displaced a distance which may
be large with respect to the dimensions of the inlet. Inertial effects of this type
can result in an underestimation of actual particle mass concentrations.
Based on these possible biases, various criteria have been developed for
the design of air samplers. Davies (1968) developed inlet criteria based on
parameters of nozzle inlet diameter, sampling flowrate, and particle
aerodynamic diameter. Agarwal and Lu (1980) predicted less than 10%
sampling losses if the probe diameter (Dt) meets the following criterion
De > 20 r2 g (4-1)
where r is the particle relaxation time and g is the gravitational constant.
Equation 4-1 was used to predict inlet losses of the 0.88 inch nozzle
used for still air sampling of the lead-in-air monitor. Results showed that
particles up to 60 micrometers aerodynamic diameter would be collected
without significant error. Particles greater than 60 micrometers would be
collected with less than 90% efficiency. The equal diameter of the nozzle the
inlet section ensures that particle transport losses should be minimized.
Sampling from Ducted Air Streams
Unlike sampling from still air, sampling from ducted airstreams can result
in significant sampling biases particularly with regard to the high velocity
airstreams and large particle distributions characteristic of the battery industry.
Moreover, the horizontal arrangement of ventilation ducts requires that a
captured aerosol must be transported a short distance to the instrument which
also involves a change in the aerosol's flow direction. As a result, particle
losses within the aerosol transport system may significantly reduce the mass
concentration detected by the lead analysis section. The factors affecting
particle sampling and transport biases will be discussed.
The purpose of the aerosol sampling system is to obtain a representative
aerosol sample from the airstream of interest. In ducted airstreams, this
involves use of a sampling nozzle and probe to extract a representative aerosol
sample and transport it to the instrument's collection section. Sampling bias
can occur, however, due to particles' inertia in flowing airstreams. Similar to the
sampling from still air, sampling criteria have been developed for efficient
sampling from ducted airstreams. These criteria are generally of three types.
First, sharp-edges inlet nozzles must be used to obtain representative air
samples. Blunt-edged orifices can disrupt the airflow entering the nozzle
resulting in nonrepresentative particle collection and high inlet losses (Vincent
and Gibson (1981), Vincent et al. (1982), Vincent et al. (1985), Stevens (1986),
Vincent (1987), Vincent and Gibson (1988)). The second criterion is that the
nozzle must be oriented directly into the oncoming airstream. Changes in the
sampled air direction due to nozzle misalignment are a function of particle
inertia, the misalignment angle, and the probe diameter (Durham and Lundgren,
1980). Lastly, the sampling should be performed under isokinetic conditions
(nozzle inlet velocity equal to freestream velocity). Anisokinetic sampling can
result in significant large particle sampling biases whose magnitude and
direction depend on the velocity ratio and the particle Stokes number. Note
that isokinetic sampling requires prior measurement of the airstream's velocity
at each of the desired sampling points. When sampling, achieving desired
nozzle inlet velocities requires proper combination of nozzle diameter and
Achieving representative aerosol sampling does not ensure efficient
transport of the aerosol from the inlet nozzle to the particle collector. Particle
losses can occur both in the inlet nozzle as well as in the transport section.
Deposition can take place due to molecular diffusion, thermophoretic effects,
electrostatic effects, gravitational effects, and inertial effects. For the sampling
of aerosols of interest in the battery industry, only gravitational and inertial
mechanisms will result in significant transport losses.
Numerous theoretical and experimental studies have been conducted in
relation to understanding particle transport in sampling tubes. In general, these
studies agree on the recognized parameters of importance (such as particle
Stokes number, sampler geometry, and fluid conditions) but often vary widely in
terms of their conclusions and recommendations. Theoretical predictions of
transport losses requires extensive mathematical modeling of both fluid flow
fields and particle trajectories within the flow fields. The validity of these models
is often not confirmed. Differences in sampler geometries and flowrates can
result in significantly different predicted losses. Experimental loss
measurements are generally more practical but differences in sampler
geometries, operating flowrates, and particle properties make comparison of
separate studies difficult.
In the lead-in-air monitor's sampling system, particle transport losses
during duct sampling can occur either in the inlet nozzle or in the flexible
sampling probe. To predict nozzle inlet losses, a brief review of pertinent
literature was made. Although the actual loss will depend on nozzle diameter,
sampling flowrate, and particle size, the literature suggests that large particle
losses may exceed 5% by mass. In the lead-in-air monitor, additional nozzle
losses might also be expected due to the airflow's slight expansion from the
nozzle outlet diameter of 0.75 inches to the 0.88 inch diameter of the sampling
probe. The magnitude of the losses in the expansion section will depend on
sampling flowrate and particle size.
Losses in the sampling probe can more accurately be predicted than
those associated with the inlet nozzles. Considering the flexible probe oriented
as in Figure 4-1, the probe can be divided into three main sections for analysis:
an 8 inch long horizontal section, a 23 inch long section with a 14.5 inch radius
of curvature, and a 8 inch long vertical section. The particle losses in these
sections were considered separately. These calculations were simplified by the
fact that laminar flow conditions exist in the 0.88 inch diameter probe even at a
flow of 30 Ipm. Turbulent flow conditions alter the nature of the fluid flow field
and particle trajectories making accurate loss predictions more complex.
Particle deposition in the 8 inch long horizontal section occurs primarily
due to the force of gravity. During their transit time within the tube, particles
may settle a distance sufficient to be removed from the airstream. The
magnitude of these losses was predicted based on a study by Thomas (1958)
on sedimentation losses in horizontal cylindrical tubes. For laminar flow
conditions, the transport efficiency (penetration) can be predicted
P = 1 [ 2 [ 2 1- 4 2/3 + arcsin0l/3 4 1/3 /1- 2/3 ] (4-2)
3x L Vs R
where P is the fractional penetration, L is the tube length, V, is the particle
settling velocity, R is the tube radius, and Q is the volumetric flowrate through
This relationship was evaluated for the 8 inch long section as a function
of particle size and sampling flowrate and is presented graphically in Figure 4-5.
Independent of flowrate, particles less than 10 micrometers aerodynamic
diameter can be transported through this section with fairly high efficiency.
Particle penetration above 10 micrometers, however, is predicted to decrease
fairly rapidly as a function of particle size. Note also that the lower sampling
flowrate results in lower transport efficiency due to the gas' longer residence
time within the horizontal tube. Efficient transport through this section,
I Nc o m M
.O .) .
therefore, would imply the use of high sampling flowrates and tubes of smaller
Unlike losses in horizontal sampling tubes, particle losses in 900 bends
are due primarily to inertia and centrifugal forces on the airborne particles. Due
to their inertia, particles will be unable to follow their respective streamlines
directly and may be displaced a sufficient distance to impact on the duct walls.
Air flow around the bend can also establish a rotational flow pattern normal to
the mean direction of flow. This rotational profile can result in additional
particle losses around the bend. Cheng and Wang (1981) developed a
relationship for transport efficiency in a 900 bend as a function of particle Stokes
P = 1 STK (4-3)
=1- pp DP2
18 l x R3
This relationship holds for laminar flow conditions and is plotted in Figure 4-6 as
a function of particle size and flowrate. Similar to losses in the horizontal
section, losses around the bend are minimal for particles less than
10 micrometers aerodynamic diameter. Particles greater than 20 micrometers
aerodynamic diameter are difficult to transport efficiently. Note also the effect of
sampling flowrate on transport efficiency. Higher flowrates correspond to larger
/ G )
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l'*' c-L e
I/ o O L
I1 I I 0 I
S o, ,. o. 0 o .-
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-o 00 0 0
" 0 0 0 0o 00 0 0 0-- 0
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stopping distances which would result in greater particle deposition. Inspection
of equation 4-3 would dictate use of low sampling flowrates and large diameter
Note that the effects of flowrate and tube diameter on bend losses are
contradictory to those required for efficient horizontal transport. For a given
tube diameter, higher flowrates result in higher horizontal transport efficiency but
lower transport efficiency around the bend. Conversely, lower flowrates
minimize bend losses but maximize losses in the probe's horizontal section.
Combined losses within these two sections are thus relatively independent of
sampling flowrate. The magnitude of these losses depends primarily on
aerodynamic particle size.
Particle losses within the probe's final 8 inch long vertical section are
predicted to be minimal. Since the tube is oriented vertically, no horizontal
deposition is possible. Operation of the probe in the laminar flow regime also
eliminates the possibility of turbulent deposition which can be significant for
particles with high Stokes numbers in turbulent flow fields. This prediction of
low particle losses in this section is supported by the low impactor inlet losses
measured during the impactor's laboratory calibration.
It can be concluded, therefore, that use of the proposed sampling nozzle
and probe may not allow efficient transport of large particles to the lead-in-air
monitor's collection and analysis section. Combined losses within the nozzle,
nozzle outlet, and sampling probe may be prohibitively high. It should be
noted, however, that these loss calculations may significantly overestimate
particle losses. Theoretical loss calculations normally assume that when a
particle contacts a surface, it is removed from the airstream. Use of liquid
aerosols during experimental loss tests would support these calculations. In
reality, solid particles contacting solid surfaces at high velocity may experience
only fractional sticking efficiencies. Reentrainment of deposited particles also
results in higher particle transport efficiency.
It is apparent that special sampling considerations may need to be given
to the on-line measurement of ducted airstreams in the battery industry.
Conventional stack sampling techniques and equipment may not enable
representative sampling and transport of these large aerosols. As possible
alternatives, three sampling approaches could be investigated. First, the
ductwork at the monitoring point of interest could be modified to include a short
vertical section so that the entire flow is directed downward. A sharp-edged
sampling nozzle attached vertically to the lead-in-air monitor could then
isokinetically extract a representative sample with minimal inlet and transport
losses. Alternatively, a small flexible test section attached to a high volume
vacuum source could isokinetically extract a portion of the major duct flow and
direct this secondary airstream downward over the monitor's sampling nozzle.
Aerosol not sampled by the monitor could then be returned to the major
airstream or filtered prior to its exhaust to the atmosphere. This concept is
similar is design by Baron (1988) and Ball and Mitchell (1990) for efficient
transport of coarse aerosols to particle sizing instrumentation. As a final
alternative, the conventional sampling nozzle could be replaced with a
precollector designed to remove all particles greater than 10 micrometers
aerodynamic diameter. The remaining fraction could then be transported
efficiently to the monitor's collection section. In the human respiratory tract,
particles less than 10 micrometers represent the size fraction capable of
penetrating to the alveolar region and thus represent the best measure of the
actual lead exposure hazard. Particles larger than 10 micrometers are
deposited in the upper regions of the respiratory tract and are absorbed in the
bloodstream with much lower efficiency. Recent developments in
10 micrometer size selective inlets indicate that this design approach is feasible.
Use of a precollector to evaluate worker exposure, however, must first meet
with approval by the Occupational Health and Safety Administration.
DESIGN OF LEAD ANALYSIS SECTION
As discussed, accurate evaluation of lead-in-air concentrations requires
efficient aerosol sampling, transport, collection, and accurate analysis of the
collected aerosol's lead content. Previous sections have outlined the
techniques required to ensure representative aerosol sampling, transport, and
collection. This chapter will outline design criteria for the lead analysis
section, briefly review available analytical techniques, and describe in detail the
analytical technique chosen for incorporation into the lead-in-air monitor.
Selection of the instrument's lead analysis technique was based on
careful consideration of several design criteria. First, the technique must be
specific for elemental lead and capable of accurately quantifying lead
concentrations ranging from 50 ppb to 9000 ppb. It was desired that the
technique's response be fairly linear over a wide range of concentrations and
the technique display adequate repeatability. Because the collected aerosol
sample's composition is not entirely lead, the analytical technique should
possess minimal physical and chemical interference from these other
components. The technique must also be physically compatible with the
chosen particle collection substrate. It was important that the selected
analytical technique require little periodic maintenance or recalibration. Finally,
the selected technique must meet cost requirements and be adaptable to
Based on these criteria, a thorough review of pertinent literature was
made with regard to selection of the instrument's analytical technique. There
exist a wide variety of analytical techniques available for the quantitation of lead
either in solid or liquid phase. These techniques include both conventional and
graphite furnace atomic absorption spectroscopy, ion chromatography,
inductively coupled argon plasma spectroscopy, x-ray and electron techniques,
colorimetry, neutron activation analysis, and electrochemical techniques. For an
automated device, most of these techniques were eliminated from consideration
either due to their high cost, level of complexity, inadequate lead detection
capabilities, or failure to adapt themselves to convenient automation. For
example, conventional atomic absorption is incapable of detecting sub-ppm
levels of lead in solution. Although graphite furnace atomic absorption
possesses the necessary detection capabilities, its use would have greatly
increased the complexity and cost requirements of the monitor. In addition, the
response of both conventional and graphite furnace atomic absorption displays
a fairly limited range of linearity. Other techniques such as colorimetry involve a
number of chemical handling steps which would have been inconvenient to
Ultimately, an electrochemical method of lead analysis was regarded as
most appropriate for the lead analysis. Because metals display high
electroactivity, their presence is solution can be detected over a wide range of
concentrations. Electrochemical techniques display high linearity and
repeatability. In particular, the electrochemical technique of polarography was
selected as that which best met the outlined design criteria. In forthcoming
sections, the principle of polarography will be reviewed and its application to
lead-in-air monitor described.
Description of Polarographic Analysis
Electrochemical analysis is based on the observed behavior of
electroactive species in solution when subjected to an electric potential gradient.
For the detection of a particular species of interest (such as metals), a varying
potential is applied to the solution through a conductive electrode. If at a
particular potential a species is oxidized or reduced, an electric current will flow
at the electrode. Since the rate of electron transfer is a function of the number
of redox reactions occurring at a given potential, the measured current
produced is proportional to the concentration of that component in solution.
Because electroactive species display characteristic redox potentials, the
observed potential at which the redox reaction occurs can be used to identify
the component of interest. By applying a range of potentials to a sample and
noting the potential and resulting current of several redox reactions,
simultaneous quantitation of several solution components is possible. This
analytical technique is commonly referred to as voltametry.
Although the study of voltametry is not new, recent advances have led to
the development of reliable, commercially available instruments suitable for trace
metal detection. Current designs now incorporate mercury drops as the active
working electrode surface. Since a fresh mercury drop is produced for each
separate sample analysis, carry over contamination of the working electrode is
virtually eliminated. Recent advances in microprocessor technology have also
led to increasing measurement sensitivity and instrument capabilities.
Commercially available analyzers allow the user to select from various applied
potential waveforms which optimize detection of the component of interest. For
many metals (including lead) part per billion detection capability is routine.
Based on a review of available electrochemical detectors, an EG&G
Model 303A static mercury drop electrode was purchased along with an
accompanying Model 264A control unit. The Model 303A contains a mercury-
based working electrode, a silver/silver chloride reference electrode, and a
platinum counter electrode which are all immersed in a static cell containing the
solution of interest. The Model 303A also contains a 5 Ib capacity mercury
reservoir and a delivery system for dispensing individual mercury drops of
Control of the Model 303A electrode system is provided through
communication with the Model 264A control unit. The Model 264A sets the
Model 303A operating parameters, provides for automatic sequencing of the
analysis steps, receives and conditions the resulting output signals, and
communicates the current versus voltage scan results to an x-y recorder. The
user is able to select from a variety of a scan types including normal direct
current, normal pulse, and differential pulse polarography as well as direct
current and differential pulse stripping analysis. Although these techniques
possess varying degrees of capabilities and applications, differential pulse
stripping analysis provides for the most sensitive detection of lead in solution.
This mode, therefore, is used exclusively for lead quantitation in the lead-in-air
Quantitation of lead in solution by stripping analysis typically involves the
following sequence of steps. First, high purity nitrogen is bubbled through the
liquid sample at a flowrate of approximately 40 cc per minute for approximately
3 minutes. This deaeration step removes the electroactive interference of any
dissolved oxygen in the solution. The Model 303A then dispenses a single
mercury drop of selectable size which hangs on the tip of a glass capillary
immersed in the solution. A constant electric potential is applied to the mercury
drop whose magnitude is more negative than the half-wave potential of the
analyte of interest. This results in a electro-reduction of the analyte onto the
surface of the mercury drop. To ensure repeatable mass transfer rates during
this step, the solution is stirred and deposition is performed for a fixed time
period (such as 3 minutes). The preconcentration step is responsible for the
technique's low detection limit for many electroactive species.
At the conclusion of the deposition step, the stirrer is turned off and the
electroactive species is stripped from the electrode by making the electrode
more positive and oxidizing the species back into solution. In this stripping
step, varying the potential at a constant rate (such as 10 mV/sec) allow
different species to oxidize at different times. In differential pulse stripping
analysis, the current measured during each species' oxidation reaches a
maximum value that is proportional to the concentration of each species that
As an example, Figure 5-1 presents the results of a typical analysis of
40 ppm lead, cadmium, and copper standards in 0.1N nitric acid. The position
of the peaks (with potentials referenced to the Ag/AgCI electrode) identifies the
species present in solution and the peak height for each species is proportional
to its concentration in solution.
To ensure reliable test results of trace metal analysis, special
experimental techniques were observed during all polarographic analyses. To
minimize sample contamination and species absorption losses, all polarographic
cells were made of teflon and were acid washed with 6M nitric acid prior to use.
Metal standards were prepared from 1000 ppm stock metal standards certified
by the manufacturer to within 1% accuracy. Serial dilutions were performed
I n I
o C .
o c a
0 0 > I
Q I- I-- I cI S
(V 90-30 L) -ua-0in
using positive displacement pipettes with disposable tips rated to within 2%
accuracy. Dilute nitric acid solutions were prepared using trace metal grade
concentrated nitric acid diluted with distilled, deionized water with a measured
resistivity of 17 x 106 ohm-cm. Only certified, triple distilled mercury and ultra-
high purity nitrogen were used in the polarograph.
If polarographic stripping analyses are performed under carefully
controlled conditions, repeatable test results can be obtained over a wide range
of concentrations. Figure 5-2 presents the instrument's response as a function
of lead concentration in 0.1N nitric acid. Each data point represents the
average of 6 separate tests performed under identical test conditions. The
technique's precision (as expressed by the relative standard deviation) was
measured to be approximately 2%.
Considerable attention was devoted to identification and subsequent
optimization of the parameters which affect the polarographic's response to
trace lead concentrations in solution. Factors affecting the deposition step were
identified to be initial deposition potential, mercury drop size, deposition time,
and solution stirring dynamics. The instrument's response during the stripping
phase was found to vary with the potential's rate of change (scan rate) and the
height of the differential wave applied to achieve oxidation. The overall
response was also found to vary with the condition of the Ag/AgCL reference
electrode, solution temperature, and the concentration of the nitric acid solution
o c c
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C, 2o o
o o c a,
Within limits, these variables can be controlled by proper selection of the
polarograph's operating parameters. However, complete control of all of these
variables in an automated monitor is impractical to achieve. In stripping
analysis, the instrument's response to a given lead standard was noted to vary
as much as 50% during successive test series. Ultimately, this variability was
virtually eliminated by including an internal standard (200 ppb copper) in the
nitric acid electrolyte.
Use of the internal copper standard was developed based on numerous
observations of the polarograph's response to several species in solution.
During the analysis of equal mass concentrations of lead, copper, and
cadmium, it was noted that the ratios of the resulting peak heights were
essentially constant regardless of the operating parameters of the instrument.
For example, although reducing the deposition time during stripping analysis
reduced the magnitude of each element's peak, the respective peak ratios
remained essentially constant. Similar behavior was noted by changing the
mercury drop size, nitric acid concentration, analysis scan rate, solution
temperature, and solution stirring dynamics. Thus, if the instrument response to
a known concentration of an analyte is measured, the concentration of a
different species can be calculated from its observed peak height and the
known peak ratio. During the course of the project, the instrument's response
to equal lead and copper concentrations was measured as a function of
concentration and instrument's operating parameters. For 156 separate
measurements, the ratio of the copper to lead peak height was measured to
average 2.33 with a relative standard deviation of 1.8%. Assuming the use of a
200 ppb copper internal standard, the concentration of lead in solution can thus
be calculated using the following relationship
Lead Cone. (ppb) = Lead Peak Height X 2.33 X 200 ppb
Copper Peak Height (5-1)
Use of an internal standard in the lead-in-air monitor offers several
advantages. First, the need for periodic calibration of the analytical section is
eliminated since the instrument essentially calibrates itself during each sample
analysis. Analytical accuracy is no longer sensitive to variables which would be
difficult to control within required limits. Lastly, use of the internal copper
standard provides an internal quality control check during each analysis. If the
instrument's response to the copper standard is outside the expected range, it
serves as an indication that either the electrolyte was prepared incorrectly or
that a possible instrument malfunction may exist.
To determine the value of this design approach, lead standards of
concentrations ranging from 0 to 8000 ppb were prepared in 0.5M nitric acid
with a 200 ppb copper internal standard. The instrument's response to these
solutions was then measured and Equation 5-1 used to predict the actual lead
concentration. For each concentration, six separate measurements were
performed under identical test conditions. Figure 5-3 presents the results of the
0 to 200 ppb lead measurements in comparison to the instrument's ideal
50 100 150 200
Actual Lead Conc.
Figure 5-3. Polarograph response to 0-200 ppb lead concentration using
200 ppb internal copper standard.
response. It can be seen that agreement between the measured and ideal lead
concentration is quite good for lead concentrations greater than or equal to
approximately 20 ppb. The observed response at 0 ppb is due to the presence
of hydronium ions in solution which provide a residual current overlapping the
position of the lead peak. At a 0 ppb lead concentration this interference
typically provides a calculated lead concentration of 5 to 15 ppb. For lead
concentration above 20 ppb, however, the contribution of the hydronium ions is
insignificant. A minimum quantitation limit of 20 ppb lead is thus reported for
the monitor's analytical section.
Figure 5-4 presents the results of the 0 to 8000 ppb lead response tests.
Note that use of the internal copper standard is accurate even over this wide
range of concentrations. A summary of these test results is tabulated in
Table 5-1 including the technique's precision as expressed by the relative
standard deviation. For lead concentrations ranging from 20 ppb to 8000 ppb,
the precision was calculated to be approximately 2%.
Analysis of Field Bulk Samples
In any quantitative chemical analysis, it is important to ensure that the
analytical technique chosen be compatible with the sample to be analyzed.
Physical and chemical interference associated with the sample's measurement
should be noted and effort made to minimize them in order to achieve reliable
test results. In the case of the lead-in-air monitor, it was important to ensure
0 1000 2000
3000 4000 5000 6000 7000 8000
Lead Conc. (ppb)
Figure 5-4. Polarograph response to 0-8000 ppb lead concentration using
200 ppb internal copper standard.
Polaroaraph Response Using Copper Internal Standard
Actual Lead Conc. (DDb)
Lead Conc. (ppb) RSD(o%)
that the expected aerosol's composition be compatible with polarographic
During battery manufacturing, airborne particles are produced primarily
by mechanical breakup (dispersion) of lead oxide powder, lead oxide paste,
and the lead-based plate material. It can thus be expected that the chemical
composition of the aerosol produced from these materials is virtually identical to
that of its parent material. Bulk samples of lead oxide powder, paste, and plate
material were obtained at the battery plant and returned to the laboratory for
analysis in order to identify any potential analytical interference.
During these tests, a known mass of each material was dissolved in nitric
acid solution and analyzed using the polarograph. For comparison purposes,
these samples were also analyzed using a graphite furnace atomic absorption
(GFAA) spectrophotometer manufactured by Perkin-Elmer (Model 5100). The
GFAA is more widely used for trace metal analysis than the polarograph and
thus represents an accepted criterion upon which to base the polarograph's
accuracy. For each test series, the GFAA was calibrated for lead
concentrations in its linear operating range of 0 to 40 ppb by autodilution of a
100 ppb lead stock standard solution. For comparison purposes, the diluent
consisted of 0.5M nitric acid with a 200 ppb copper internal standard. To
minimize matrix effects associated with lead measurement by GFAA analysis, a
fixed quantity of a matrix modifier (0.2 mg NH4H20PO4 + 0.01 mg Mg(NO3)
was injected with each 5 pl sample.
The results of these comparison tests are presented in Table 5-2.
Analysis was first performed on certified laboratory samples of lead nitrate, lead
monoxide, and lead acetate. Based on the known formula weights of these
materials, each sample was prepared to obtain an expected lead concentration
of 100 ppb. As shown in Table 5-2, both the polarograph and the GFAA
measured these solutions with reasonable accuracy. Based on the three
replicate analyses for each material, the polarograph displayed somewhat better
repeatability than did the GFAA.
Similar solutions were prepared from the bulk materials collected at the
battery plant. Although the lead content of the lead monoxide powder can be
assumed with some confidence, its actual percentage in the lead paste can only
be estimated based on discussions with the battery plant personnel. Results
shown in Table 5-2 generally verified these estimates. Similar to the analysis of
the certified laboratory samples, these results indicated fairly good agreement
between the polarograph and the GFAA. More important, no interference was
observed during the polarographic analysis of these battery plant samples. It
was concluded, therefore, that no interference can be expected during analysis
of any airborne particles produced from these bulk materials.