Atomic emission and atomic fluorescence spectrometry in inductively coupled plasma

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Material Information

Title:
Atomic emission and atomic fluorescence spectrometry in inductively coupled plasma
Physical Description:
ix, 231 leaves : ill. ; 28 cm.
Language:
English
Creator:
Nikdel, Seifollah, 1942-
Publication Date:

Subjects

Subjects / Keywords:
Spectrometer   ( lcsh )
Emission spectroscopy   ( lcsh )
Fluorescence spectroscopy   ( lcsh )
Genre:
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis--University of Florida.
Bibliography:
Includes bibliographical references (leaves 225-230).
Statement of Responsibility:
by Seifollah Nikdel.
General Note:
Typescript.
General Note:
Vita.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 000097475
notis - AAL2915
oclc - 06572087
System ID:
AA00003484:00001

Full Text











ATOMIC EMISSION
AND ATOMIC FLUORESCENCE SPECTROMETRY
IN INDUCTIVELY COUPLED PLASMA















BY

SEIFOLLAH NIKDEL


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY














EDUCATION


FOR INDEPENDENT


THOUGHT


is not enough


to teach


man


a specialty.


Through


it he


come


a kind


of useful


machine


but not a harmoniously


developed


person-


ality.


It is essential


that


the student


acquire


an understanding


of and


a lively


feeling


for values.


He must


acquire


a vivid


sense


of the beau-


tiful


and of the morally


good.


Otherwise


he--with


his specialized


knowledge--more


closely


resembles


a well-trained


than


a harmoniously


developed

beings, t


person.


heir


He must


illusions,


learn


and thei


to understand

r sufferings


the motives


in order


of human

acquire a


proper


relationship


to individual


fellow-men


and to the community.


ALBERT


EINSTEIN


From


York


Times


October


1952














DEDICATION


This dissertation is sincerely dedicated to my wife,


Christine Nik-


del, and my mother,


who have given me their


love and encouraged me to


obtain the endless knowledge within higher education.


In their debt


will always be.














ACKNOWLEDGEMENTS


wish


take


this


opportunity


express


my regard


to the friends


and colleagues


who,


through


their


interest


and discussions,


helped


make


this


time


pass


so easily.


like


to take


particular


pleasure


in express-


special


gratitude


research


director,


Graduate


Research


Profes-


sor Dr.


James


. Winefordner,


for his constant


sincerity


steady


guidance,


and kindly


inspiration


throughout


this


work.


Special


thanks


Drs.


Omenetto


Epstein


and R


Reeves


their


valuable


help


during


course


this


work.


extend


my appreciation


to the University


of Florida


Graduate


School


the financial


support


during


this


time.


And,


the patient


comprehension


and love


wife


Christine


typing


the first


draft


of this


dissertation


has made


this


effort


worth


while.


Her continuous


cheerfulness


and undeniable


sacrifices


offered


inspiration


when


was most


needed.
















TABLE OF CONTENTS



Page


ACKNOWLEDGEMENTS.......................... ... .. .................. iv

ABSTRACT........... ................................................. vii

CHAPTER

ONE INTRODUCTION......................... ..... .. ...... 1


- Atomic Emission..............
A.1 History of the ICP.....


.................
. .... .*..
* C C C C C .


- Theory and Mechanism of Excitation
in ICP.................... ........
- Sensitivity and Growth Curve in ICP


- Atomic Fluorescence..............
B.1 History..................
B.2 Theoretical Considerations.
B.3 Noise and Detection Limit..


*.... 3


C.............
* C .C.C.. ..


MOLECULAR EMISSION SPECTRA AND ATOMIC
EMISSION SPECTROMETRY IN ICP........................


- Molecular Emission Spectra...
A.1 Introduction...........
A.2 Experimental...........
A.3 Results and Discussion.
- Rare Earths Analysis.........
B.1 Introduction...........
B.2 Experimental...........
8.3 Results and Discussion.


THREE


...... .... .a. ..
S.a....... ..... C



a...... ...C.C.C.C..
...... ... C *Ce .*
*.C.....C ..* ..* .....*. *


ATOMIC FLUORESCENCE SPECTROMETRY IN THE FLAME
USING ICP AS A NEW EXCITATION SOURCE "ICP-


EXCITED FASF"


- ICP Diagnostics Using the ICP-Excited FAFS..


A. 1 Introduction...................


S..... ..











- Introduction...........


- Experimental.....................
- Results and Discussion...........


- Applications...........


FOUR


... 109
... 110
... 116


. 128


ATOMIC FLUORESCENCE SPECTROMETRY IN ICP WITH
DYE LASER EXCITATION "LASER-EXCITED AFS IN ICP"


- cw Dye Laser


as an Excitation Source in ICP


- Introduction...........
- Experimental...........
- Results and Discussion.


S.. 135
... 135
... 136
... 141


- Relative Spatial Profiles of Barium Ion
and Atom in the Argon Inductively Coupled


as Obtained by Laser Excited


Fluorescence..........................
B 1 Intr oduction
S1 Introduction... .. .. . ......
B.2 Experimental.. ...................
B.3 Results and Discussion..........


- Pulsed Dye Laser
in ICP..........


..... 155
..... 155
....... 158
....... 163


as an Excitation Source


- Introduction...........
- Experimental.........
- Results and Discussion.


FIVE


S..... .. 209


CONCLUSIONS AND FUTURE WORK ON INDUCTIVELY


COUPLED PLASMA EITHER AS ATOM/ION CELL OR AS
AN EXCITATION SOURCE..... ........................... 218


REFERENCES............. ..... ...... ..... ........ ... ...........


BIOGRAPHICAL SKETCH..... .. * .4.. .. 231


Plasma


Page














Abstract


Diss


of the Univer


ertation


sity


Pres


Florida


ented


to the Graduate


in Partial


Fulfillment


Council
of the


Requirements


for the Degree


Doctor


Philosophy


ATOMIC


SSION


AND ATOMIC


FLUORESCENCE


SPECTROMETRY


IN INDUCTIVELY


COUPLED


PLASMA


Seifollah

December


Nikdel

1979


Chairman:


James Dudley


Winefordner


Major


Department


Chemi


stry


Molecular


emission


spectra


native


species


and NH)


in the


argon


inductively


coupled


plasma


(ICP)


have


been


studied


as a function


height


in the plasma,


input


power


and adjacent


environment


to the


plasma.


Similar


studi


were


carried


out with


an ICP into


which


is introduced


moderately


concentrated


solutions


elements


forming


stable


monoxides,


such as Sc,


, Gd,


and Lu


Using


a medium


power


.25-1


ICP as an excitation


source


atomic


emission spectrometry


with


a glass


concentric


nebulizer


, aqueous


solutions


rare


earth


elements


pure


solution


and combined


mixture


all the


rare


earth


elements


are measured


except


Analytical


lines


characteristics


for such


measurements


are given.


T aa L 21-2- -


1:-


,-C. -I


-C c, r^ ~ -- i i,


T~,, 1,,,,, L,,:,


L








qualitatively


detected.


In such


means


urements


the flame


is used


as a reso-


nance


detector.


The results


show


that


self-absorption,


while


present


severe


and that


self-reversal


is absent


in the observation


zones


com-


only


used


for analytical


measurements.


Furthermore


use


of the ICP


as excitation


source


in flame


atomic


fluorescence


spectrometry


permitted


low limits


of detection


to be achieved


with


a simple


experimental


set-up.


An inductively-coupled


argon


asma


(ICP)


is used


as a narrow


line


radiation


source


for the excitation


of atomic


fluorescence


in several


analytically


useful


flames


(nitrogen


-separated


air/


acetylene


and nitrous-


oxide/


acetylene).


Detection


limits


14 elements


are compared


to atomic


fluorescence


detection


limits


using


other


radiation


sources


and to those


other


atomic


spectrometric


techniques.


Dominant


noise


sources


which


limit


measurement


precision


and high


concentrations


and the


sig-


nificance


and correction


for the scatter


problem


are di


scussed.


reduction


spectral


interference


observed


in ICP-emission


demonstrated


for the determination


zinc


in unalloyed


copper


SRM-394


and 396)


The technique


is also


applied


to the determination


zinc


SRM-1633


cadmium


and zinc


in simulated


fresh


water


(NBS


SRM-1643


copper


zinc


in orange


juice.


Atomic/ionic


fluorescence


in the ICP


using


a cw dye


laser


as an ex-


citation


source


has been


studied


Detection


limits


elements


such


and V


are given


Also


the results


show


non-Local


Thermo-


dynamic


Equilibrium


excitation


conditions


in our low powered


ICP.


Relative


ionic


and atomic


fluore


science


profiles


for barium


have


been







procedure,


with


a volume


resolution


of approximately


. The profiles


are given


along


the excitation


axis


as well


as along


the observation


axis,


different


heights


above


the coil


and different


input


powers.


At low


heights


ion profile


resembles


a hollow


pencil


with


a typical


double-


peaked,


asymmetric


distribution,


while


the atom


profile


seems


to be


com-


elementary


to the


ion profile.


Some


scatter


from


water


is also


evident


at low heights.


tuning


the laser


at two excitation


transitions


shar-


a common


upper


level


the temperature


the plasma


could


be evalu-


ated.


The atomic


and ionic


fluorescences


iron


barium


and indium


excited


flashlamp-


and nitrogen


laser-pumped


pulsed


lasers


in the


inductively


coupled


plasma


(ICP)


are studied.


Noise


sources


are investi-


gated


and detectability


is compared


to the techniques


of ICP-emission


laser-excited


atomic


fluorescence


spectrometry.















CHAPTER


INTRODUCTION


- Atomi


- History


Emission


the ICP


Earl


developments


ectrodel


ess


charges.


The low-pressure


electrodel


ess


charge


been


known


since


1884.


was discovered


first


Hittorf1


experiments


on the conduction


electricity


through


the noble


cases.


Mavrodineanu


and Hughes


traced


the rf-excitation


of low-pressure


ases


for spectrochemical


Thomson,


1927


analysis


published


to the work


experimental


Tesla


results


in 1891


along


Then


with


theory


the electrodeless


discharge


expressing


the magnetic


and el


field


spatial


distributions


Bessel


functions


, assuming


constant


asma


conductivit


across


the column.


Later


Babat,


in 1942,


discov-


ered


that


a ring


discharge


can be


maintained


while


pressure


creased


modern


up to atmospheric


inductively


evel.


coupled


Thus


asma


Babat


discharge.


the inventor


However


of the


major


terest


developed


only


after


Reed


success


fully


showed


that


one


can pro-


duce


an "Induction


Torch"


where


the plasma


is observabi


above


the torch


useful


as a heat


source.


ec-


* U


. .








engineers,


and second


as a spectrochemical


excitation


source


ana-


lytical


chemists.


main


reason


for the delay


developing


a stable


Inductively


Coupled


Plasma


(ICP


after


Babat's


publication


was the de


sign


a re-


liable


method


cooling


and heat


protection


of the plasma.


Water


cooled


wall


water


jackets


porous


dielectric


walls


were


applied


but Reed


s approach


a gas cooling


system


proven


most


effective


is now


universally


accepted


for spectrochemical


applications.


Reed


s work


showed


three


attractive


properties


for the plasmas


which


were


high


gas temperature


(ii)


capability


of being


sustained


noble


environments


(important


from


free-atom


lifetime


consideration)


iii)


freedom


from


contamination


electrodes


which


were


quired.


After


Reed


groups


Greenfield


and Wendt


and Fassel7


independently


investigated


the potential


use of


induction-heated


plasmas


as an excitation


source


. vaporization


, dissociation,


decomposition


cell


free


atom/


reservoir


for atomic


ssion


spectrometry


Fassel


and co-workers


studied


a great


number


variabi


veloping


group


an analytically


had experienced


useful


problems


ICP system


introducing


Until


sample


1969


into


Fassel


the high


temperature


zone


of the plasmas


. At that


time


Fassel


and Dickinson


were


able


"punch


a hole"


through


the center


of the plasma


intro-


during


high


velocit


carrier


in the sample


injector.


However


same


solution


had been


used


since


1964


Greenfield


in England







results


, (ii)


to be sensitive


give


detection


limits,


(iii)


need


sample


pretreatment


cover


a broad


range


elements


. ideally


measure


all elements


in the periodic


table)


possess


a large


namic


concentration


range


and (vi)


to allow


simultaneous


multi


-element


determination.


Looking


to the field


instrumental


analysis


the optical


tech-


niques


seem


be the most


appropriate


for analytical


purposes.


They


are favored


their


speed


ease


operation


as well


as simple


sample


pretreatment.


The possibilities


handling


many


samples,


giving


high


sensitivity


and of


determining


numerous


elements


simultaneously


are the


other


important


characteristic


relating


this


field.


The inductively


coupled


radio-frequency


argon


plasma


(ICP)


being


an extremely

capability


efficient


atomic/


and the high


chemical


interference


ionic


kinetic

s than


emission


source


temperature


lower


makes


temperature


has multi


less


sources


-element

susceptible


such


flames.


general


chemical


effects


are 1


than


10%.)


pre-


cision


is comparable


flame


atomic


absorption


-5%)


The reported


linear


dynamic


range


approaches


the capabilities


modern


electronic


equipment


which


are indeed


needed


to take


advantage


of this feature


and exceeds


the linear


dynamic


range


flame


atomic


absorpt


cons


erably.


The limits


of detection


reported


most


widely


the literature


ssion


spectrometry


with


the ICP usually


lie i


range


of 0.01-


are one


to three


orders


of magnitude


lower


than


with


high


I-,








phase by the excitation source,


atoms or


i.e.


concentration in condensed phase to


ions per unit volume in the gas phase c-nM;


transition of a


certain number of these atoms/ions


from the ground state to the excited


state


(or metastable state) M-4M


conversion of the excitation energy


into the spectral line radiation M -4M+hhv;


selection of the radiation of


the specific line from the total radiation of the source by the spectral

instrument and its transformation by the recording system into a measur-


able signal


from the line under study


(e.g. photomultiplier photocurrent


in case of photoelectric recording etc.).


A unit volume of the luminous source contains nM atoms or


the analyte.


ions of


The elementary phenomenon leading to the excitation of


ground state atoms/ions and the quenching of the excited atoms/ions


(M+M


are rather varied.


These are collisions of atoms/ions with free


electrons and heavy particles,


the gas in the source,


atoms,


as well as,


ions,


electrons and molecules of


in some cases,


chemical reactions


with the participation of M atoms/ions.


In practical spectral analysis,


resonance


lines of atoms/ions,


corresponding to the transition from the nearest excited level to the


ground state are often measured.


In sources with a high concentration of


electrons, e.g.,


in plasmas,


the basic phenomenon which lead to the


citation of resonance levels and also to atomization/ionization are col-


lisions with electrons.


Collisions with heavy particles are


less effec-


tive.


In the rf-excited


, inductively coupled argon plasma,


the population


ex-







at a given height


(=15 mm) above the induction coil.


Let us assume that


LTE conditions

population of


(T=6000 K)


in AV are disturbed by the creation of


argon metastable atoms


(outside of


an over-


AV) and representing


this by a net inflow of metastables


from the surroundings to AV.


This


inflow has a rate constant k


<(cm
m


-1 and its value depends on exter-
) and its value depends on exter-


nally controllable factors such


as power


input


in the coil region


(energy


addition region),


the plasma gas


flow conditions,


type of ICP (two-flow


argon ICP or a three-flow


ICP with nitrogen as the plasma gas and argon


as the carrier and auxiliary gas)


and the height of


AV from the induc-


tion coil.


Conservation of


energy


charge and mass balance require


16,18


(i) a


net outflow of


argon ions


and electrons


from the coil region having


a rate constant k.(cm
:1


for ambipolar diffusion;


(ii) a net outflow


of argon atoms


(Ar)


in the ground state from the coil region having a


rate constant ka (cm
a


for diffusion;


and (iii)


the following in-


terrelationships to apply


ki=(V
1


k =1-(V
a


where V
m


/V.)k =0.73 k =1-k ,
m/Vi m m l-k



m/V.)k =0.27 k =1-k.,
m i m m i'


is the weighted average excitation potential of the metastable


atoms at 6000 K


V =11.58 eV),
m


the ionization potential of


argon


(V.=15.76 eV), and k
I-


is the rate constant of the inflow of


argon metas-


table atoms


from their surroundings to the volume








as an ionizant,


i.e.


as an early ionizable constituent, eq.


as an


ionizer, eq.


having assigned the "dual role"):


+ 2e,


* +
MP


+, (3a)


+ e + Ar


+ 2e.


These rate constants are interrelated,


for local


thermodynamic con-


editions,


12/k21


-1/cm


34/k43'


where KAr


and KAr


are the Saha equilibrium constants in (cm


defined


=n n /nAr
Ar+ e Ar


=4.83x1015(cm


-3/2)


3/2
T


(Zi/gm)


m)/T)


I' rm/


-/cm


-3/cm


10(-5040(E.i


r\ / v


I kn .^*_^^


/ jkrq







argon and ground state argon


dimensionlesss);


m(eV)
m e


the excitation


energy of the relevant metastable level;


the excitation energy


argon.


For a steady state of the rate equation for Ar+ and electrons from


equations


and 4,


one would obtain the


following:


/dt=0=k +k21nAr+
m 21 Ar+


2n
n -k n
e 12 Ar


dne/dt=O=k1 nAr


n +k n n -k 2nr n
e 34 Ar e 21 Ar+ e


-k4 nA
43 Ar+ e


(10)


Using the quasi-neutrality condition,


ne =nAr+, combining the equations


and 10 would give:


(11)


The general solution of


equation


is an expression for n


in terms of


both KAr


and k /k43.
m 43*


Two special


cases


are of


interest in the


k /k3 >>KA nA n
m 43 Ar Ar e


, this applies to an


ICP; when


T=6000 K, n e1015
e


-1016


, in which the second term in equation


to n


11 is negligible compared


and therefore:


ne=(0.27 km/k43)1/


, (non-LTE),


(12)


for km=0,
m


equation


yields the Saha value of n


(KAr Ar1/2
e Ar Ar


(LTE).


(13)


dnA
Ar+


-k..
1


n-K Arn n -0.27(k /k 3)=0.
e Ar Ar e m 43


LTE).







to be a useful postulate,


as it permits a qualitative understanding of


the high sensitivity of


ionic lines,


the high electron number density,


and the negligible ionization interference observed in ICP's under op-

timum operating conditions.


- Sensitivity and Growth Curve in


Four analytical


figures of merit of


an atomic emission spectrometer


are:


the sensitivity,


the signal-to-noise ratio,


the line to background


ratio,


and the detection limit.


The detection limit is usually expressed


as the concentration which produces a signal equal


standard deviation of the blank.


to three times of the


The standard deviation in the blank


is a difficult quantity to predict


theoretically.


Neglecting the noise


in the detection system,


the following noise sources can be considered


in emission


ICP studies:


source flicker noise,


variation in the aerosol


production,


in the solvent evaporization,


the dry particles,


and instabilities of the plasma


and in the volatilization of


(electrical or fluid


dynamic).


The ICP has the properties of


arc and the flame.


a dynamic source


in common with the


It has a stationary high temperature zone


(coil re-


gion


10000 K)


in a fast


flowing gas stream maintained by forced and/or


free convection.


Sample material injected into the gas stream quickly


passes into the high temperature zone.


Nevertheless,


the ICP


as an ex-


citation source,


has several disadvantages:


(a) concentration of the element in the condensed phase and the

atom/ion population in the excitation zone in the oas chase do not have







and so it has to be supplied at a low constant rate for maintaining a


steady signal;


also material


that has been injected into the source can


not be used for


further


investigation and observation;


incomplete


mixing, unstable


flow and turbulence introduce fluctuations in signal


and background,


which raise


the detection limit;


and (d)


it is almost


impossible to understand the detailed performance of


because of the transient nature and the


a dynamic source


interdependence of the mixing


heating,


dissociation,


excitation,


and ionization processes to which the


analyte is subjected,


combined with the non-uniform velocity and temper-


ature fields of the source.


The sensitivity,


which is generally defined in analytical chem-


istry as the change of the signal per unit change of the analyte concen-


tration is inversely related to the detection limit.


For a linear cali-


bration curve,


it is


the slope of this curve,


which relates,


in its


simplest algebraic expression


14),


the line radiance,


, in (W cm


-1) with the analyze concentration in the gas phase (cm
) with the analyte co ncentration n,. in the gas phase (cm


-3).


It is


clear that to convert solute concentrations to emission of light a great


number of


intermediate physical processes are needed.


B=SnM


14)


In order to inject the sample solution to the plasma,


a means of


transport is needed.


Thus,


the solution is transported up by a nebulizer


and is converted into a fine spray of


droplets.


The larger droplets


fall


into the drain tube and the smaller droplets are carried away to the








Actually


the amount


of liquid


delivered


unit


time


(2.5


mL/min)


diluted


carrier


unit


time


L/min)


a factor


=0.005.


The smaller


droplets


are introduced


into


the high


tempera-


ture


zone


the plasma.


The solvent


which


usually


water


, evaporates


carrier


the free


expansion


model


, expands


with


a factor


where


the kinetic


gas temperature


is room


tempera-


ture.


fraction


the solvent


in the droplets


that


can


be evaporated


depends


on the kinetic


temperature


gradient


which


droplets


must


always


pass


before


reaching


the observation


zone.


Also


the droplet


size


distribution


is important


because


complete


evaporation


of large


droplets


takes


more


time


than


small


ones


This


transit


time


related


linear


velocity


of the


carrier


the temperature


gradient


as well


the droplet


size


and composition


, because


expansion


of the


will


duce


the velocity.


In the literature


, no experimental


results


are avail


able


locate


the height


the plasma


where


all droplets


are


just


evaporated.


However


similar


experiments


flames


suggest


that


sol-


vent


evaporation


will


be completed


rapidly


in the ICP


The next


step


is volatization


in which


a high


kineti


gas temper


ature


is needed


to disintegrate


the particle.


If only


analyte


jected


the dry


particles


will


very


small


However


an excess


another


salt


as a matrix


is present


the dry


particles


will


be larger


The boiling


point


this


salt


can


deviate


greatly


from


that


of the salt


the analyte


Thus


the amount


of analyte


released


in the


presence







As the solid


particles


are transported


deeper


into


plasma


high


temperature


zone)


their


temperature


will


rise


and start


to boil


come


into


gas phase


state.


At this


point


the relationship


between


the solution


concentration


and the total


vapor


concentration


analyte


species


has been


established.


From


here


the most


important


processes


are dissociation


, ionization


and excitation


, which


are all temn-


perature


(and


time)


dependent.


Let us consider


each


process


separately


when


the molecules


original


salt


are in the


gas phase


, they


will


dissociate


into


free


atoms.


Because


the solvent


is water


oxygen


atoms


from


dissociation


of water,


could


react


with


the free


analyte


atoms


to form


substantial


monoxide


spe-


cies


which


been


observed


for stable


monoxides


such


as rare


earths


laboratory


and will


shown


Chapter


The equilibrium


can be


shown


as follows


M+0.


The degree


of atomization


can be calculated


as follows


=nonM/nMO


(nO+Kd


15a)


, (16)


where


according


to Boumans


equilibrium


constant


can be calcu-


lated


from


-5040Ed


=nM/(nO+nMO)


) (


, nT,


=nM/( nM


nT=nM+nMO,


+ nMO


1024







water


is completely


dissociated


, the


value


is the


same


as the


number


density


water


molecule


introduced


nebulizer


into


plasma


x1016


The next


parable


important


to the dissociation


hase


except


process

that t


ionization


he products


which


are ions


com-


and elec-


trons.

here w


The degree


ie are interested


ionization

in atoms


defined


in the


expression


same ma

n for (


inner


-8.)


Because

is used.


. the


fraction


elemental


species


not ionized


+ K.)
I.


(18)


where


Saha


constant


and its numerical


form


.=4.83x1015
1


n )
a e


-5040Ei
i1


(19)


where


the ionization


energy


and Z


temperature


in (K)


are the partition


the ionization


functions


for ion and atom


dimensionlesss


The result


for (


-6.)
1


shown


Table


combining


the relation


these


between


processes


solute


assuming


concentration


complete


and the free


volatili


atom


zation


number


densit


the pi


asma


can be expressed


=apc8,


-si)


and for


ion i


given


nl+=pc8 .


(20b)


where


conversion


factor


in (


atoms


is given Oy


-i1


+ nM+


=nM/(nM








NA=Avogadro's number,


atoms


(molecules) per mole,


6. 0x1023


(dimen-


sionless);


MA=atomic (molecular) weight of


analyte,


(g/mole)


are in (cm


(ug/mL),


respectively.


The next important process in the gas phase is excitation,


which is


the process which transfers the atoms/ions from the ground state to the


excited state.


Upon returning to the ground state


light is emitted,


which the relation of


emitted light and total number density of free


atoms is given by:


J=I1n exp(-E


(21)


ex),


where E


is the excitation potential


in (eV),


is the excitation


temperature


(K),


J the emittance expressed in


(W cm


k is the


Boltzmann constant


(eV K


and 11


is an element dependent


(W sr


(see


Table


which is given by the equation 21a:


(21a)


a)Aul


where


g =the statistical weight of the upper


level,


dimensionlesss)


h =the Planck constant,


uul the
ul


(erg s) or


frequency of the transition u-l,


=the partition function of the atom,


Aul the probability of the transition ul1,


A very important aspect is the quantity,


dimensionless)


the light emitted per sr


nM and


-1),


11=(hul /4)(gu








entire


depth


the plasma.


For a radially


symmetric


plasma


at the


cen-


ter of the plasma,


the radiance,


emitted


a narrow


beam


observa-


tion


given


J(r)dr.


(22)


The complete


expression


is found


substituting


the equations


and 21


into


the equation


B=2IclP


(r)(1-8


(r))exp(-E
1


(23)


/kT
ex


where


the radial


dependence


dissociation,


ionization


and excitation


has been


indicated.


For ionic


lines,


expression


23 changes


to 24:


B=2I c
1


IR 8i(r)exp(-E
0


(24)


It is possible


to calculate


the sensitivity


with


the aid


complete


equations


23 and 24,


but ignoring


the radial


variation


of the


parameters


(see


Table


And the growth


curve


is given


in Figure


the log-log


plot.


high


the slope


1/2,


and for


slope


Under


some


circumstances


, it is logical


use the ionic


line


stead


of the atomic


line


to take


advantage


of the


high


fraction


ionized.


Experimentally


this


has been


recognized


and it


appears


that


applied


a great


number


of elements


(e.g. ,


rare


earths).


I I I I I % \


can


, Mg,


+ r


i I


I 1


|


SI *


* *


I




















C

X

3 Ln
N
**
"1 C11


N
I
CM

3 O
- r-
< X
**
- rn


CO 0
II C


C
cc
*0' Q
^r -r^

o
CO

s3- 0
O -a
COC
II CI)



-D



=3C



x ~ci
D'O
XO
11 -4.

O






**
. II
-X
4 0
-l e


A


































Figure


Growth Curve for Atomic Emission.





17















Js










/





Log nM








plasma,


such


as a change


power,


volume


ratio,


and observation


height,


must


be known;


from


a practical


analytical


point


view,


ignor-


ance


these


relevant


data


is not desirable.


- Atomic


- History


Fluorescence


of Fluorescence


Previous


Investigations.


Although


the fluorescence


atomic/ionic


vapors


was investigated


several


physicists


in the


19th


and 20th


cen-


tries


major


advances


began


1955


when


Boers,


Alkemade,


Smit


used


atomic


fluorescence


for studying


the principle


of physical


and chemical


processes


in flames.


Alkemade


also


suggested


potential


use of atomic


fluorescence


in analytical


spectroscopy.


Although


the principles


of atomic


fluorescence


have


been


known


many


years


its first


use in analytical


chemistry


for spectrochemical


analysis


was b


Winefordne r


and Vickers


1964.


Much


of the credit


for this is attributable

near's group in the U.S.A.


to two research


University


groups i

Florida)


n particular,


and West


s gr


Wineford-

oup in


England


(Imperial


College).


In atomic


fluorescence


, just


as in atomic


absorption


, the sample


atomized


and then


optical


pumping


with


a suitable


external


light


source


radiationally


excited.


Here,


however


the radiation


emitted


deactivation


excited


atoms


is usually


measured


at 90


to the incident


beam.


In atomic


fluorescence


the intensity


linearly


depends


upon


number


of excited


atoms/ions


if there


is nealiaoible


'a .,


1 f-absorotion.


I


_ _








AAS.


It has been


shown


that


the fluorescence


intensity


depends


linearly


on the excitation


source


intensity


and the quantum


efficiency


of the


transition


involved.


It should


be mentioned


that


these


linearities


are valid


only


intensity


sources,


such


as Hollow


Cathode


Lamps


(HCLs)


metal


vapor


dis-


charge


lamps


xenon


arcs,


and Electrodeless


Discharge


Lamps


(EDLs


fact


use of high


intensity


sources,


such


as tunable


lasers,


result


in a near-saturation


upper


energy


level


involved


in the


fluorescence


transition.


Because


of saturation


the fluorescence


radi-


ance


two level


atomic


system


does


not depend


on the


source


inten-


sity

tion


or quantum

is minimize


Tunable


efficiency


the corre


are fairly


lasers


the transition.

ct illumination


expensive


Furthermore,

geometry is u


are


now


useful


sel


ised.


only


f-absorp-
28-31


for slow


sequential


single


element


determination.


Therefore


laser


excited


atomic/


ionic


fluorescence


with


an inductivel


coupled


plasma


is not present


useful


analytical


tool


because


the single


element


limitation


and the


high


cost


both


sources.


However,


use of


lasers


for diagnostic


purposes


in the ICP


promising,


particularly


for profiling


the temper-


ature,


electron


density,


and velocity


of the plasma,


means


of laser-


induced


atomic/ionic


fluorescence


spectrometry.


21,3


The rf-excited


inductively


coupled


argon


plasma


operating


at 36


a maximum


power


output


of 2


was used


for the first


time


Hus-


sein


and Nickless


as an excitation


source


for atomic


fluorescence


spectrometry


in conventional


flames


with


an unsheathed


air/propane


flame








air/acetylene nitrogen-separated flame,


which will be discussed in Chap-


ter 3.


In Chapter 4,


tunable dye laser excitation with an


ICP as


atomizer/ionizer will be considered in more detail.


B.2 Theoretical Considerations


Types of Fluorescence


Transitions.


Fluorescence spectrometry is


based upon the absorption of


radiation of


a specific wavelength by an


atomic/ionic vapor and deactivation of the excited atoms/ions,


called "atomic/ionic fluorescence"'


which are


Both absorption and the measured


atomic/ionic emission processes occur at wavelengths that characterize

the atom/ion species present at the reservoir giving an excellent selec-

tivity to this phenomenon.

Resonance fluorescence occurs when excited species absorb and re-


emit radiation of the same wavelength,


i.e.,


the same two levels are in-


volved in the excitation-fluorescence processes.


is the basis for the


This type of transition


"resonance monochromator" described by Sullivan and


Walsh.


Resonance


fluorescence has been the most useful in analytical


atomic fluorescence spectrometry because resonance line intensities are

significantly greater than the intensities observed with other types of


atomic fluorescence.


A variation of


resonance


fluorescence called ther-


mally assisted resonance


fluorescence occurs when the


lower state


is not


the ground state,


but a metastable,


thermally populated state.


Direct line


fluorescence occurs when an atom or ion radiationally is


excited from the ground state to a state and then undergoes radiational


de-excitation to a lower excited state and emits a photon of


energy


less







predominant source of noise.


All non-resonance processes can be Stokes


or antistokes.

Direct line fluorescence variation called thermally assisted direct

line fluorescence occurs when the excitation process originates from a


state above the ground state;


the fluorescence process occurs to the


ground state and is an antistokes process.


Excited direct line fluorescence


involves all excited states in the


excitation and fluorescence processes;


fluorescence transition can


therefore be either Stokes or antistokes with respect to the excitation

process.


Stepwise


line fluorescence occurs when an atom or


ion is radiation-


ally excited to a state above the ground state, then is collisionally de-

excited to some intermediate state from which it then radiationally de-


activates,


and emitting a photon with lower energy


(Stokes) than that


which is absorbed.


Stepwise


line fluorescence


finds its analytical use-


fulness when excitation source scatter


is the predominant noise source


and there is no possibility for direct line fluorescence or when the


transition probabilities are not


favorable.


A thermally assisted stepwise line fluorescence variation is also


possible.


An atom or


ion is radiationally excited to some upper


level


and while it is in this excited state


(short lifetime),


it is thermally


excited to a more energetic upper state from which it


deactivates to the ground state


then radiationally


(the emission can be either Stokes or


antistokes with respect to the excitation process).








The final


type


atomic


fluorescence


is multiphoton


fluorescence.


more)-photon


absorption


will


observed


only


with


coherent


sources


The last


Figure


excitation


processes


shows


with


have


the variety


extremely


little


intense


analytical


atomic


non-coherent


usefulness


fluorescence


sources)


present.


transitions


which


described.


Several


fluorescence


processes


shown


in Figure


lb have


been


observed


with


laser


excitation


some


transition


elements.


Basic


Fluorescence


Signal


and Analytical


Curve Growth ExDress ions.


The theory


absorption


radiation


atomic


radiation


the excited


fluorescence


can be described


the atomic


atoms


vapor


and the return


in three


parts:


emission


of the sample


(fluorescence)


into


atomic


vapor


within


the volume


observation


These


relationships


well


treated


in the


literature


39-41


21.37


The radiation


absorbed


an atomic


vapor


given


sA[1


-exp


-k z)]


(24a)


where


radiance


absorbed


atomic


vapor


is the


solid


angle


of exciting


radiation


collected


upon


absorption


cell


in(sr


is the


source


spectral


radiance


in (


the atomic


absorption


coefficient


for analyte


atom


at wavelength


the length


of the


absorption


cell


(cm)


see


Figure


Evaluation


integral


after


assuming


source


type


and optical


density


leads


the following


formulas


(see


pages


Continuum


Source


Optical


Destis


When


using


the continuum


Curve


Growth


ExToo n~
L pU o -Lfl


A= A


Densities.


When


I


__


111


I


I

























Figure


atomic


uorescence


transitions
dicative of
their process
okes direct
t line fluo


e-
om).
tate
ce;
i-
okes
s-
e;
te
sted
ng
r
n);


tion
the
(D=d


pt


radiation); h
photon process
are even less


or; nvE=excit
two photon e
es involving
probable than


n wne
velen
m, s
A=acc
hvF=


xcitation
more than
the two


-fluorescence
two identical


photon


process'


, respect
tized
r; A*=
rescence


(multi-
photons
es).











































( -


i


I


(h)


~-

-
I,


1
I






3


--2


1



(1)

I
I


i____









i
(j)







- I _
i


D + hvE


--- a* a


-- A*


A + ,
--vE


3
L 3i I














































*


a


rrr
U

a)



o
0.



S

a,
U
CO


c,
U

CO
-3
U-l
0
D



*rl
0
'4
0
CD

4-i
C


0'


CH

'4j
CO


-C

(Si



-o
r





26




C.l

CD -

/J -,--- ---_^^






// //
/ //








la
1
/















/-
/ / s


^.-,^
< ^ '"'-^








BF=C2Bc


KOnM


YAXD( A/4 ) flu


(25)


where


=continuum source spectral radiance at temperature of


source and at


wavelength x

C2 =(/in2)1/2/2


(W cm


dimensionlesss)


=absorption oscillator strength


=(mc/8r e


dimensionlesss)


(26)


1.51(gu/gl)A ul
uV I ul 0


1=(11'L/A


)=absorption or emission path length (in cm)


A =211'+21L+2Ll'=total surface area of the cell
S
A ,=Einstein coefficient of spontaneous emission


Y=fluorescence quantum efficiency


(see Figure


dimensionlesss)


nM=concentration of species at lower


level


(in cm


AXD=Doppler half-width


(in nm)


QA=excitation source solid angle


K =modified absorption coefficient,
0


impinging in absorption cell


k /nflu
o M lu


(in sr)


(in cm


k =absorption coefficient
0


oM lu


for pure Doppler broadening


2
=XF /cAx C (in cm
so D 2


(in cm


o =peak absorption or emission wavelength


c=speed of light


(in nm)


(in cm


m=mass of


electron


(in g)


=source factor to account


for saturation of


energy


levels


(dimen-


sionless)


2)(gu/g )Aul O=


ire2/mc)=3.0x10-4







Line


Source


Low Optical


Densities.


When


source


half-width


is much


smaller


than


the absorption


line


width


fluorescence


line


radiance


given


BF=B K


onMYllu (A
oM lu A


'4)flu


(27)


where


=radiance


line


source


=factor


to account


finite


line


width


source


compared


absorption


line


(dimension-


ess)


the equations


describing


fluorescence


spectral


radiance


pre-


sented


so far


are involved


absorption


radiation


atoms


ground


state.


Figure


centration


the log


are called


-log


growth


plots

curves.


spectral


To convert


radiance

growth


vs atomic


curves


con-

ana-


lytical


calibration


curves


two other


terms


must


considered.


first


one relates


the second


solution


one relates


concentration


fluorescence


c to number


radiance


to detector


atoms n

output


while

signal


The relationship


c is given


nM=1019(Ra


-8i)


(28)


where


c is the concentration


of analyte


solution


in (ug/mL


solution


uptake


rate


in (


min)


the atomization


efficiency


mensionl


ess)


is the aspiration


efficiency


(dimens


ionl


ess)


flow


rate


gases


into


the flame


plasma)


L/min)


pension


factor


r/"nTT


where


are the number


moles of


ex-



























log


/ ,b


r


Sb


fa








The relation


between


the fluorescence


spectral


radiance


and the


output


signal


given


cP= 4aAA
F


syWHT
s x


BFRT


Where


the signal


in (


is the


area


of the sample


cell


in (cm


the fluorescence


spectral


radiance


in (


W and H


are the


width


height


of the


slit


, cm)


is the transmittance


of the


monochromator


wavelength


dimensionlesss),


is the spectral


band-


pass


of the monochromator


is the photodetector


sensitivity


is the electronic


transfer


function


in (


B.3 Noise


and Detection


Limits.


The signal


measurement


in an optical


spectrometer


is limited


existence


fluctuations


on the signal.


However,


the quantum


nature


the radiation


produces


fluctuations,


which


are called


"photon


noise"


more


general


"shot


noise."


Shot


noise


adds


error


to the desired


that


only


be predicted


statistical


In addition


to the shot


noises


predicted


from


statistics,


additional


fluctuations


occur


"excess


low-frequency"


noise;


such


a noise


a spectral


noise


power


inversely


proportional


to the


frequency


and is called


"flicker


noise.


cause


of these


noise


sources


can be found


in light


sources


absorb-


medium,


detectors,


and electronic


measurement


teams


used


in an op-


tical


spectrometry.


There


are two types


errors:


"systematic


errors"


which


arise


from


procedural


inconsistencies


errors


(sample


nrcnonro nnn


04- n


k ar ,l- T kirnrn ,-1


r%%\ I I I- .-


., a~- L


n 4 T


I I I


~, C,, L,,


I


*


AI-I







analytical


which


figures


reciprocal


of merit,


are "relative


signal-to-noise


ratio


standard


"limit


deviation"


of detection"


which


is the detectable


analyte


concentration


with


a certain


confidence


level;


and "the


sensitivity"


corresponding


to the slope


the analytical


calibration


curve.


The limit


of detection


is definedl9


cL-


-Xbl )
b1


dS/dc)cl
lim


=kabl


/S=(


(30)


which


link


the limit


detection


(concentration,


and the sensitiv-


, S=(dS/dc)
lim


and the


noise


level


(random


errors


in the blank,


is obtained


from


a number


or more


consecutive


readings


mea-


surements


of the blank,


i.e.


are the


average


of the blank


and standard


deviation


the blank,


and k


is a protection


factor


give


a desired


confidence


level


Lim


=k=3


is chosen


which


gives


99.86%


confidence


level).


When


one works


near


the limit


of detection,


one usually


applies


paired


readings,


the background


at t=t


and the signal-plus-back-


ground


is the


s+b'


sampling


time.


The signal


reading


corrected


background


, is given


AX=X


or in another


form


AX=x


o+ )+(dxb (t
a s b


o+r)-dxb(t
o s b


where


dx,(t


is the


statistical


fluctuation


in the meter


deflection


)-xb(t


S/N)lim(abl/S),
1im bl


s+b(t
stb








S/N=x


Ax (32)


with


(dx (t


o+~s)-dxb(t


]1/2


From


equation


variance


can


be expressed


=dx (t
b


+ dxb(t
b


- 2dx (t
b


o+r )dxb
o s b


(34)


For stationary

the time-independent


background

variance


fluctuation

of dxb(t).


, the va

In order


riance


which


to be able


compare


signal-to-noise


ratios


obtained


with


different


types


of noise


with


different


measuring


procedures


and to find


optimum


values


of the


various


characteristic


times,


one


would


take


advantage


of the relation


between


the auto-correlation


function


and the spectral


noise


power.


The auto-correlation


function


a continuously


fluctuating


signal


dx(t)


is defined


=dx(t)dx(t+T),


S=2a
Ax b


-2dxb(t
b


o+r)dxb (t
o s b


0)-iF


36()


where


)=2(


+rs)/0


x(r)),


/


I







To calculate a


, the auto-correlation function is expressed in


terms of the spectral noise power S.
b


(f) of the background current fluc-


tuations and from the Wienner-Khintchine theorem-

a,


(f)cos(2n fT


s)df,


(37)


where


(f)=S.
xib


(f) G(f)j


and G(f

Since,


)


is the frequency response of the


the noise power is a squared quanti


(linear) measuring device.

ty, only the square of the ab-


solute value of the frequency response is needed.


G(f)


Then,


we have


cos(2Trft


(38)


One would obtain the relation between the variance and the spectral


noise power


as follows:


o S.
o b


G(f) 2(1-cos(2wfT


(39)


Because cos(2nfT


sampling time r


for T


is therefore a function of the


T s0O both (ax
S AX


approach zero.


The spectral noise power


(noise power per unit


frequency


interval)


for shot noise in terms of


current


fluctuations


is given by


T )=







electron count rate in (count

as a function of frequency f


-1).


The spectral noise power considered


is called the noise spectrum.


The dimen-


sion of S.
I


is (A2


The bars denote average values.


Noise spectra for


an ICP have been evaluated in this laboratory.


The major sources of flicker noise (or


1/f noise)

and detect


excess


involve random drift of light sources,

on. The spectral noise power for flic


low-frequency noise,

analyte production,


:ker noise in terms of


current fluctuations is given by


Si)fl(f)=


2 -2
K /f)i.
j j


2 -'2
(K ./f)Rte
j J


(41)


where


is the frequency


, K. is a constant with unity dimension which
J


describes the low-frequency stability of the noise source and i. and R.

are as before.

There are peaks occurring in the noise power spectrum which are due


to oscillations


(organ-pipe)


in the plasma torch system.


They may extend


to the audible frequency range and are called "whistle noise"'


(specially


for fluorescence torch configuration).

Another source of noise existing in electrical circuits and compo-

nents but insignificant compared to the other noises when photomulti-


pliers are used,


is the "thermal


(Johnson) noise" which are due to the


random thermal motion of electrical charge carriers in any conductor.


When combining all

noise expression, care


the noises


from different origins into a total


must be taken in the method of addition.


ex-







where C is a correlation coefficient,


and its value is unity,


in the


case of


complete statistical correlations


(both have a common origin),


and its value is zero,


in the case when both noises are completely un-


correlated.


For systems which are shot noise


limited,


modulation of the signal,


regardless of the modulation frequency will not improve the signal-to-


noise ratio.


For systems which are additive


flicker noise


limited,


is necessary to "modulate" only the signal and not the background.


must keep in mind about


the suppression of "excess low-frequency noise"


that the gain in signal-to-noise ratio by modulation is limited to that


modulation frequency at which


frequencies above this limit,


shot noise


(S/N)shot

additive


x (S/N)


flicker.


At modulation


flicker noise still decreases,


is unaffected by modulation and sets a


final limit to the S/N


gain by modulation.


In conclusion, one could say


in practice the absolute magnitude of


both signal and the noise


in the signal have little use.


However,


signal-to-noise ratio can be useful and is directly related to the appli-


ability of any method for an analysis.


The signal-to-noise ratio allows


the limiting detectable concentration to be defined and is used to deter-


mine the values of the experimental parameters at which the limit of


tection as well as the maximum signal-to-noise ratio at higher concen-

trations occur.


Detection limits


(atoms per cm


of hot gases are given for AFS,


AAS and AES21


Table


The conversion factor from atoms/cm













Table


COMPARISON OF CALCULATED DETECTION LIMITS FOR SEVERAL

FLAME/PLASMA SPECTROSCOPIC METHODS21


LIMIT OF DETECTION, atoms cm
C2H2-Air H2-02-Ar ICP


ATOMIC FLUORESCENCE


Laser, Typical operating values
(x =300 nm)


Detector noise limit
Detector noise limit


2 -1
: R =102s-
SD10s1
4 -1
Ds


Background noise


limit


102
10
3x104
3x10


x104


Laser,


Saturation (x


--------- o
Detector noise limit
Detector noise limit


=300nm)


: R =10 s1
D- s
4 -1
: R =10 s
D-


2x10


2x100


Background noise limit


2x10
3x101


2x10


3x101


ATOMIC ABSORPTION (Source noise
limited o =300nm)
0


Xenon Arc Source


Hollow Cathode Discharge


4x108
7x107


8x108
1x108


4x10
7x108


ATOMIC EMISSION (Background noise
limited)


=300 nm
=600 nm


3x103







NA=Avogadro's number, atoms


(or molecules) per mole,


6.0x1023


dimension-


less


MA=atomic (molecular) weight of analyte,


g/mole


F=transport rate of sample solution as determined by nebulizer, cm


e=efficiency of nebulization and desolvation


(fraction of sample solu-


tion transported to chamber and flame which actually gets into flame


or plasma and becomes desolvated particles)

B =efficiency of vaporization of particles in


submicroscopic species,


, dimensionless

flames or plasma to produce


dimensionless


Ba=efficiency of atomization,
a


(atoms, molecules,


i.e.,


fraction of


ions) which end up


submicroscopic species


as atoms, dimensionless


Q=flow rate of unburnt gases


(at room temperature)


into


flame,


cm3/s.














CHAPTER


MOLECULAR


EMISSION


SPECTRA


AND ATOMIC


EMISSION


SPECTROMETRY


IN ICP


- Molecular


Emission


Spectra


- Introduction


rf-excited,


inductively


coupled


argon


plasma


ICP)


is becoming


increasingly


popular


as an excitation


source


for atomic


(and


ionic)


emis-


sion


spectrometry.


standing


advantages


of this


plasma


include


long-term


stability


and the


very


high


temperatures


achieved


throughout


a considerable


volume


of the


plasma.


The temperatures


appear


be suf-


ficiently


high


to bring


about


essentially


complete


decomposition


of almost


all molecules


introduced


into


plasma


in the form


aqueous


aerosols.


a result,


emission


spectroscopy


using


the ICP is relatively


free


those


types


of interference


well


known


in flame


spectroscopy


that


related


to stable


chemical


compound


formation.


various


regions


of the plasma


however,


molecules


exist


and their


emission


spectra


can


be observed.


When


an aqueous


aerosol


containing


organic


salts


is aspirated


through


the center


a toroidal


plasma,


many


processes


(desolvation,


possibly


hydrolysis,


melting,


sublimation


vaporization,


and molecule


decomposition)


take


place


in a manner


analogous








ions


in the


region


where


emission


measurements


are generally


made


25 mm)


above


the coil.


Higher


in the


plasma


oxides


are again


formed


metal


atoms


react


with


entrained


atmospheric


oxygen


with


oxygen


produced


from


water


dissociation).


Other


sources


molecular


emission


from


the plasma


include


OH emi


sion


, arising


from


incomplete


decomposition


of aspirated


water


emis-


sions


arising


from


the entrainment


atmos


pheric


gases


starting


where


the plasma


"tail


flame"


emerges


from


the plasma


torch


ssions


result-


from


atmospheric


gases


and their


reaction


products


include


those


NH and NO.


The conditions


under


which


molecular


emissions


can be


served


from


argon


plasmas


are di


scussed


in thi


chapter


Particular


tention


has been


paid


to the y-band


system


which


extends


from


nm, a wavelength


range


in which


many


elements


have


their


most


use-


emission lines.


Molecular


emissions


are also


observed


when


gases


other


than


argon


used


for plasma


flows


, such as in


nitrogen-cooled


argon


plasmas


when


organic


solvents


are aspirated


or when


various


gases


or vapors


are intro-


duced


as samples.


- Experimental


The instrumentation


used


study


described


n Table


schematic


diagram


the experimental


system


shown


in Figure


present


study


involved


the use of both


the conventional


short)


torch


a long


(for


fluorescence


studi


torch


(see


Figure


. 8


extending


ni lrn- 7


*ha lnnM


rn nil


in fhM lnnn


Fnrr'h


ml


i Y i nn


nF amhi nf


f-mns-


are


fihP


ahnvo


I












a,
I
.4-i


3
O
C u
i; 2


O 0
o n
o Cl


'.
c< '-
C


N
'-C
*H c


10 -
U^ 4 C

Or- V


O r


*- 4. 0
.C4- 0

CC
- -i '

re -


2 0 0
S--


XIr-
2 -z
022
-0 U O
iri o r
LJ W
I X
D uj a


C
CO O
0 0


SW a
050
mac
CO C
03 n
C3

OLE 2
O 0
I-or
Of
0 -
0 0
00 E
1 10
0,110
4JUI. f-^
OCa-
wtEIcf
Ia-Cd


I

-o
r
03
0 i
'r-

W fl
C
CO ^
0 0
O .



D, v
OLs.
4-i *
00 L
cUO )
003
o c
CO
-1


oE
C >E
0 C
-C ON C
03 0

ro Ec
a o


O* N
4 -O -
CO C
0M 00
OO\O


*o
03
c-
0
+J -

C
0 0


x p
>01
0 LL.
.4-
W a

o ~
0 O
C


10
,O
I-a
-CO






02
CO C
O c CD


OW N C
OX: C E "^
c1 Ca s- c
C 2N X


C 0 C

S- ON .j
I I
L Ur


Ya
- c
C
NW I

>
40 0
O3 ^
D0
O
2


I
3


C


c


H)C
O3 0
i- o




SC
-H X


00
4-il
0, -
E C
O O
e c
4J C
C 3
CO
4.)
0
S-p
-c
SC


2:































CO
03
'-I

ID
V



C
0
-4
CO
'(1
CO)
*-
E
W

C
0)
CO
03
'-4
C-



C
-4



a
I

a,


ul


C
0






S
0-I
3
'i-4
03





a
x
LUl
w






cI-
0

E
'V
'-4
03
-4
X



C 0,

OT



CO
.l

-(fl







43









O


O
0
U


CI




ff:
->w



H>
z
-o
C-































Figure


Schematic
thickness


Diagram
of 1 mm


of ICP


Torches


(all


quartz


tubing


has wall


Short
Long


(Conventional)
("Fluorescence")


Torch
Torch

















mm-.


PLASMA


(TANGENTIAL


mm ID


4 mm ID


10
nm
1u


INLET
(TANGENTIAL)


XILIARY


INLET







are of


the short


variety.


Unfortunately


mixing


ambient


occurs


where


the torch


is terminated


- Results


and Di


scussion


Molecular


Emissions


from


Unsalted


Plasmas.


The emission


from


a pure


argon


plasma


consis


ts essentially


Ar atom


lines


superimposed


on a back-


ground


continuum


due to Bremsstrahlung


and ion-electron


recombination


processes.


We have


observed


all recorded


Ar I


lines


range


340-


A weak


Truitt


Ar II line


and Robinson.


at 480.6


Most


nm has


commercial


been


argon


observed


contains


in the plasma


several


core


ug/mL


carbon


as low


molecular


-weight


hydrocarbons


CO and CO


These


traces


lead


to C


lines


193.09


nm and 247.86


but emissions


molecular


species


derived


from


these


trace


impurities


have


been


observed


only


high


-powered


plasmas


such as that


of Greenfield


Emission.


The OH molecule


the most


ubiquitous


molecular


emitter


unsalted


plasmas.


Even


when


water


is not injected


as an aeroso


"after


-flame


above


the plasma


torch


shows


emission


from


the A


band


in the vicinity


of 306.4


nm.


This is apparent


in published


background


emission


spectra


46-49


whether


water


been


injected


or not,


both


for low-powered


and for


high-powered


plasmas.


Where


no water


is aspirated


the emission


arises


from


entrained


atmo


spheric


water


vapor.


OH emission


minimized


when


no water


is aspirated


long


plasma


torch


is used


the plasma


being


observed


through


the torch


wall


made


of Spectrosil


quartz


in our case


. The


band


of the


same


tem i


usually


also


apparent


through


the wavelength


range


from


. CN)


from


I v


__








emission.


most


prominent


emission


band,


which


is at 336.0


been


reported


in the high-powered,


nitrogen-cooled


asmas


Greenfield


et al


and is recorded


as a weak


emission


in the


"after flame"


low-powered


air is introduced


observed


argon-cooled


into


NH emission


plasmas


the coolant


in an argon


NH emission


stream


plasma


also


kW Ar pi


operated


occurs


asma


at low plasma


when


We have


flow


(10-1


L/min)


at 0.55


viewed


18-20 mm


above


the induction


coil


with


ambient air


and with


air being


flushed


through


the pi


asma


pre-


vent overheating


In Figure


, spe


ctra


are given


OH and NH (


306-338


obtained


under


these


conditions


power


L/min


, and


mm ob-


servation


height)


NH has


a very


strong


sharp


branch


at 336.0


influ


ence


ambient


versus


flushed


air on the variation


the NH emission


piration)


signal


as a function


to background a

of observation


t 336 n

height


m (resulting


above


with


the load


water


coil


and for


power


level


is shown


Figure


case


the lower


power


plasma


ambient


air surrounding


the plasma


results


in a rather


sharp


maximum


whereas


flushed


air surrounding


the plasma


spreads


max-


Imum


over


about


mm.


This


is apparent


a result


of considerable


air entrainment


over


a greater


observ


ation


height


in the


latter


case


com-


pared


to the former


case


At higher


input


powers


kW in


our case


the extent


air entrainment


is greater


in both


cases


than


for lower


power


maximum


occurs


at about


same


height


25 mm),


and the


breadth


maximum


are approximat


same


in both


cases


. In













I E
*' E

C CO <
0 (- II
C S

CO



(U CL *r-t
S




6UU


*-1 **
ocrz I
(1 =i~
-IS-
cgr1-


O > **
O E
C 1Q3
O 0
'rf O
0 CI II
(U 5C 11
*rM
f Of
C DJ
cp Uc -


& Cr
O -1 II
l) C)
a,
cn u ^
0-c
1-. 4 ..
C 4C

o ca
Len *.

E *r4 1
L.U .


































Figure


NH Emission


Signal


to Background


Signal


at 336.0


nm Band


Head


vs Observation


Height.


W=25


perimental
F=2 mL/min


Conditions:
(water).


R=15


L/min;


Solid


line


Broken


line


a Air flushed


into
(hou


box (housing)


sing)


containing


containing


torch.


@ Ambient


air in box


torch.

















- -


/
I
I
I
I
I

i/
I

I/

r

i'
/


II

I/I
'\AI


I


































Figure


NH Emission
vs Nebulizer


Signal


Pressure


to Background


Signal


Experimental


at 336.0


Conditions


nm Band


same


Head


as above


training


except
torch.


z=20


mm and


air flushed


into


box (


housing)


con-







53




















10




E 9
C


\.
8-
z








O


C
C










Z

26
o



(11-






1-
h-4









UJ
or 6-


ui


^;i







nitrogen


, which


is then


passed


into


a low-powered


plasma.


Only


very


weak


NH emission


is seen


when


concentrated


ammonium


salt


solutions


are aspirated


directly


into


asma.


emission


emission


which


from


air entrainment


plasma


flame


been


noted


Truitt


and Robinson


and b


cott


Strasheim


major


bandhead


the y-band


system


NO (


at 247


the fine


structure


several


bands


can


be observed


from


-280


nm in


a plasma


operated


kW and viewed


a height


above


the coil.


In Figure


8a, spectra


are shown


in the


-band


tem of


NO (


200-280


and the


band


the A


transition


-300


obtained


under


the above


conditions


slow


scan


expanded)


the 232-248


nm region,


including


the two strongest


NO bands


shown


in Figure


and the (1,0)


band


210-21


shown


detail


in Figure


The 1


of these


bands


been


suggested


as a


possible


cause


of difficulty


in determinations


zinc


using


the Zn I


line


at 213.86


The superimposition


of this


line


on some


of the fine


struc-


ture


of the NO emi


ssion


been


noted


son


et al.


In Figure


continuum


background


spectra


are given


for 3


input


power


level


Ar emission


is observed


through


the quartz


wall


long


torch


eliminate


ambi


air entrainment.


At 0


the back-


ground


signal


is virtually


the photomultiplier


dark


current


level


With


an increase


in input


power


the background


signal


level


increases


as one


would


expect


for a "blackbody"


radiator


In Figure


the variation


NO emission (at


214.9


to background


emiss


ion (also


at 214.9


with





























Figure


Emission


pectra


(y-band)


NO and A2


Band


perimental


Conditions:


P=1 kW


L/min


z=24 mm


W=50


F=2 mL/min


Spectral


Range


of 200-280


nm Showing


y-Band


of NO and A


Band


psi;


































































WAVELENGTH






























en
0]
C

z

CM
N

a -o
0 C



a a1
C 0




ca -I

3 3
o a

<-C

ET E
C C





IrI
N C
N N
o o






0' 0'
E
C C







CO u

Cu CM
I I
CM 0








a a
-r o \




o 0
-D C
cn c
C C







CO Cu
a a


Q3 03


unJ wa



SU L
* *



-a u









58


























U
S-9
N


3









-*4
r'j
















N















MI
am








^i~






















one




M4'































Figure


Argon
Tubing
L/min;


Plasma


Long


Continuum


Torch
H=1


Background


vs X.


N=40


Observed


perimental
si; F=2 mL/


Through


Conditions:
min (Water)


Quartz


z=20


55 kW;


The dark


current


level


this


a P=1.5
studies


x 10


mm.


.0.5


-lOA)






60



















20



4
r&


18-




16-




14-




12





a ~10









6-
ts
-4-





in ,
U 1


20 10-


q
c






6-








0 iri"""- -\






-^nn 11^ IA ii ^-itt-ti*^t-.






























Figure


NO Emission


Signal


Background


Ratio


(1,0)


Band


Head


at 214.9


R=15


nm vs Obse


L/min;


o Argon


W=25


flushed


rvation


urn; H
into b


Height.


mm: N=40


(hou


sing)


Experimental
i; F=2 mL/min


containing


Conditions:


(water).


torch.


W Air flushed


A Ambient


into


air in box


box (hou


(hou


sing)


sing)


containing


containing


torch.


torch.


Key:


Solid


line


P=1 .0


Dashed


line






















1.0




0.9




0.8


0.7 -




0.6



0. -



0.4-.


-aN.


-0 C -r --


// -
S",/

I,' /
/ /


-t n- n -r -. .. -- ---


0 5 10 1S 20 25 30


* ~







reaches


a maximum


at =25


mm for


all "atmospheric


conditions


" the


maximum


having


greatest


amplitude


for ambient


air and to lowest


an Ar


flush.


Other


Molecular


Emissions


Other


emission


spectra


are readily


served


when


various


gases


, vapors


and liquids


are injected


into


the plasma


include


those


and 0


(from


oxygen


),47


and N


(from


nitrogen),


46,47


CN and C


(from


hydrocarbons


CHC1


CC14


and CO)


CO (from


aspira-


tion


of methanol


into


a nitrogen-cooled


asma)


(from


PC13)


and SO


(from


SO 2).


Molecular


Emissions


from


Salted


Plasmas


The existence


metal


oxides


both


in the central


region


a few


millimeters


above


the rf coil


in the outer


part


of the "tailflame"


at heights


of 25-50


can


be demon-


strated


clearly


aspirating


moderate


concentrated


solutions


ments


with


rather


stable


monoxides


, e.g.


1000


of Y


and Zr


In all of these


cases


the molecular


emissions


from


monoxide


appear


a different


part


of the vi


sible


spectrum


from


atom


lines


of the


same


element


is apparent


visual


observation


inductively


coupled


argon


plasma


that


at 1


east


four


rather


distinct


zones


exist


a preheating


=0-10


where


poor


analyte


atomic


or ionic


signal


background


ratios


occur


but with


a very


bright


background;


(ii)


narrow


zone


(=10


-15 mm)


where


fairly


intense


atomic


and ionic


ssion


occurs


but al


where


monoxide


emission


seen


(iii)


analytical


zone


(the


pencil


=20-30


where


ionic


and atomic


emission


measurements


are generally


made


because


the excellent


line-to-background


ratios


I t


ion


Ir *


r I --


*








depends on


a complex


fashion


upon


power


level


, plasma


gas flows,


nebulizer


pressure,


torch


configuration


, and analyte


species


Although


many


workers


use the ICP


as an emission


source


are aware


these phenomena


little


information


has appeared


in the literature


concerning


presence


lecular


mas--see


emission


above


produced


discussion)


salt


into


introduction


the ICP


well


and therefore


as unsalted


the 1


plas-


knowledge-


able


worker


might


attempt


measurement


of radiation


(especially


emission,


but also


absorption


or fluorescence)


under


conditions


where


the signal


-to-


background


ratio


is far from


optimal


In Figure


10a-f


, typical


monoxide


emission


spectra


several


species


observed


in the plume


region


(%45


are given.


It should


be stressed


that


these


spectra


are the result


aspirating


high


concentrations


specific


species


into


the plasma


and of


observations


in the plume


region


Therefore


such emission


will


rarely


affect


analytical


measurements


AEICP


Neverthel


monoxide


emissions do


occur


and workers


should


aware


existence.


In Figure


11a and


the variation


emission


signals


height


several


are shown.


than


which


species


For Y


occurs


(atomic


maximum


ionic


a higher


and molecular)


for Y(II)


height


occurs


than


with


observation


a higher


For Lu,


height


maxi


for Lu(II)


Lu(I)


occurs


was rather


a higher


height


low and constant


over


than


: the


the observation


emission


height


signal


range


10 to


- Rare


Earth


Analys


mo-


mum


mm.


I


I 1~












I
X
SLJ
E
S
E
EU
* <
II
1 1
C N
"O **
IE
0\
CA E


a a
OOr
e t


0 0
000000 00000


000000 000000
\ O M LA 0 COOui
s~td Ol Mr f^PLFt iAn


C *
*H
EC
-JO10
Er
LIC
rO
*r


-4

0L

Ca



C


1** S
*H [J 0

Zn C
1C kCO

n E ra
15O C Q
Z Il C


*^l co E

0CO C C S M
l- > |0 s 0

*H E CD 3 I
0 ^ -f^ 0 0
(D J SO C7
a u) IC 0
I \0 C C N
F-Q) Ir( C: IA
-$ il





L *"O CD
(U ^- -a c O
4 s-i .oC
- -f 0 (C V

II .- CO C
O03 O C 0
S**4 0 *5
+j ( E *M CO
CJ *- 0E 0C
n as cu *
a-I c ao
or o o
*M CJ C
c -r C o


VC C
C400
HU
J ^


OC
-0 C
'X E(
X E
n


-HD
x o
0O
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So (0 fC
OO N NQ N03' o oc c
00 0 CN < lA f \ \ rCMU
S* C C a a C
Oc ^n 0r~rOrr^~ co v \ooc
ON N ON rrFl
CNN CCrt-t-t- C\o-r-r-
\O \f 000

C\o- 0rlm oo\r- 0 0CO\OnONo
10oMMr^O~O\ oMfCIr oOsr\M~
c^ C r- r^^rc (jCoc csfi cD 5r


** 4 C C
CC \o r r r r
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C C S *4
ac^ e< -t- CM


f-4


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CLi
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(^
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-Do0
C o
0 3


CO C0O-
C- D
0 (1))
E
SaE E
C Qa


3
O
C-i
'S


C
000
000
C C C
CCC







































Figure


10f.


Background


in 590-650


nm range.







68











10






9-






8






U 7
z
=

'C
>




<~ 6

a

z
2

- 5








33












2J
r(
3 6-







iiCf

rcl-'""""^~---5

























Figure


10b.


1000


Lu (LuO


emission


bands


in 465-475


nm range)


Background


in 465-475


nm range.


Monoxide


Band


Element


Heads


(nm)


Intensity(a)


.175
.416
.800


.231
.546
.500


,310
,270
.100


Same


as fig.


10 a,c,d






70







0!o




9-




(b)
8-






Z



4< 6

4<





z

C,,






3



(Ce)
2





I
0lr5e~~~r





























Figure


Emission Signals of Several Species


Experimental Conditions:


Same


vs Observation Height.


as Figure


500 ppm Y
o YO band head at 614.84 nm
o Y atom line at 619.17 nm.
A Y ion line at 508.74 nm.


(band head intensity=1100)
(atom line intensity=1200)
(ion line intensity=1100)






72











100



90







70-



1 60






30.0






20-
0 10












5 10 15 20 25 30 35 40 45


OBSERVATION HEIGHT (mm)































Figure


11b.


o LuO


band


head


at 466.18


(band


head


intensity=630)


e Lu atom


A Lu


line


ion line


at 500.11
at 499.41


(atom
(ion 1


lin


ine


e intensity=800)
intensity=800)











































































































































































| I. I I







spectroscopic


methods


which


have


been


extensively


used


the quantita-


tive


determination


rare


earth


metals.


Flame


spectra


rare


earth


ele-


ments


were


observed


Rains


et al.


who aspirated


non-aqueous


solutions


these


elements


into


an oxy-hydrogen


flame.


Reducing


flames


aid in


atom


production


were


used


Fassel


et al.


were


successful


in obtain-


analytically


useful


line


spectra


these


elements


in fuel


rich


flames


introducing


ethanol


solution


of the elements.


Further


studies


Mossotti


Fassel


and others


60-62


on these


elements


provided


were


over


successfully


one thousand


used


absorption


quantitative


lines


in the optical


determination


region


rare


earth


which


ele-


ments


atomic


absorption


spectrometry.


Amos


and Willis


in their


study


observed


that


when


higher


tempera-


ture


flames


were


employed


as an absorption


cell,


the degree


of ionization


rare


earths


became


significant.


Determination


of traces


rare


earths


atomic


absorption


with


electrothermal


atomization


and by


d.c.


arc emis-


sion


spectroscopy


was discussed


Dittrich


and Borzym.


Dickinson


and Fassel6


with


an ultrasonic


aerosol


generator


and de-


solvation


facility


in their


ICP determined


detection


limits


La and Ce.


Later


Fassel


and Kniseley


reported


detection


limits


rare


earths


which


were


superior


to those


from


flames.


Souilliart


and Robin


used


high


power


(6.6


ICP and ultrasonic


nebulizer


in the study


rare


earth


metals.


In this


study,


we have


used


a conventional


moderate


power


.25-


ICP with


a glass


concentric


pneumatic


nebulizer


for the determina-


tion


detection


limits


(LOD)


and linear


dynamic


ranges


(LDR)


rare







- Experimental


Preparation


Standards.


Individual


stock


solutions


1000


ug/mL


each


rare


earth


element


as well


as mixtures


elements


were


pre-


pared


dissolving


reagent


grade


pure


oxides


(ignited


at 6000C


4 hr)


in hot 3


M HC1.


Successive


dilutions


were


made


before


each


determination


using


distilled-deionized


water


A solution


hydrochloric


acid


ioni


water


was used


as the


blank.


Apparatus


Instrumental


given


Figure


and a 2


ductively


radio


coupled


frequency


plasma


generator


(Plasma T

was used


herm


Kresson


in conjunction


with


with


a glass


a 27 MHz

concentric


neublizer.


The nebulizer


solution


flow


rate


was controlled


with


a syringe


pump


Instruments,


Div.


Orion


Research


Inc.,


Cambridge


list


major


instrumental


components


the ICP


stem


used


in thi


study


given


n Table


Limits


detection


(LOD)


and linear


dynamic


ranges


(LDR)


were


measured


integrating


the electrometer


signal


was taken


that


concentration


equivalent


a signal


three


times


that


the standard


deviation


of 16


consecutive,


integrated


blank


readings.


Experimental


Conditions.


The plasma


was operated


argon


flows


specified


the manufacturer


The ICP


power


was maintained


.25-


throughout


all measurements


the nebuli


zer


solution


flow


rate


maintained


2.2 mL


and the cooling


was controlled


16-20


-1 A
A


spherical


lens (image


placed


in a 3"


metal


tube


, was used


to focus


emission


beam


to the monochromator


entrance


slit.


was














Table


SPECIFIC


COMPONENT


OF EXPERIMENTAL


SYSTEM


Component


Model


Company


Torch


asse


mbly


Plasma


Therm


Kresson


RF Generator


HFP-


500D


Nebulizer


T-220-A2


Meinhard


Associates


Santa


Anna,


Monochromator


1870


SPEX,


Metuchen


Photomultiplier


R-818


Hamamatsu


Corp.


Middl


esex


High


voltage


power


supply


eithly


Instruments,


Clev


eland,


Current


voltage


convertor


Keithly


Instruments


, Cleveland,


Integrator


Lab Constructed







observation


height


could


be adjusted


with


respect


to the


monochromator


optimum


plasma


observation


height


was found


to be


mm above


the load


coil


of the ICP.


- Results


and Discussion


The observed


detection


limits


and linear


dynamic


ranges


rare


earth


elements


with


the lines


used


for measurement


(also


energies


of level


values


are given)


are summarized


in Table


The linear


dynamic


ranges


were


obtained


measuring


the relative


emission


signals


selected


anal


ysis


lines


vs concentration


metal


ions in solution.


analytical


cali-


bration


curves


are linear


over


a concentration


range


s5 orders


mag-


nitude for

calibration


a


11 of

curves


the elements


for La,


being


, Dy


studied.


As representative


and' Lu


are presented


examples,

in Figures


Comparison


of the results


of this


work


with


that


of other


authors


indicates


that


the condition


employed


study


have


resulted


simi


lar or better


limits


of detection


as others


reported


in the literature


13,66


It i


clear


from


the LODs obtained


in the mixture


elements


that


matrix


interference


are negligible


in the ICP


In the present


intense


analytical


studies


, only


ion lines


measurements


rare


were

earth


found

metal


to be


sufficiently


Ion lines


(see


Table


were


selected


on the basi


maximum


of intensity


minimum


spectral


interference


and background


The detection


limits


obtained


study


in pure


or superior


aqueous


to those


3,66


solutions


reported


or rare

in two


earth

previo


mixtures a

us studies


re similar

of ICP exci










O

E
cia

Cu

*H


C (.
Cc
Co

CD
-J
*I


03


O








C~3x
S0 w-

u .
0*






Q -
.. X

I
V


C













.j)
0 c








31C
I<
ClO0
















-J






'It
*P (U











EcD




>) Y



33



r

I E


0 O


N O\


ON\
I
c m

OM
iu i


f^ r\
CM a\



* *


Nor-

0 00 M
r^l F<~
CM CMCMV
NC'.c'
I I I
COC


r\OO

r r


v- 00

cj \o
\ J \O
rr-rfl

4 II
I L\ fM
r^ M r


I r\ lu
ONCN
* *
\a CM Vn











































































O r-






SN*
0r-c


CM CN
SO O


I I
SC







0 O
*\ *































Figure


Analytical


Calibration


Curves


for Lanthanum


and Cerium.


* single


element


x in mixture



































LOb'


" GLOO


10 1' 10 103 104 105 1


ANALYTE


CONCENTRATION


(ng. mi)































Figure


Analytical


Calibration


Curves


Europium


and Dysprosium.


* single


element


x in mixture



































LOD f


/-0LO D


S10 1 104 10 106


ANALYTE


CON CENTRATION


(ng. ml'')































Figure


Analytical Calibration Curves for Holmium and Lutetium.


* single element
x in mixture





86






10







10

S3. Ho
10- x
SyxLU


, I


i 10
LOD Q /

LOODx
1i b5 o1' 1o2 3 ioi io5 160
ANALYTE CONCENTRATION (ng. ml")















CHAPTER THREE

ATOMIC FLUORESCENCE SPECTROMETRY
IN THE FLAME USING ICP AS A NEW EXCITATION
SOURCE "ICP-EXCITED FASF"


- ICP Diagnostics Using the


ICP-Excited Flame AFS


- Introduction


Many physical parameters of the inductively coupled plasma


(ICP)


source


for analytical spectroscopy have been investigated.


review by Barnes


contains an extensive list of


The recent


references covering fun-


damental operating principles and methodologies.


several elements fc

by Human and Scott,


ometer to obtain the p


Emission profiles of


>r several experimental conditions have been reported


who used a pressure-scanning Fabry-Perot interfer-

rofiles. These authors concluded that the spectral


profiles of the lines emitted for the elements investigated


(Ca, Sr, Ar)


depended upon the height of

tion and self-reversal were


observation and that,


observed at certain he


although self-absorp-

ights, these phenomena


did not occur for a range of


at least three orders of magnitude (1-1000


pg/mL)


indicating the excellent characteristics of the


ICP as an emission


source.

This work reports preliminary results showing how several limiting


characteristics of the


ICP emission profiles can be derived if the


ICP is


I __ ___ ___ _1 ~_ ~_


II








suitable


monochromator


mixture


70-73


flame.


This


Therefore


procedure


the flame


is capable


acts


as a resonance


providing


in a very


simple


way qualitative


but unequivocal


information


about


the line


profile


the ICP


emission


without


sophi


sticat


instrumentation


such as high


resolution


monochromators


Fabry-Perot


interferometers.


These


latter


approaches


are the only


ones


giving


complete


quantitative


information


about


the true


line


profiles.


However


, the fluorescence


technique


only


clearly


detects


self-absorption,


is especially


sensitive


cipient


self-reversal


emission


line


profile.


This information


is obtained


from


the experimental


log-log


plot


three


curves


growth


the first


one


is the excitation


curve


growth,


in which


a fixed


concentration


used


the flame


while


increasing


concentrations


are aspirated


into


the flame


acts


as a


resonance


monochromator);


second


fluorescence


curve


growth


, (i.e.,


conventional


curve


growth


obtained


with


a fixed


high)


concentration


a selected


element


in the ICP


while


aspirating


increasing


concentrations


of the


same


element


into


the flame


and the


third


one


is the


common


emission


curve


of growth


which


emiss


from


the ICP is


directly


plotted


vs the concentration


of the analyte


The results


obtained


for the elements


, Mg


and Ca verify


pected


behavior


these


curves.


- Theoretical


Considerations


_








in which


integral


is extended


over


the entire


absorption


line


width


(i.e.,


width


over


which


k (x)
f


differs


markedly


from


zero).


Here


terminology


as follows:


= spectral


irradiance


of the ICP as a function


of wavelength


a given


height


and at


a given


analyte


concentration,


evaluated


at wavelength


a (x)


= fraction


radiation


absorbed


any wavelength


dimension-


less;


kf(x)


= absorption


coefficient


of analyte


atoms


in the flame,


usually


given


product


(peak


absorption


coefficient


purely


tion


Doppler
m-1 for
m : for


broadened


a given


profile)


resonance


and the Voigt


line


and under


profile


given


func-


condi-


tions


and flame


temperature,


is proportional


flame


atomic


concentration


over


all the absorption


line,


all values


= interaction


spectral


length


irradiance


for the absorption


the ICP


process


can


in the flame,


be regarded


as the


exe


product


of the blackbody


spectral


irradiance


at the wavelength


considered


and at the ICP emission


temperature


and the total


absorption


factor


which


a function


of the concentration


in the ICP


and of its emission


depth


in the direction


flame.


When th

fluorescence


e interaction


emission


process i

the flame,


.s studied


then


means


the fluorescence


of the resulting


radiance


cm.








BFa4s
abs


1-exp[-k (x)L}dX


exc


41-exp[-kf(x)f] dx / ~C


k (x)dx] ja


(45)


abs


(X) 1-exp[-k (x)L]dX{[A (n ff)]/[C


Equation 45 is equal to Equation 44 multiplied by a term which takes into

account the self-absorption of the fluorescence radiation leaving the


flame.


Here,


At(n i)


1-exp[-kf(x)z] d),


is the total absorption fac-


tor for the fluorescence radiation in the flame, nf is the flame atomic


concentration,


and t is the


(homogeneous)


fluorescence depth.


Equation


45 simplifies


for the two usual limiting cases of line and continuum ex-


citation sources and for negligible fluorescence self-absorption.


fact,


if nf is low,


then the second factor at


the right hand side of


Equation 45 is unity.


If the


ICP acts as a spectral line source,


ered constant and equal


to its peak value,


maxf


the kf(X) can be consid-

Equation 45 then re-


duces to Equation 46.


BF~~ -exp[-k
F


maxf


L]} E


(x)dx


(46)


exc


Thus the


ICP may act as a line source if


its overall spectral emission


profile is narrower than the absorption profile


in the flame,


even though


its higher temperature would cause the Doppler halfwidth to be larger


than that in the flame by a


factor of


approximately the square root of


Iabs


k (A)dx]










BF,:E


1l-exp[1-kf(x)L]IdX~aE


o)At(nfL)


(47)


in which


is the constant


irradiance


of the


over


the absorp-


tion


line


profile.


Therefore,


when


the interaction


between


the ICP


emission


and the


resonance


absorption


profile


in the flame


is monitored


the fluorescence


emitted


from


a constant


concentration


of flame


atoms,


a slope


of unity


concentrations


in the ICP and a limiting


__


concentrations


in the ICP


should


be obtained.


This


limiting


zero


slope


will


hold,


irrespective


of the amount


of self-absorption,


as long


as self-


reversal


does


not affect


emission


profile.


When


this


happens,


i.e.,


when


the ICP emission


profile


starts


showing


a dip


in the center,


this


will


negative


slope


be reflected


will


immediately


be observed


in the fluorescence


in the experimental


signal


plot.


On the other


tainted


hand


aspirating


the ICP-excited


the flame


fluorescence


increasing


curve


concentrations


of growth


analyte


should


give


the following


information:


if the asymptote


at high


a slope


near


-0.5


then


ICP,


like


at that


a line


particular


source,


height


i.e.,


and atom


spectral


density


profile


behaves


narrower


than


the absorption


profile


in the flame:


(ii)


when


self-absorption


is large


in the


ICP emission,


this


curve

1.. tt^ n s


of growth

S A ,r, t i .


should

4Lt. J- A-


approach


I -


a region
- C_ 1 -


zero


i L I


slope,


.I 1


slooe


r of h i q h


zero


El









These


results


which


are summarized


Table


can


be obtained


from


Equations


45-47


, by


assuming


that


the self-absorption


factor


for the fluo-


rescence


cannot


be considered


negligible


and remembering


that


At(nfL)


At(nrf)

values.


vary


linearly


with


at low


values


and with


at high


- Experimental


Figure


shows


the experimental


arrangement


used.


The ICP


source


(ICP


1500


Plasma


Therm.,


Inc.,


Kresson


was focused


on the


aper


ture


a chopper


and subsequently


onto


the flame


means


spherical


quartz


lenses.


order


to minimi


pre-


and post-filter


effects


a rec-


tangularly


shaped


burner


was used


care


taken


to optimize


properly


illumination

possible inh


and observation


omogeneities


geometries.


in temperature


Furthermore, i

and composition,


n order


the flame


avoid

was


shielded


another


similar


flame


which


could


in turn


be surrounded


an inert


flow.


The fluorescence


was collected


a spherical


quartz


lens and di-


rected


onto


the entrance


slit


a small


monochromator


(Jobin-Yvon


, H-10,


reciprocal


linear


dispersion);


the slit


width


and height


were


unless


otherwise


stated


at 50


mm, respectively.


The out-


signal


a photomultiplier


(Hamamatsu


was fed


a lock-in


plifier


(840


Autolock


, Keithley


Cleve


land


Ohio)


then


a chart


corder.


For the emission


measurements


the ICP


image


was focused


directly


onto


the entrance


slit


on a 0.35-m


monochromator


reciprocal


re-