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S p e c t r o c h i m i e acta, 1055, Vol. 7, pp. 108 to 117. Pergamon Press Ltd., London

The application of atomic absorption spectra tochemical analysis

A. WALSH

Chemical Physics Section, Division of Industrial Chemistry,Commonwealth Scientif ic and Industrial Research Organization, Melbourne, Australia

(Recei ved 18 January 1955)

S u m m a r y -T h e t h e o r e t i c a l f a c t o r s g o v e r n i n g t h e r e l a t i o n s h i p b e t w e e n a t o m i c a b s o r p t i o n a n d

a t o m i c c o n c e n t r a t i on a r e e x a m i n e d a n d t h e e x p e r im e n t a l p r o b l e m s i n v ol ve d i n r e c o r d i n g a t o m i c

a b s o r p t i o n s p e c t r a a r e d i s c u s s e d . O n t h e b a s i s of t h e d i s c u s s i o n s , i t i s s h o w n t h a t s u c h s p e c t r a

p r o v id e a p r o m i s i n g m e t h o d o f c h e m i c a l a n a l y s is w i t h vi t a l a d v a n t a g e s o v e r e m i s s i o n m e t h o d s ,

p a r t i c u l a r l y f r o m t h e v i e w p o i n t o f a b s o l u t e a n a l y si s . I t i s a l s o s u g g e s t e d t h a t t h e a b s o r p t i o n

m e t h o d o ff e r s t h e p o s s i b i li t y o f p r o v id i n g a s i m p l e m e a n s o f i s ot o p i c a n a l y s is .

1. ntroduction

The application of at omic spectr a to chemical an alysis h as proved so successful

over such a wide field th at th ere is a ten dency to overlook some of th e basic

limita tions of existing meth ods. In spite of th e rema rka ble advan ces in technique

which ha ve resu lted in pres s-butt on an alyses of high precision at fant ’ast ’ic speeds,

th ere ha s been practically no progress what soever in solving th e fun dam enta l

problem of devising an absolute met hod, i.e., a met hod which will provide an an alysis

without compa rison with chemically ana lyzed sta nda rds or synthet ic sa mples of

kn own composition. In rout ine a na lysis for production cont ,rol th is problem is of

little consequ ence, since it is only necessar y to ha ve a limited nu mber of st an da rd s,

an d in such work m odern direct-readin g met hods leave little to be desired, except

on th e score of complexity of equipm ent an d ass ociat ed expense. When a na lyses of

miscellan eous ma terials ar e required, th e ta sk of providing th e required ra nge of

sta nda rds becomes insur mounta ble an d th e spectr ochemical meth od then loses

its accur acy, since accur at e an alyses generally necessitat e th e use of sta nda rds

which a re closely sim ilar in composition to th e sam ple for an alysis. In some

ana lyses it is also essential tha t the sample and sta ndar ds be similar as regards

physical condition. For exam ple, th e intensity of th e spectr um of a meta l or alloy

ma y vary with th e met allur gical hist ory of th e sam ple. This difficulty ma y be

overcome by tak ing th e sam ple int o solution, but accur at e an d sensitive meth ods

of an alyzing solut ions ar e only available for th e limited ra nge of element s, ha ving

a low excita tion potent ial, which can be estim at ed by flam e photomet ry. In th is

met hod, also, it is necessar y to use sta nda rd solut ions ha ving compositions closely

similar to that of the test solution.

The possibility of ada pt ing an y of th e existin g met hods to absolute an alysis

does not a ppea r t’o be promising. In the first place, t her e seem s to be little pr ospect

of deGeloping a light source wh ich is such th at th e emission spectr um of a given

element is not affected by th e pres ence in th e at omic vapour of at oms of oth er

element s. Secondly, even if th ese int erelemen t effects were eliminat ed, th ere

rema ins the problem of absolut e intensity measu remen t an d th e associated problem

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The application of atomic absorption spectra to chemical analysis

of det,ermining th e distribution of at oms over th e various energy sta tes. In pra ctice

th ere is th e th ird difficult y th at electr ical dischar ges of th e type now in use do not

give a sta ble outpu t of ra diation; modern met hods conceal th ese erra tic variat ions

in outpu t by int egrat ing, photograph ically or photoelectr ically, th e ra diat ion over

a period of several seconds . Finally, th ere are oth er problems ar ising from self-

absorpt ion an d self-revers al, an d from th e fact tha t th e pr ocesses of vaporizationan d excita tion ar e not isolat .ed from ea ch oth er. A review of th ese and oth er

as pects of source behaviour ha s been given elsewh ere [l].

At t,he present sta ge of its development th ere is no doubt th at th e major

obsta cle to furt her progress in th e techn ique of spectrochem ical an alysis is th e

occurr ence of int erelemen t effects, sin ce if th ese could be elimina ted it would be

possible to use th e sam e set of sta nda rds for the determina tion of an y one element

in an y ma t)erial. With existing emission meth ods th e intensity of a given spectrum

line due to one par ticular concentra tion of an element in different ma terials varies

great ly. For exam ple, PROKOF’EV [2] reports t ha t for th e sam e concentra tion of

silicon th e int ensit y of th e silicon lines in th e spa rk spectru m of steel is eight tim esas great as in brass, an d in dura lumin th e intensity is even less. Many other

examp les of int erelemen t effects ha ve been publish ed.

No sat isfactory explana tion of th ese effects ha s yet been given, nor can one be

expected, since th e phenomena occurr ing in th e ar c an d spar k discha rges used as

light sources a re far too complex to perm it of an y th eoret ical an alysis, an d th e

appr oach to th e subject seems likely to rema in essentially empirical. It seems

possible, h owever, to ar rive at certa in broad conclusions. Since int erelemen t

effects ar e usu ally of th e sam e order of ma gnitu de for different spectru m lines of a

given element , corr esponding to tr an sitions between different energy sta tes, they

probably ar ise from cha nges in th e concentra tion of at omic vapour ra th er tha nchan ges in th e excita tion condit ions. However, such chan ges cann ot occur in

sources in therm al equilibrium at a consta nt temperat ure. Thus, if therm al

equilibrium is assu med, th en interelement effects m ust necessarily be due to a

cha nge in th e tem perat ur e of th e at omic vapour. Whilst th e behaviour of th e

electr ical dischar ges used in spectrochem ical an alysis shows th at it is not just ifiable

to assu me ther ma l equilibrium or to ascribe a temper at ur e to th e discha rge, it is

inst ru ctive to consider th e effects of chan ges in tem pera tu re on a mass of atomic

vapour in th erma l equilibrium.

Consider th e emission of a spectr um line due to th e tran sition from an excited

sta te j, of excita tion ener gy Ej, to a groun d sta te of energy E, = 0. Then if P j

and P, ar e th e statistical weights for the excited

tively, th e nu mber of at oms in th e excited sta te,

at oms in th e groun d sta te, N,, by th e relation

stat e and ground stat e respec-

Nj, is related to th e number of

an d, neglecting self-absorpt ion an d indu ced em ission, t he int ensit y of th e emit ted

line is proport,iona l to Nj.

In order to illustra te th e ma gnitude of N,/N,, th e calculated values for resonance

lines of various elements at different temper at ur es ar e given in Table 1.

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R e m n a n c e

line

Cs 8521 A

Na 6890 A

Ca, 4227 A

Zn 2139 A

-

-

-

A. WALSH

Table 1. Valu es of N j/ No.for various resonance linea

Transition

2sl/2 -

2P212

25l/2 - 2p2/2

‘S, - ‘P,

IS, - ‘P,

-.

-

pjIpOT = 2,OOO”K T = 3,OOO”K

2 4.44 X lo-’ 7.24 X lo-9

2 9.86 x IO-6 6.88 x 10-4

3 1.21 X lo-’ 3.69 X 10-h

3 7.29 x lo-15 5.58 X lo-‘0

- -

T = 4,OOO”K 1T = 5,OOO”K

2.98 X 1O-2 j 6.82 X 1O-2

4.44 X 10-s 1.51 X 10-2

6.03 X 10-4 3.33 X 10-s

1.48 X lo-’ 4.32 k 1O-6

It will be seen th at in near ly all cases the number of at oms in the first excited

sta te is only a small fra ction of th e nu mbers of at oms in th e groun d sta te. The

fraction only becomes app reciable at high tem pera tu res for st at es of low ener gy.

Since most elements ha ve th eir str ongest resonance line at wavelengths below

6,000 A, an d since we sha ll be concern ed ma inly with flam es or furn aces ha ving

tem pera tu res below 3,00O”K, we ma y rega rd N, as negligible comp ar ed to No.

The fra ction of at oms in higher excited sta tes is mu ch less th an th ose given above,

an d th us CN, is also negligible compa red with N,, an d th e latt er can be considered

as equal t o th e total nu mber of at oms, N.

Thus, whilst the num ber of excited at oms varies exponent ially with temper a-

tur e, the num ber of atoms in the ground stat e remains virtua lly constan t and

th erefore t he integrat ed absorption J K, dv due to tr an sitions from th e groun d sta te

is independent of tem perat ur e. (This discussion only applies to at oms ha ving a

ground st at e well rem oved from th e lowest excited sta te; th e case of at oms

ha ving a multiplet groun d sta te is discussed in Section 2.)

On th e basis of th e above discussion it would appea r th at at omic absorption

spectr a would ha ve import an t advan ta ges over emission spectr a as a mea ns of

chemical analysis. It is th erefore surpr ising to note th at th e resear ch in th is field

ha s been devoted alm ost exclusively to emission spectr a; th e an nu al review [3] of

pr ogress in spectrochem ical an alysis is, in fact, given un der th e genera l title of

“emission spectroscopy.” Apart from t he special case of estimat ing th e cont am ina-

tion of room an d laborat ory at mospher es by ‘mer cury va pour, th e app licat ion ofat omic absorpt ,ion spectr a t o chemical an alysis appea rs t o ha ve been confined to

ast rophysical work on th e deter min at ion of th e composition of th e solar an d stellar

atmospheres.

The pur pose of th is paper is to exam ine th e th eoretical factors governing th e

relat ionship between at omic absorpt ion an d at omic concentra tion an d to discuss

th e experimen ta l problems involved in recordin g at omic absorpt ion spectr a. On

th e basis of th ese discussions, it is shown th at such spectr a, provide a promising

met hod of chemical an alysis with vital adva nt ages over emission m eth ods,

par ticular ly from th e viewpoint of absolute an alysis. It is also suggested th at th e

absorpt ion met hod offers th e possibility of providing a simple m ean s of isotopicanalysis.

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The application of atomic absorption spectra to chemical analysis

2. Variation of atomic absorption with atomic concentration

The relat ionships between at omic absorption an d at omic concentra tion, un der

var ious condit ions, ar e fully discussed in several pa per s, ma inly in ast rophysical

jour na ls, an d in sta nda rd reference books [4-s]. For th e pur pose of th is discussion

it is sufficient to consider only the most fun dam ent al of th ese rela tionsh ips. In

th is section th e discussion will be fur th er restr icted by assu ming th at a tomicabsorption lines possess n o fine str uctur e; th e case of isotopic hyper fine str uctu re

is discussed lat er in Section 4.

Consider a par allel beam of ra diat ion of int ensit y,I,, at frequ ency v incident on

an atomic gas or vapour of th ickness 1 cm. Then if I, is th e intensity of th e tr an s-

mit ted bea m, the absorpt ion coefficient K, of th e vapour at frequ ency v is defined

byI, = I,” emKJ (2)

The depen dence of K, on v, i.e., th e sha pe of th e absorpt ion line, is deter min ed

by th e na tu re of th e tr an sition involved in th e absorption an d on th e physicalconditions such as temper at ur e, pressur e, an d electr ical fields, to which t he at oms

are subjected during the measurement .

Accordin g to class ical dispersion th eory, t he rela tionship between absorpt ion

an d concent ra tion is given by

I K,dv = n$=N,ff

wher e e is th e electr onic cha rge, m th e electr onic ma ss, c th e velocity of light ,

N,, th e number of at oms per cm3 which ar e capa ble of.absorbing in th e range v to

Y + dv, and f, th e oscillator stren gth, is th e average num ber of electr ons per at om

which can be excited by th e incident ra diat ion. Equation (3) is not valid for str ong

absorption lines, since it assu mes tha t the refractivejndex is of th e order of 1 over

th e breadt h of th e absorption line.

For a resonance line due to a tr an sition from a groun d sta te wh ich is well

sepa ra ted from th e’lowest excited sta te, N, can be consider ed as equa l to N, th e

total nu mber of at oms per cm3 (see Table 1). If, however, th e tr an sition does not

origina te in th e ground sta te, or if th ere is a multiplet ground sta te, th en the nu mber

of at oms capable of absorbing is given by

where i denotes th e initial stat e involved in th e tr an sition, an d th e summ at ion in

th e denomina tor extend s over all possible en ergy sta tes. In pra ctice, of cour se, th e

sun ima tion can be rest ricted to th e low-lying levels.

In term s of tr an sition probabilities th e equat ion corr esponding to equat ion (3)

is

s,dv =z.r 1

N J ,, (1 - $$)I

(5)

where I is th e wavelength at th e centr e of th e absorption line, Pi and P, are the

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A. WALSH

sta tist ical weights of th e lower an d up per sta tes, respectively, involved in th e

transition, and Aji is th e Ein stein coefficient of spont an eous emission for th e

j -+ i tr an sition. N, is th e nu mber of at oms in th e upper electr onic sta te, a nd will

genera lly be negligible comp ar ed with N,, and in th is case equa tion (5) redu ces to

th e well-kn own relat ion:

sK, dv = s, N ,A,, (6)

97 ,

In th e gener al case N, is given by (4), but for resona nce lines is equa l to N. Similar ly,

if th e tr an sition from th e jth to ith sta te is th e only one which can occur , as in the

case for resona nce lines, Aj, = l/r, where 7 is th e mean life of an at om in the

excited sta te j.

Equ at ions (3) an d (6) provide sim ple rela tionsh ips between absorpt ion an d

concentra tion, an d it is now necessar y to investigate wheth er th ey can be applied

to a pra ctical met (hod of spectrochem ical an alysis. Since th e int ensit ies of spectru m

lines ar e usu ally expr essed in ter ms of oscillat or str engt hs, it is convenien t toconsider equation (3).

Fir stly, it is necessar y to kn ow wheth er suit able absorpt ion lines occur in

regions of th e spectr um which a re am enable to measu remen t. In term s of sensi-

tivity, it is obviously desira ble to use th e str ongest resona nce lines, an d in genera l

th ese will corr espond to th e str ongest lines occur rin g in emission spectr a. These

ar e listed in a paper by MEGQERS [7], an d reference to th is shows tha t with th e excep-

tion of th e ra re gases, hydrogen, mercury, th e ha logens an d th e meta lloids, all

element s ha ve th eir m ost sensitive lines in th e region 2,000-9,000 ip. Thus th e lines

for a ll th e more comm on elements all lie in regions of th e spectr um where mea sure-

ments are simple t o make.The oscillator str engths of some of th ese lines ha ve been determ ined, an d ar e

listed in Table 2. Theoretical calculat ion [S-14] of f-valu es is possible for a toms

having simple electr onic str uctur es, but has not yet been carried out for heavy

at oms ha ving a complex str uctur e. BATES and DAMGAARD [15] ha ve described a

simplified t heoretical meth od an d have published ta bles from which t he absolut e

str engths can be ra pidly obtained. The meth od ha s been shown t o give accur at e

results for a ll tr an sitions in th e lighter simple systems, but there ar e insufficient

experiment al dat a to enable one to judge to what extent th e met hod can be

ap plied to th e more complex electr onic str uctu res.

It is interest ing to note th at th e oscillator str ength for th e str ongest‘copper

line is approxima tely th e same as for the alkali met als, in spite of th e fact tha t th e

closed 3d shell of th e copper at om is not n ear ly so tight ly boun d as th e inner

electr on shells of th e alka li meta ls. By compa rison with th e f-values of other

element s in Group 1, it seems probable th at r ubidium, silver, and gold will have

f-values of th e order of 0.7 for their st .rongest resona nce lines. Similarly, th e Group

2 element s Zn and S r ma y be expected to ha ve f-valu es of th e order of 2 for th eir

strongest lines.

It‘ does not a ppear possible at the present time to ma ke a ny corr esponding

estim at es of th e oscillator str ength s of spectra l lines of oth er element s, par ticular ly

th ose such as iron and cobalt, with complex electr onic str uctur es an d mu ltiplet

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T h e a p p l i c a t io n o f a t o m i c a b s o r p t i o n sp e c t r a t o c h e m i c a l a n a l y s is

ground states. The possibility of deter min ing ap proximat e f-values for such

element s by a simple experimenta l meth od is discussed in Section 4.

For those elements for which t he oscillator str engths ar e known, th e at omic

concent ra tion can be deter min ed from th e int egrat ed absorpt ion coefficient , usin g

equa tion (3); th e experimen ta l problems involved in mea sur ing su ch coefficients

ar e discussed below.Ta ble 2 . Lis t o f f -va lu es

Reso n a n ce l i n e

-

L

-

Tra n s i t i o n f Re f e ren ce

Li 6708 A

Na 5890

K 7065

cu 3247

cs 8521

Be 2349

Mg 2852

Ca 4227

Cd 2288

Ba 5535

Hg 1849

Tl 2769

Cr 4254

Ni 3415

Fe 3720

2sl/2 - 2p2/2

25l/2

- 2P312

25l/2

- 2P212

2sl/2

- 2P3/2

2s I la - 2P312

lS() -'P,

‘Se - ‘P ,

‘S, T lP ,

‘S , - ‘P ,

IS, - lP ,

‘Se - lP ,

2Pl/2 - 2*2/2

'52 - ‘P i

3D, - 3F!j

a6D4 - z5F,

r

-

0.50

0.70

0.64

0.62

0.66

1.82+t

1.74*

2.28* 7

1.20

2.10

1.19

0.20

0.084

0.02

0.013

-

-

-!_

PI

Pel

[171

[181

[I91

[ll, 13, la ]

[13, 141

[12, 13, 141

VW

WI

c221

~231

~241

~251

rw

* Theo r e t i ca l va lues .

7 BI E RM A NN a n d T RE F F T Z [ 13 ] s t a t e t h a t t h e s e v a l u e s s h o u l d b e co r r e c t e d a c c o r d i n g t o t h e m e t h o d

desc r ibed in re f . 14 . See a l so th e va lues quo ted i n re f . 30 .J ’oooo tno t e .i n c e t h e p u r p o s e of t h i s t a b l e i s t o i n d i c a t e t h e o r d e r o f t h e f -v a l u e s f or v a r i o u s l in e s , t h e

v a l u e s o b t a i n e d b y d i ff e r e n t o b s e r v e r s a r e n o t i n c l u d e d . T h e y a r e d i s c u s s e d b y K O RF F a n d BRE I T [2 6]

an d by MITCHELL an d ZEMANSKY [a ] .

3, Experimental determination of atomic absorption coefkients

The shape of an at omic absorption line is determ ined by (a) th e na tu ra l width of th e

line due to th e finite lifetime of th e excited sta te; (b) th e Doppler cont our due to

th e motions of th e at oms relative to th e observer; (c) pres sur e broaden ing, eith er

by at oms of th e sam e kind giving rise to resona nce broaden ing or to foreign gases;

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A. WALSE

and (d) Sta rk broadening due to extern al electr ic fields or to neighbourin g cha rged

pa rt icles. The nat ur al width of an at omic spectr al line is of th e ord er of lo4 A,

an d for t he pu rposes of th is discus sion is negligible comp ar ed to th e width due to

other causes.

The Doppler width of a line is given by

where R is th e un iversal gas const an t an d M is th e atomic weight. Typical values

of D, ar e given in Ta ble 3.

Table 3. Valuer, of DA at various ternperaturea

Element

-

-

-

1 M

DA

i

I

1,OOO”K j 2,OOO”K 1 3,OOO”K

Na

cu

Zll

-____

6890

3247

2139

22.3

63.0

66.4

0.028A o.oxa A O-048 A

0.0092 A 0.0013A 0.016 A

I0~0060 A / 0.0085 A o-010 A

I I-If we as su me th at . a t emper at ur e of 2,OOO”K s required to produce su fficient

v&pour , th en th e Doppler width is of th e order of 0.01 A. The a ccur at e m easu re-

ment of the profile of such a line would require a resolution of about 500,000,

which is beyond th e perform an ce of most spectr ogra ph s. In a ddition, if it isdesired to use photoelectr ic met hods of intensity mea sur ement , then it is scar cely

feasible to use a continuous source, since the energy emitted over such a small

spectr al slit-width would be too sma ll to give a high en ough signal/noise ra tio.

In t he pa st th is difficult y h as often been overcome by us ing th e met hod of tota l

absorption, in which t he energy removed from th e incident beam is measu red.

This method has the advantage tha t the m easurement is independent of the resolu-

tion of th e monochr omat or, but su ffers from t he disadva nt age of giving a comp li-

cated relation between N and f, according to the region of the curve of growth in

which t he mea sur ement is ma de. However, if th e absorpt ion is so str ong tha t it

is not possible to m ak e an accur at e m easu rem ent of th e absorption coefficient, a8in as tr oph ysical work , then t he cur ve-of-growth met hod is th e only one ava ilable.

The m eth od ha s been successfully applied t o the measu remen t of oscillator st rength

from furnace absorption spectra by KING [18] and by ESTABROOK 24, 251.

From th e point of view of spectr ochem ical an alysis, a more at tr active met hod

appea rs to be to m easu re th e a bsorpt ion coefficient at th e centr e of the line, using

a sha rp-line sour ce which emits lines ha ving a mu ch sma ller ha lf-width th an th e

absorption line. If the sh ape of the latt er is determ ined en tirely by Doppler

broadening, we have [27]

-

K

212

J

G-2 TV9

max - -- .DA ,r a N f (8)

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The application of atomic absorption spectra to chemical 8ndysia

where D is th e Doppler width. Thus in th is case also th ere is a linear relation

between a bsorpt ion an d concentra tion.

If such a sha rp-line source is used , it is now no longer necessar y to use a spectra l

slit-width of th e sam e order a s th e ha lf-width of th e absorption line. The require-

ment now is th e ability to isolate a selected line from oth er lines emitted by th e

sour ce. Thus spectr ograph s ha ving th e sam e resolut ion as th ose used in conven-tiona l emission meth ods ar e adequa te. In many cases it ma y be sufficient to use

filters.

Various m eth ods of producing such sha rp-line sour ces ar e ava ilable. In our

work it ha s been foun d convenient to use hollow-cat hode dischar ge tu bes, an d it

ha s proved possible to ma ke sea led-off tu bes a bout th e size of a photomult iplier

tube.

Ther e is one oth er experimen ta l difficulty. In ma ny cas es vaporization of th e

sam ple will result in th e emission of ra diation at exactly th e wavelength where it is

desired to ma ke th e absorpt ion mea sur emen t. This difficult y can be overcome by

modulating th e incident ra diation before it reaches th e at omic vapour an d am plify-ing th e out put of th e detector by an am plifier tu ned to th is modulation frequency.

Thus the ra diation emitt ed by th e at omic vapour , which is not modulated, produces

no signa l at th e out put of th e amplifier.

So far it ha s been assu med th at th e line sha pe is determ ined solely by Doppler

broadening, an d this is sensibly tr ue if th e vapour is produced by a vacuum

furn ace, such a s tha t used by KING, an d if th e vapour pressur e is so sma ll tha t

resona nce broaden ing is negligible. Anoth er convenient met hod of vaporizing

th e sample is to atomize a solution of th e sam ple into t he air supply of a Meker

burn er, as in emission met hods of flame photometr y. In th is case th ere is broad-

ening due to foreign gases, an d, although we have not ma de accur at e measu remen tsfor th e flame we ha ve used, its ma gnitude is probably of th e same order as th at

due to th erma l motions. Once th e broadening due to pressur e is known, th e

corr esponding correction to equa tion (8) can be app lied, u sing th e ta bles published

by ZEMANSKY [28]. As an examp le, if th e pres sur e broaden ing width is equa l to

th e Doppler width, th e ma ximu m absorption is 43 per cent of th at due to Doppler

broadening alone.

IV. Discussion

The above discussion of at omic absorption spectr a ha s indicat ed th e at tr active

possibilities of usin g th em for chemical an alysis an d of developing a met hod which

will provide a useful complement to emission met hods an d in ma ny cases ma y well

supersede them. One of th e ma in at tr actions of th is absorption meth od is th at ,

th eoret ically, it is expected to be mu ch less sus cept ible to int erelemen t effects.

In so far a s effects observed in emission ar e due to variat ions in th e distr ibution

of at oms over the tarious excited sta tes, th ey would ha ve no count erpar t in

absorption where this is due to a tr an sition from the ground sta te. Similar ly,

absorption will not be critically dependent on th e temper at ur e of th e at omic

v&pour , since th e Doppler width only varies as Tlj2,hereas small changes in

temper at ur e produce large cha nges in th e intensity of th e emitted ra diation.

In add ition, th e int egrat ed absorpt ion coefficient is indepen dent of wavelength , in

ma rk ed cont ra st to th e emission inten sity, which will vary according to Plan ck’s

law.

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A. WALSH

Whilst th e discussion ha s been limited to th e ideal case of a vapour in th erma l

equilibrium, in pra ctice th ere m ay be effects due to a shea th of cool vapour .

But wher eas in emission spectr oscopy th is causes self-reversa l which redu ces th e

peak intensity, in absorption it will cont ribute to th e absorption an d will in no

sense reduce th e sensitivity. Incident ally th e self-reversal observed in emission

spectr a provides a good indicat ion of th e sensit ivity of th e absorpt ion met hod.Anoth er cause of deviation from ther ma l equilibrium is chemilum inescence;

th is may well produce a nu mber of excited at oms, which is compa ra ble with tha t

due to th erma l excitat ion, but will not produce an y significant cha nge in th e num ber

of un excited at oms. Thus th e effects on th e absorption spectr um ar e negligible

compa red to th ose on th e emitt ed ra diation.

On th e experiment al side, th e import an t advan ta ge of th e absorption meth od

lies in th e fact tha t th e mea sur emen t of th e absorpt ion coefficient consist s of

measu ring th e ra tio of two int ensities, which is mu ch simpler t o achieve th an t he

measu remen t of emission int ensities in absolut e un its.

The possibilities of absolute an alysis h ave been discussed an d th e use of arelat ive absolut e met hod ma y also be noted. For exam ple, if th e sam ple solut ion

is at omized int o a flam e, then a calibra tion for one element would serve to dedu ce

th e appr oxima te calibration of oth er element s, provided th e oscillat or str engths

ar e known. In th is connection it would a ppear t ha t, using sta nda rd solutions,

th is meth od could be used to determ ine oscillator str engths, at least to within

an order of magnitude.

Fina lly, th e absorpt ion met hod ma y prove suit able for isotopic an alysis.

If sources emit ting spectr a of only one isotope ar e used , th en a n an alysis for this

isotope can be obta ined directly, since th e oscillator str engt h is th e sam e for each

componen t of th e hyperfine str uctu res of th e excited level. If no pur e isotope isavailable, th en sources ha ving different concent ra tions of isotopes ma y be used .

Alter na tively, an isotopic filter conta ining th e vapour of one or more isotopes

may prove satisfactory.

The results of some preliminar y experiment s have been in full accord with th e

conclusions ar rived at in th is paper, an d fut ur e paper s by J. P. SHELTON an d th e

au th or will describe th e const ru ction of an at omic absorpt ion spectrophotomet er

an d its app licat ’ion to various an alytical pr’oblems.

References

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[19] MINKOWSKI, ,R. and M~~HLENBRUCH, W.; 2. Phy.9ik 1930 63 198.[20] ZE%%ANSKY,M. W.; Z. Phyaik 1931 72 87 .

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[26] KORFF, S. A., and BREIT, G.; Rev. Mod. Phys. 1932 4 471.

[27] See ref. 4, p. 100.

[28] ZEMANSKY, M. W.; Phy8. Rev. 1930 36 919; ee lso ref. 4, p. 329.

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