METAL TRACE ANALYSIS BY FLAME/GRAPHITE FURNACE OG SPECTROSCOPY

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METAL TRACE ANALYSIS BY FLAME/GRAPHITE FURNACE OG SPECTROSCOPY

I. Bykov, A. Skvortsov, Yu. Tatsii, N. Chekalin

To cite this version:

I. Bykov, A. Skvortsov, Yu. Tatsii, N. Chekalin. METAL TRACE ANALYSIS BY

FLAME/GRAPHITE FURNACE OG SPECTROSCOPY. Journal de Physique Colloques, 1983, 44

(C7), pp.C7-345-C7-352. �10.1051/jphyscol:1983732�. �jpa-00223289�

JOURNAI- D€ PHYSIQUE

Colloque C7, supplCment au n O 1 l , Tome 44, novembre 1983 page C7-345

METAL TRACE A N A L Y S I S BY FLAME/GRAPHITE FURNACE OG SPECTROSCOPY

I.V. Bykov, A.B. Skvortsov, Yu.G. T a t s i i and X.V. chekalinr

V . I . Vernndr;ky I n s t i t u t e of C e ~ c h e m i s L r y qnd AnaigLicnl L%ernist,r~2,

U S F Xcclderny g f S c i e n c e s , !4gscow, U.S.S.R.

R6surn6 - A l ' a i d c d c d i f f g r c n t s s c h e m a s d ' e x c i t a t i o n l a d E t e c - t i o n o p t o g a l v a n i q u c d c s t r a c e s d 1 6 1 6 m e n t s a 6 t E e f f e c t u E e d a n s d e s s o l u t i o n s d ' c a u p u r e a i n s i clue d a n s d e s m a t r i c e s c o m p l c x e s p a r a t o m i s a t i o n d c s 6 c h n n t i l l o n s d a n s u n e flamme e t un f o u r 6 l c c t r o t h c r m i q u e c n g r a p h i t e .

Abstract - The optogalvanic d e t e c t i o n of t r a c e elements with

- various e x c i t a t i o n schemes i s i n v e s t i g a t e d f o r pure water s o l u t i o n s and f o r matrix i n flame and electrothermal graphite furnace.

I n t h e reviews /l-3/ t h e perspective8 of optogalvanic d e t e c t i o n of atoms i n flames were shown f o r t r a c e element a n a l y s i s . Now it i s important t o i n v e s t i g a t e t h e p o s s i b i l i t i e s of t h e method more care- f u l l y and t o f i n d t h e ranges of i t s optimum application. I n t h i s work we s t u d i e d various o p t i c a l schemes of e x c i t a t i o n of Na, I n , CS, Yb atoms i n air-acetylene flame and determined t h e conditions of e f f e c t i v e i o n i z a t i o n of atoms i n water s o l u t i o n s and i n matrix. The matrix i n t e r f e r e n c e s were i n v e s t i g a t e d f o r I n i n SnC12 and SnSO4 matrices and f o r CS i n NaCl matrix. The experiments with t h e e l e c t r o - thermal g r a p h i t e furnace a r e a l s o described.

Experimental Apparatus. The experimental apparatus i s schematically shown i n f i g . 1. IPwo various systems were used a s e x c i t a t i o n sources:

1) dye l a s e r s with a prism telescope pumped by N2-laser; 2 ) grazing incidence dye l a s e r s , pumped by t h e second harmonic of Nd-YAG l a s e r . The dye l a s e r r a d i a t i o n was amplified and could be doubled i n KDP c r y s t a l . The main c h a r a c t e r i s t i c s of both l a s e r systems a r e shown i n t a b l e ^1. An air-acetylene flame was used a s a source of f r e e atoms.

Table 1 C h a r a c t e r i s t i c s of l a s e r s used

Nd-YAG Grazing i n c i - 2nd harmo- N l a - Dye l a s e r l a s e r dence dye n i c of dye 2-ser with prism (532 nm) l a s e r l a s e r 337 nm telescope Pulse energy

E~ ^{1 0 0.1-1 .O 0.05-0.1 4 0.03-0*1}

Pulse dura-

t i o n 'Zrp ,ns 15 13 11 8 5

Line width

~3 , cm-' - ^0.35 - - ^0.5

Repetition

r a t e f , K 4 2-50 1-12

*Mr. C h e k a l i n w a s unfortunately u n a b l e t o come to the C o l l o q u i u m but has w r i t t e n the lecture h e w o u l d have presenred w h i c h is published in the Proceedings.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983732

JOIJHNAIL Dt PHYSIQUE

Fig. l - The experimental apparatus.

Dissolved samples were a s p i r a t e d i n t o a standard premix s l o t burner l1 cm long. Two collimated dye l a s e r beams were d i r e c t e d c o l l i n e a r l y along t h e burner a x i s by a system of mirrors. The i o n s a r i s i n g in t h e r a d i a t e d zone were c o l l e c t e d on t h e watercooled s t a i n l e s s s t e e l e l e c t r o d e , being under t h e negative p o t e n t i a l (-800 - -1200 V). The e l e c t r i c a l l y grounded burner head served a s an anode. The electrode had a form of p a r a l l e l e p i p e d 4x20fl20 mm and could be placed e i t h e r in t h e flame, o r on one s i d e of t h e flame. Ion c u r r e n t p u l s e s were removed from t h e capacitance e l e c t r o d e - burner head (-- 25pF), then they preamplified and a f t e r passing through t h e cut-off HF f i l t e r s were observed on t h e C-1-70 oscilloscope o r processed with a box-car averager, having 1 ~ gate width. The time constants of t h e f i l t e r s s provided t h e i n t e g r a t i n g of input s i g n a l s .

Results and discussion. The e x c i t a t i o n schemes f o r elements used and corresponding d e t e c t i o n limits (DL) (36 c r i t e r i a ) a r e given i n Table 2. Let us consider t h e r e s u l t s f o r p a r t i c u l a r atoms i n more d e t a i l . Indium. The schemes of e x c i t a t i o n a r e shown on f i g . 2. To compare

various excitation-ionization schemes we can, besides of DL, use t h e e f f i c i e n c y of i o n i z a t i o n , determined as a p a r t of e x c i t i n g atoms N~~ converted i n t o ions. When t h e o p t i c a l s a t u r a t i o n was reached, t h e N~~ number could be e a s i l y estimated. When t h e r e were no o p t i c a l

s a t u r a t i o n we could get only t h e lower estimation of

.

For one-step e x c i t a t i o n (A =303.9 m) t h e A B value ( s e e Table 2) i s much higher than kT=0.22 eV. Yet t h e e f f i c i e n c y of i o n i z a t i o n from 5 d * ~ ~ , - l e v e l and 5 d 2 ~ % l e v e l , which i s probably i n equilibrium with t h e former i s high enough (- 4%) and provides t h e low DL f o r t h i s scheme. The cross-section of a l a s e r beam i n which t h e o p t i c a l satu- r a t i o n was reached i s 0.08 cm , but a s one can see from f i g . 3 , t h e cross-section of t h e flame i n which atom concentration i s s u b s t a n t i a l i s much l a r g e r . The use of more powerful l a s e r s w i l l r e s u l t i n t h e s i g n i f i c a n t lowering of DL i n t h i s case.

Note t h a t DL on 1 =303.9 nm f o r t p = l l n s ( t h i s work) a r e n e a r l y equal t o DL f o r Z p =800 n s from CMX-4 flash-lamp l a s e r /3/, i n s p i t e of t h e f a c t t h a t T p values d i f f e r more than 70 times. It would be more

c o r r e c t , of course, t o compare t h e e f f i c i e n c y of i o n i z a t i o n , but it

i s impossible t o do on t h e b a s i s t h e d a t a /3/.

Table 2. The experimental conditions and results obtained - efficiency of atomization in acetylene-air flame; Ei - energy of ionization; dE=Ei - zex ; DL - limit of detection; E, and E:, - laser pulse energies for 1st and 2nd steps; rp - the time width of laser pulse.

E ,v 1.7 0 .6 0.6 0.4 0.4 0.3 0.3 1.17 -0.25 0.86

DL pg/a 7 3 1 l0 3 30 7 2 100 I 0.6

A2 - 57208 571.0 526.3 525.4 502.3 501.8 - 581 82 581 82 568.2 568.8

3 I nm 303.9 451 ,I 451. 'l 451 1 455.5 555 6 589.6

Element In C S Pb N a 7pP3/T4d2Dj,2,5/, 1 589.0 I

%p as 11 5 5 5 5 5 13 5

&I W 70 27 27 30 30 30 30 120 200 35 35

Excitation scheme 5$Pk - 5d2~y2

8p2p?2 5~2 P72- 8p2 p%

S p2 ?$P%- 6s2S1/ 2'gp2 p,/2 ,l0 p2Py2 sp2pi/,- 6~2s4/2\ 40 p2p7/2 6s2S,1/2-7p2Py2 4fd4('s)6s2 - 4f 14(' s)6~6~ - ~-F'''GF$&Js*~~~/~ - AI, level 3 p2~ii2-4d2D 3/2 ~S~SI/~

d.lo2 8.5 2 100

62 yJ 65 65 52 52 47 47 - 500 35 35

Ei eV 5.78 3.89 6-25 5.l4

C7-348 JOCJRNAI- DE PHYSIQUE

4eVl In even .,., ... .

Fig, 2 - The schemes of e x c i t a t i o n f o r I n and Yb,

C , , -4i

I

Fig. 3 - Space p r o f i l e of I n atoms d i s t r i b u t i o n i n air-acetylene flame, obtained by monitoring t h e l a s e r beam ( 3 =303,9 nm) over t h e flame and detecting t h e a r i s i n g ions. The electrode-burner head distance i s 11 mm.

I n two-stepped schemes we investigated t h e e x c i t a t i o n of I n atoms on 8p, gp and l o p l e v e l s with a corresponding diminution of A E values ( s e e t a b l e 2). We could not reach t h e o p t i c a l s a t u r a t i o n of these l e v e l s - it was reached only f o r t h e first s t e p of e x c i t a t i o n ( f i g , 4). Under t h e s e experimental conditions we don't observe t h e increase of t h e a n a l y t i c a l signal with a growth of e x c i t a t i o n level.

It i s d i f f i c u l t t o compare t h e population of exciting l e v e l s with- ou-j s a t u r a t i o n , but t h e estimation of i o n i z a t i o n e f f i c i e n c y f o r 8p P312 l e v e l brings t o 0.8 (we suppose t h a t t h e r e i s no mixing between 5p2pdI2 and 5p2~3/2 l e v e l s during t h e l a s e r pulse z p =5 n s , and t h a t 8#p3/,level i s i n saturation. We a l s o suppose t h a t t h e efficiency of atomization f o r I n i s 0.085). It i s c l e a r t h a t even f o r aE=0.6 eV t h e e x c i t i n g l e v e l i s i n equilibrium with t h e continuum system of l e v e l s , A f u r t h e r r i s e of t h e e x c i t a t i o n l e v e l i s e f f e c t - l e s s , because t r a n s i t i o n p r o b a b i l i t i e s f a l l quickly with n ( l e v e l

numbec) . So, t h e 8p2p3/2 l e v e l i s optimum f o r t h i s two-step scheme

and p::ovides m i n i m u m DL. It i s i n t e r e s t i n g t o estimate f o r t h i s case

t h e minimum detected number of atoms and ions. Using t h e expression

Fig. 4 - Saturation curves f o r I n and Yb.

f o r atom concentration i n flame from /4/ we found t h a t m i n i m u m i o n number i s - ^100, minimum atom number -- 250 and minimum detected concentration - ^{5.10 cm-'} f o r our experimental conditions.

A s one can see from f i g . 4 d i f f e r e n t a n a l y t i c a l s i g n a l s were observed from doublet components 8p2~4/2 ( A =572.8 nm) and 8p2p3,, ( 1 2571.0 m). The r a t i o of this s i g n a l s i s equal t o 2.8 f o r t h i s doublet.

Analogous r e s u l t s were a l s o obtained f o r 9p and l o p doublets. It i s evident t h a t + e $ r a t e s of c o l l i s i o n a l mixing of t h e components a r e l e s s than ^C$ but what mechanisms bring about t h e r a t i o observed, i s not c l e a r . T h i s problem i s now under consideration.

Ytterbium. The e x c i t a t i o n scheme i s shown on f i g . 2. I n t h i s scheme except of c o l l i s i o n a l i o n i z a t i o n f ram 4f '3 (2F712 )6s2 6p

l e v e l (two- s t e p e x c i t a t i o n ) , t h e r a d i a t i v e t r a n s i t i o n i s a l s o possale from t h i s l e v e l t o autoionizing ( A I ) l e v e l with E=6.5 eV. The wavelength of t h i s t r a n s i t i o n coincides with t h e second s t e p wavelength (581.2 m) so only two dye-lasers a r e needed f o r such three-step excitation. I n general t h i s scheme i s more complex and demands t h e use of t h r e e l a s e r s , but it provides higher s e l e c t i v i t y and r a t e of ionization.

This scheme can be used when c o l l i s i o n a l i o n i z a t i o n i s e f f e c t i v e l e s s .

For Y b atom t h i s scheme provides a low DL-O.l ng/ml i n s p i t e of t h e

f a c t t h a t atomization of YbCl5 i n air-acetylene flame i s verg poor.

The e x c i t a t i o n conditions a r e near t h e optimum - t h e o p t i c a l satu-

r a t i o n having been reached i n a l l t h r e e steps. The estimated cross-

section of A I t r a n s i t i o n i s 6 gIcv10-~cm2.

JOURNAIL D€ PHYSIQUE

Sodium. The minimum d e t e c t e d concentration i s l i m i t e d by blank (Na -nation i n deionized water being-'l0 ng/ml). The DL obtained by e x t r a p o l a t i o n of a s i g n a l t o n o i s e l e v e l i s low enough, being

l e s s t h a n 1 pg/ml f o r two-step e x c i t a t i o n . The o p t i c a l s a t u r a t i o n i s reached e a s i l y f o r both steps. The l i n e a r p a r t of c a l l b r a t i o n curve extends t o 3yg/ml. For higher concentrations t h e n o n l i n e a r i t y appears due t o t h e space charge e f f e c t s . The space charge b r i n g s about t h e broadening of t h e e x c i t a t i o n s p e c t r a and i o n c u r r e n t p u l s e s /6/. The broadening of t h e l a s e r p u l s e s can be used f o r c o n t r o l of t h e e f f e c t .

Note, t h a t v a r i o u s two-step e x c i t a t i o n schemes (through t h e 3 ~ ~ ~ 1 1 ~ and 3p2~5/2 l e v e l s - see t a b l e 2) provides d i f f e r e n t magnitudes of

a n a l y t i c a l s i g n a l s . This f a c t shows, t h a t mixing time f o r t h e s e l e v e l s i n flame i s more t h a n T p =5 ns,

Caesium. The one-step e x c i t a t i o n was used and t h e o p t i c a l s a t u r a t i o n was reached. The e f f i c i e n c of c o l l i s i o n a l i o n i z a t i o n from 7p2P3/!

l e v e l i s very high ( -- 'l.0y and i n s p i t e of l a r g e n E value and low e f f i c i e n c y of atomization of CS i n air-acetylene flame ( 2%) t h e DL i s t h e same a s i n /5/ f o r propane-butane-air flame.

The matrix i n t e r f e r e n c e s . The low DL i n water s o l u t i o n s doesn't quarantee bhe low DL i n r e a l samples where matrix i n t e r f e r e n c e s occur. W e i n v e s t i g a t e d t h e determination of I n i n presence of Sn having high i o n i z a t i o n p o t e n t i a l and influence of Na on a n a l y t i c a l s i g n a l of CS. For In-Sn system t h e matrix i n t e r f e r e n c e s depend on t h e e x c i t a t i o n scheme. The s o l u t i o n s of SnCle and SnSO i n I M a c i d s were used.

A. One-step e x c i t a t i o n ( 3 =303.9 v). For SnC12 s o l u t i o n s t h e con%inuous background w a s observed I n t h e e n t i r e tuning range of t h e dye used (285-310 nm). The magnitude of this background depends t o a l a r g e extent on a p o t e n t i a l U of electrode - it r i s e s a s U6 and t h e n i s s a t u r a t e d . The second i n t e r f e r e n c e f a c t o r i s supression of s e l e c t i v e I n s i g n a l when matrix concentration a r i s e s ( f i g . 5).

For 10 @;/l of Sn t h e s i g n a l of I n drops t o 30%,and when Sn concen- t r a t i o n a r i s e s it i s completely recovered, A s a r e s u l t of t h e s e i n t e r f e r e n c e s t h e DL of I n i n SnCl2 s o l u t i o n (10 @;/l Sn) i s 1m

times higher t h a n it i s i n water s o l u t i o n . The n a t u r e of continuous background i s unknown. It may be connected with l a r g e q u a n t i t i e s of

& C l 2 molecules i n flame a s only about 4% of molecules a r e atomized.

The wide W absorption band of t h e s e molecules overlapps t h e tuning range of t h e dye l a s e r and multiphoton i o n i z a t i o n i s possible. Even i f t h e e f f i c i e n c y of t h i s process i s 10-7thi.s i s q u i t e enough t o provide t h e observed amplitude of t h e background. The SnC12 can a l s o serve a s t h e e f f e c t i v e quencher of e x c i t i n g I n atoms and b r i n g s about suppression of t h e a n a l y t i c a l signal. For SnS04 molecules t h e continuous background i s absent.

B. WO-step e x c i t a t i o n . The matrix i n t e r f e r e n c e s a r e much smaller i n t h i s case ( f i g . 3 ) . For SnS04 t h e suppression of t h e a n a l y t i c a l s i g n a l only about 10% w a s observed up t o Sn concentration 10 g/l.

This suppression r i s e s t o 15% f o r 45 g / l Sn. For SnCl:! s o l u t i o n s

t h e suppression i s higher - up t o 25%. No continuous background i s

observed. The c a l i b r a t i o n curves f o r pure I n s o l u t i o n s and f o r

( I n + 10 g / l Sn) s o l u t i o n s a r e given i n Fig. ^6.

Fig. 5 - The matrix i n t e r f e r e n c e s in In-Sn system.

M * .

Iridium

DL /IniSn)&D L (In)

Q3

PI", %t

Fig. 6 - The c a l i b r a t i o n curves.

The curves a r e p a r a l l e l and DL f o r water s o l u t i o n s and matrix a r e p r a c t i c a l l y equal. T h i s provides t h e determination of I n t r a c e i n Sn up 60 10-~%. The s y p l e s of Sn, which w e i n v e s t i g a t e d so far, had I n contamination-10- 1. The r e a l measurable concentration of I n a r e l i m i t e d i n our experiments by burner vmemoryw and a r e equal t o 10 pg/

m l - more t h a n order of magnitude higher t h a n DL.

Cs+Na ^S stem. Both atoms e f f e c t i v e l y i o n i z e i n flame, c r e a t i n g t h e

& n e a r t h e electrode. The range, where CS a n a l y t i c a l s i g n a l can be detected, diminishes, when Na concentration increases.

This e f f e c t depends on t h e e l e c t r o d e configuration and i t s p o s i t i o n i n flame ( s e e a l s o /3/. I n our experimental conditions t h e a n a l y t i - c a l s i g n a l from 20 ng/ml CS independent on Na was observed up t o I pg/ml Na. I n concentration range 1-200 pg/ml N a t h e 18% i n c r e a s e i n s i g n a l w a s observed. (Che f u r t h e r Na concen r a t i o n increase give8 dropped t o 1096).

r i s e t o suppression of CS s i g n a l ( f o r 2.10 X )~g/ml th e CS s i g n a l

Experiments with flameless atomizer. The first attempt of optogal-

vanic d e t e c t i o n with g r a p h i t e c r u c i b l e w a s reported i n / 7 / . We used

two modifications - t h e g r a p h i t e c r u c i b l e and g r a p h i t e furnace

C7-352 JOURNAL DE PHYSIQUE

HGA-72. The c o n f i g u r a t i o n of apparatus i s presented i n f i g . 7. For g r a p h i t e c r u c i b l e t h e s t a i n l e s s p l a t e w a s ^used as a e l e c t r o d e a t t h e

+U

- .

with sampt~

d i s t a n c e 4 mm above c r u c i b l e . The l a s e r beams propagated 4 mm above c r u c i b l e n e a r t h e e l e c t r o d e n o t touching i t , The s i g n a l s were obser- ved f o r p o s i t i v e and negative p o t e n t i a l up t o 1 kV. The experiments were c a r r i e d i n a i r ( t h e c r u c i b l e being changed a f t e r 2-3 experi- ments) and n i t r o g e n atmospheres.

For g r a p h i t e tube t h e t u n g s t e n rod (diameter 1 mm) along tube exis was used a s anode. The l a s e r beams propagated along t h e rod. Maximum p o t e n t i a l on t h e rod i n t h a t c a s e was l e s s t h a n 300 V t o avoide t h e breakdown between r o d and furnace. The volume of t h e sample t o be analysed was 20-30 rl. The e q e r i m e n t s were c a r r i e d out i n gas-stop mode, i n argon atmosphere.

The Na, CS, I n , Yb water s o l u t i o n s were i n v e s t i g a t e d with one-step and two-step e x c i t a t i o n s . It w a s found t h e a n a l y t i c a l s i g n a l t o be 10-100 times h i g h e r , compared t o flame atomization, but; t h e p r e c i s i o n was notj4very ood. Evaluated a b s o l u t e d e t e c t i o n ^{are 10-} - 10- g, and p r a c t i c a l l y t h e r e i s no memory e f f e c t . The a limits of Na and I n r e s u l t s obtained a r e promising but it i s necessary t o improve p r e c i - s i o n f o r r e a l samples a n a l y s i s .

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