Atomization and Excitation 37 1.4 ELECTROTHERMAL ATOMIZATION The year under review could be considered as one o f consolidation r a t h e r than one o f innovation. published and presented a t conferences. the development o f the techniques o f ETA up t o 1982, w h i l s t Dawson (C1440) has presented a review o f background correction techniques. reviews were concerned w i t h guiding the user through the maze o f interference data (see Section 1.4.3.1).The one area o f growth, however, has been i n the use o f magneto-optical systems both f o r background correction and f o r a n a l y t i c a l detection. Real progress i s now being made, demonstrating the p r a c t i c a l u t i 1 i ty of these o f t e n m i sunderstood techniques. This was r e f l e c t e d by the number o f review a r t i c l e s both Ottaway (C1411, 2036) has reviewed The more p r a c t i c a l 1.4.1 Atomizer design The advances o f the l a s t few years towards producing a more isothermal heating environment i n ETA has l e d t o modifications such as platforms and probes gaining "state-of-the-art" acceptance.workers t o investigate unusual modifications t o conventional pulse-heated atomizers, w i t h the sole object o f achieving isothermal atomization. simple, and y e t arguably the most v e r s a t i l e , modification t o conventional furn- aces u t i l i z e s a graphite probe t o i n s e r t pre-dried samples i n t o a heated furnace.Ottaway e t a l . (1544) have described such a system, whereby the r a p i d heating o f the sample during atomization (>4000 OC s-'), r e s u l t s i n minimal vapour phase interferences.Detection l i m i t s , i n both absorption and emission modes, were excel l e n t and the technique has found p r a c t i c a l a p p l i c a b i l i t y i n c l i n i c a l analysis (C304, C2075). Frech and Johnsson (C2192, C2424) have improved t h e i r convoluted design of a constant temperature furnace.samples were placed i n a graphite cup positioned beneath the sampling aperture o f the graphite tube. temperature followed by separate heating o f the cup. This has acted as an impetus f o r many groups o f The most I n t h e i r new configuration, Atomization occurred by heating the tube t o a preselected I n t h i s manner, atomic38 Analytical A tomic Spectroscopy vapour entered the preheated isothermal environment w i t h consequent improvements in performance. The atomizer has an opening i n i t s side and i s equipped w i t h a tubular attach- ment f o r controlled ashing p r i o r t o sample introduction i n t o the furnace tube.The idea of dual heating was described by Siemer (C2431) who reported a furnace construction whereby the processes of sample v o l a t i l i z a t i o n and atomization were independently controlled by using two temperature feedback regulated power supplies.whereby the preheating of the tube, and atomization, were both achieved by using two separate current c i r c u i t s . spots" i n the tube and hence reduce condensation of vapour. 36) described by Holcombe e t a l .(C117, C1432, 1571, C2193, C2293) has been further developed a s a viable a l t e r n a t i v e t o the L'vov platform furnace f o r the reduction of interference e f f e c t s . This modification, referred t o colloquially as "the plug", u t i l i z e d a cooled s i t e w i t h i n the furnace t o achieve condensation of species a f t e r i n i t i a l sample v o l a t i l i z a t i o n .Re-atomization then followed and p a r t i a l reduction i n background absorption and temporal separation of analyte and matrix was achieved. Lawson and Woodriff (1455) have described a double-walled furnace i n which a more uniform s p a t i a l temperature d i s t r i b u t i o n together w i t h an increased residence time, resulted i n lower interferences com- pared t o a conventional mini-Massmann device.However, the device was s t i l l i n f e r i o r t o both platform and probe atomization systems of l a r g e r furnaces. i n f e r i o r t o the more conventional graphite atomizers, i n c e r t a i n s i t u a t i o n s the former devices do o f f e r d i s t i n c t advantages. lower power requirement was a prime f a c t o r i n the design of a completely portable AA instrument described by Castledine and Robbens (C279).w i t h i n a graphite furnace was said by Daidoji and Tamura (466) t o o f f e r improved s e n s i t i v i t y f o r carbide forming elements. such as Co and Ni , were degraded w i t h use of such a boat, as would be expected. The use of a W filament f i r s t a s an electrode i n an e l e c t r o l y s i s c e l l and sub- sequently as the atomizer, was applied t o the determination of Pb i n seawater (1953).had potential f o r t r a c e level multi-element analysis. Falk and his colleagues have published f u r t h e r work on t h e i r configuration called furnace non-thermal excitation spectrometry (FANES) (425, C2189, C2394, C2405, C2475) (see also ARAAS, 1979, 2, 27). pressure discharge and the detection l i m i t s quoted were superior t o ETA-AAS and ICP-OES.A graphite tube atomizer has been described in a patent (378). Similarly, a German patent by Sperling (1966) described an atomizer This arrangement was said t o eliminate "cold The imaginative concept of second surface atomization [see ARAAS, 1981, lJ, Although in general the performance of metal atomizers has always been The f a c t t h a t a W ribbon has a The use of a Ta boat S e n s i t i v i t i e s f o r some elements, Atomizer designs intended s p e c i f i c a l l y f o r emission spectrometry have always The sample vapour formed by ETA was excited by a low A system consisting of a double plasma a r c w i t h i n a graphite furnaceAtomization and Excitation 39 was described by Arans e t a1 .(1079, 1088). h i g h detection power. presented. The device was claimed t o o f f e r Some theoretical d i s t r i b u t i o n s of p a r t i c l e densities were Other references o f i n t e r e s t - Reduction of carry over contamination : C2154. Separative column atomizer : 1369. 1.4.2 Fundamental studies The complexity of processes occurring within a graphite furnace i s probably only matched by the complexity of the techniques employed t o elucidate them.The use of mass spectrometry a s a means of monitoring molecular as well as atomic species has been developed by S t y r i s and Kaye (970). The apparatus described previously (see ARAAS, 1981, 71, 38) was used t o investigate the mechanisms o f vaporization of V205 from both vitreous carbon and tantalum.In f u r t h e r work, coupling t o a t r i p l e quadrupole mass spectrometer led t o a more positive i d e n t i f i c a t i o n of molecular intermediates produced when parent molecules were induced t o fragment o r react i n the second quadrupole region. and Mitchell (C2443) have questioned the v a l i d i t y of much mechanistic data and concluded t h a t the s i g n i f i c a n t shortcomings o f these studies lay i n the uncert- ainty of the postulated precursors in the atomization process.The combination of ETA w i t h MS o f f e r s a d i r e c t means of sampling such high temperature i n t e r - mediates. and matrix interference was also used by Holcombe and Rayson (C120, C2156, C2313, C2444).by ETA (C2157). compared t o platinum atomizers was noted by Guevremont e t a1 . (C2135, C2467). This obviously increases the already d i f f i c u l t task of interpretation. Hol combe and Salmon (869) have demonstrated how the appearance temperatures of several metals were affected by the presence o f oxygen i n the sheath gas (see ARAAS, 1981, 3, 38). The increases of appearance temperature w i t h increasing oxygen content were appreciable f o r Ag, Cd, In, Pb and Zn but not f o r Cu and Gay f o r which the appearance temperatures were above the desorption temperatures of oxygen from the graphite.in the graphite tube which can chemisorb oxygen, was r e i t e r a t e d by Vastola (C2153). t o be several orders of magnitude higher than could be explained by the thermo- dynamic equi 1 i brium: Sturgeon Mass spectroscopy as a means of studying mechanisms of atom formation They used the technique t o investigate the determination of Se The complexity of mass spectra obtained from graphite as The r o l e of oxygen in the atomization process i s now more widely appreciated.The concept of a c t i v e s i t e s , defined as t h a t area The p a r t i a l pressure of oxygen i n the graphite furnace i s now believed40 Analytical Atomic Spectroscopy This has l e d L ' v o v and Ryabchuk (1502) t o o f thermal d e s t r u c t i o n , a t o m i z a t i o n and d pounds.It has been suggested (1086) t h a t t h e new conclusions regarding t h e processes s s o c i a t i o n o f oxygen c o n t a i n i n g com- o n g i t u d i n a l temperature i n a g r a p h i t e tube can be described by a Gaussian d i s t r i b u t i o n .Several methods have been used t o measure temperature, and a l s o atom d i s t r i b u t i o n s . technique was developed by Human e t a l . (1495), w h i l s t an o p t i c a l pyrometric technique was p r e f e r r e d by Pelieva e t a l . (1528). h i g h l i ghted t h e d i ff i c u l t i e s i nvol ved i n " r e a l -time" temperature measurements and concluded t h a t t h e achievement o f accurate temperature measurement i n a g r a p h i t e furnace was an "exceptional challenge". l o n g i t u d i n a l atom d i s t r i b u t i o n s u s i n g coherent forward s c a t t e r i n g was described by Yasuda and Murayama (414).atomic vapour and r e l a t i v e atom d i s t r i b u t i o n s f o r Cd, Cu, Mn, N i and Pb were measured.Holcombe e t a l . (1358) have demonstrated t h a t a uniform f r e e atom d i s t r i b u t i o n w i t h i n t h e g r a p h i t e furnace does n o t e x i s t a t a l l times d u r i n g t h e atomization c y c l e and t h a t t h e degree o f non-uniformi ty appeared t o be element dependent. decreased markedly over t h e years.Russian i n o r i g i n . L ' v o v and h i s colleagues (407) have described a method f o r t h e determination o f t h e atomization energy o f DyC2 molecules, w h i l s t Katskov and G r i n s h t e i n (1085) p o s t u l a t e d t h e formation o f a c e t y l i d e s o f Ag, Ca and Cu. The same authors (1 146) have now used t h e i r p r e v i o u s l y described t h e o r e t i c a l model and experimental system (see ARAAS, 1981, 11, 39) t o study t h e vapor- i z a t i o n k i n e t i c s o f Co, C r , Fe, Mn and N i .The most s i g n i f i c a n t t h e o r e t i c a l work presented was contained i n a s h o r t paper by M u s i l and Rubeska (1555). These authors used F u l l e r ' s k i n e t i c model (see ARAAS, 1974, 4, Ref. 1131) t o e x p l a i n discrepancies i n peak shape modelling, t h e premise being t h a t i n t e r a c t i o n with, and re-evaporation from,atomizer w a l l s was a p o s s i b l e explanation o f t h e discrepancies. f u t u r e and more complete work. techniques, t h e i n f l u e n c e o f g r a p h i t e on t h e a t o m i z a t i o n r e a c t i o n was i n v e s t i g a t e d .An elegant study by Rowston and Richardson (699) noted how mixtures o f powdered g r a p h i t e and metal s a l t s underwent e x p l o s i v e decomposition a t t h e temperature where t h e f r e e metal was formed. Nagdaev and Pupyshev (1532) p o s t u l a t e d t h e formation o f Ba, Pb and Sr carbonates and noted t h a t Mg d i d n o t r e a c t w i t h carbon.The temperatures o f Ba and S r oxide formation from t h e carbonates were s a i d t o be i n f a i r agreement w i t h c a l c u l a t e d data. A p r e d i c t i v e computer Doidge and Routh (C2191) have An elegant method f o r measuring A transverse magnetic f i e l d was a p p l i e d i n t h e The number o f papers d e a l i n g w i t h t h e o r e t i c a l mechanistic models has The m a j o r i t y o f such work i s now mainly This i n t e r e s t i n g simple paper i s h o p e f u l l y a forerunner o f a I n two s i m i l a r studies, b o t h employing thermal a n a l y s i s and X-ray d i f f r a c t i o nAtomization and Excitation 41 1.4.3 Interferences 1.4.3.1 Reviews Readers who are l o s t amongst the mass o f l i t e r a t u r e on interferences may be heartened by the recent publication o f two reviews.Manning and Slavin (1910) have produced a comprehensive guide t o the l i t e r a t u r e on interferences, w h i l s t a more concise, b u t nonetheless useful, review was offered by Matousek (362). Both works review the many interference e f f e c t s i n ETA and l i s t techniques f o r t h e i r elimination and c o n t r o l . 1.4.3.2 Interference studies The use of the platform furnace w i t h matrix modification techniques i s now the accepted mode of c o n t r o l l i n g many interferences occurring i n ETA (C1433, C2155, C2294) and much l i t e r a t u r e continues t o emphasize t h i s f a c t . However, i n one o f the more i n t e r e s t i n g studies presented t h i s year, Slavin e t a l .(C64, 399, 1359) have shown t h a t when using p y r o l y t i c a l l y coated graphite platforms , the s e n s i t i v i t i e s o f A1 and T1 were degraded by HC104 i n the sample s o l u t i o n . of (NH4)2C03 as a m a t r i x modifier caused f u r t h e r problems due t o evolution o f C02. By improving the q u a l i t y of the p y r o l y t i c coating, the authors have now reported no interference from up t o 0.5M HC104 and no signal d e t e r i o r a t i o n a f t e r 150 f i r i n g s .atomic species w i t h i n the furnace, continue t o cause problems. M i l l e r - I h l i et - a l . (C102) have described a system, based on continuum source AA and a r e f r a c - t o r p l a t e , which allowed the operator t o record and s t o r e time-resolved spectral p r o f i l e s w i t h i n the modulation i n t e r v a l of t h e r e f r a c t o r p l a t e .P o s i t i v e i d e n t - i f i c a t i o n o f spectral interferences as w e l l as d e t a i l e d inspection o f structured background was thus a v a i l a b l e . e i t h e r HNo3 o r HC104 were atomized were reported by Martinsen and Langmyhr (538, 1525).w e l l as possibly S2, SO2 and NS were recorded. correction could n o t compensate f o r such interferences. Zeeman background correction i n dealing w i t h comnonly occurring spectral i n t e r - ferences was emphasized by Fernandez and Giddings (391). interference when determining As by ETA-AAS i n the presence o f A1 was reported (2006). A t the As 193.7 nm l i n e , A1 absorbed s i g n i f i c a n t l y , and the interference could n o t be removed by D2 background correction, nor by high ashing temperatures w i t h m a t r i x modifier.It was suggested t h a t A1 i o n l i n e s a t 193.58, 193.47 and 193.45 nm caused t h i s interference. The l e s s s e n s i t i v e As l i n e a t 197.2 nm d i d n o t s u f f e r from t h i s interference, though a chemical suppressive interference was encountered a t very high A1 concentrations.Use Matrix background interferences o r i g i n a t i n g from molecular o r s c a t t e r i n g Molecular species generated when H2S04 and Using a medium quartz spectrograph, emission spectra o f CS and COS as The effectiveness of Conventional deuterium background A severe spectral42 Analytical Atomic Spectroscopy Some i n s i g h t i n t o the interferences encountered during electrothermal atom- i z a t i o n o f t i n was provided by Welz e t a l .(C115). ation, o f HN03 and NH,OH as matrix modifiers, and interrupted i n t e r n a l gas flow, allowed optimum atomization o f Sn w i t h minimum ashing losses up t o 1000 'C. s e n s i t i v i t y due t o v o l a t i l i z a t i o n o f SnO species (1583), w h i l s t the a d d i t i o n o f H2 reduced such losses b u t could not, however, compensate f o r losses o f v o l a t i l e SnS formed i n sulphate matrices.The use o f platform atomir- --- The presence of O2 i n the sheath gas was also found t o decrease Sn 1.4.4 Developments i n Techniques 1-4.4.1 Magneto-opti cs Without any doubt, the l a r g e s t growth area i n applied ETA has been concerned w i t h the use o f magneto-optical e f f e c t s f o r both background correction and f o r improving detection.The performance o f an a.c. modulated Zeeman system was described by de Loos-Vollebregt and de Galan (1501). Termed "the three f i e l d system", the modification incorporates an a d d i t i o n a l i n t e n s i t y measurement a t intermediate f i e l d strength.ground absorption and s t r a y r a d i a t i o n were performed a t the expense o f halved a n a l y t i c a l s e n s i t i v i t y . I n two sequential a r t i c l e s , Ma and Feng (1913, 1920) discussed the behaviour o f s p l i t t i n g absorption l i n e s i n Zeeman AAS. o f hyperfine structure, they stated, could be minimized by optimization o f the magnetic f i e l d i n t e n s i t y .The advantages and disadvantages o f the three main configurations i n Zeeman AAS were reviewed by Broekaert (552). system i n terms o f both cost and performance was t h a t having an a.c. magnetic f i e l d around the atomizer. strength and hence resulted i n b e t t e r s e n s i t i v i t y and l i n e a r i t y . s p e c i f i c technique f o r m i n i t o r i n g simple and complex molecules was described by Hadeishi e t a1 .(1370). The technique depended on the Zeeman e f f e c t t o tune an AE l i n e t o coincide w i t h a sharp molecular absorption feature. Termed TALMS (Tunable Atomic Line Molecular Spectroscopy) (see ARAAS, 1980, 10, Ref. 795) the technique was said t o show p o t e n t i a l f o r environmental monitoring. review by Dawson (C1440). s i t u a t i o n s continuum source background correction was adequate. Situations outside the realms o f t h i s generalization were discussed by Welz e t a l . (C289, 1581) who demonstrated t h a t i n the determination o f P i n steels, spectral interferences from the i r o n matrix could only be e f f e c t i v e l y eliminated using a Zeeman system.The use o f continuum source correction techniques l e d t o an overcompensation. The u t i l i t y o f Zeeman correction i n b i o l o g i c a l applications has also been described (C58, C61, C2295) I n t h i s manner simultaneous correction f o r back- The e f f e c t The optimum This permitted regulation o f the magnetic f i e l d A h i g h l y The p r a c t i c a l u t i l i t y o f Zeeman background correction was highlighted i n a It was stressed, however, t h a t f o r most p r a c t i c a l (see also Section 2.2.1).The advantages o f the coherent forward-scattering technique were i 11 ustratedAtomization and Excitation by Debus e t a l . (132, C263). T h i s method was element capable of ETA and since a continuum used, simultaneous multi-element analysis may Hi rokawa and Nami k i have determined Mn ( 1 381 ) 43 applicable t o the detection of any i g h t source (%, xenon lamp) was be f e a s i b l e .Sn and Zn (415) in s t e e l s , and Cr Using a xenon source, in aluminium alloys (1381). an excitation source f o r coherent forward-scattering spectroscopy has a l s o been evaluated (480).The l i g h t s c a t t e r i n g i n t e n s i t y was found t o vary w i t h the gas pressure and e l e c t r i c a l power of the lamp. The application of a Grim-type glow discharge as 1.4.4.2 Sample introduction A number of sample introduction modifications have been described this year. The u t i l i t y of the graphite probe (see Section 1.4.1) f o r introduction of pre- dried samples into a heated graphite furnace has been demonstrated f o r both AAS and AES (1544) and f o r AAS alone (C2190). ferences was obtained, and ETA-AES detection l i m i t s f o r v o l a t i l e elements were improved compared w i t h conventional furnace operation. f o r introducing s o l i d samples i n t o a conventional graphite furnace (see ARAAS, 1981, 11, 41) has now been patented (1937). drying and ashing steps i n ETA was described by Dawson e t a l . (1983). Using a laboratory-constructed mini-Massmann type furnace, the authors dispensed liquid samples d i r e c t l y i n t o the furnace without a drying o r charring stage. cycle time was reduced t o c30 s and precisions of 58% were claimed. A reduction i n vapour phase i n t e r - A semi-automatic device A sampling system which eliminated The Other references of i n t e r e s t - A new AA background correction technique : C2138. ETA-AAS using s e l e c t i v e concentration on t o a W wire : 501 Increasing the l i n e a r dynamic range f o r ETA-AAS by u s i n g a photodiode array detector : C2195