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Recent developments in atomic absorption analysis
Spectrochimica Acts, 1964, Vol. 20, pp. 1185 to 1195. Pergamon Press Ltd. Printed in Northern Ireland
Recent developments in atomic absorption analysis* D. J . DAVID C.S.I.R.O., Division of Plant Industry, Canberra, A.C.T., Australia (Received 24 September 1963)
SINCE t h e T h i r d A u s t r a l i a n S p e c t r o s c o p y Conference in A u g u s t , 1961, a large a n d w i d e s p r e a d increase in i n t e r e s t in a t o m i c a b s o r p t i o n a n a l y s i s has occurred. S o m e 130 p a p e r s h a v e b e e n p u b l i s h e d bringing t h e t o t a l since its i n t r o d u c t i o n b y W a l s h in 1955 to close on 200. F o u r t e e n e x t e n s i v e r e v i e w s [1-14] a n d a n u m b e r o f m i n o r ones h a v e a p p e a r e d a n d a t o m i c a b s o r p t i o n h a s t a k e n a m a j o r place in a t l e a s t s e v e n s p e c t r o s c o p i c a n d a n a l y t i c a l conferences. T h e n u m b e r of r e v i e w s is p r o b a b l y s o m e w h a t larger t h a n is n e c e s s a r y to c o v e r d e v e l o p m e n t s o v e r such a s h o r t p e r i o d , b u t a p a r t i a l e x p l a n a t i o n for t h e a p p e a r a n c e o f so m a n y is t h a t t h e r a p i d r a t e of a d v a n c e h a s r e s u l t e d in s o m e of t h e m being o u t o f d a t e p r a c t i c a l l y as soon as t h e y were p u b l i s h e d . T h e b e s t of t h e s e r e v i e w s are t h o s e of WALSI~ [1] for t h e t h e o r e t i c a l a s p e c t s , WILLIS [12] for t h e biological a p p l i c a t i o n s a n d GILBERT [14] for a n e x h a u s t i v e a n d r e a s o n a b l y r e c e n t c o v e r a g e o f all w o r k in t h e field. One b o o k on a t o m i c a b s o r p t i o n a n a l y s i s [15] h a s a p p e a r e d a n d I u n d e r s t a n d t h a t t h e a u t h o r s of it are a t p r e s e n t p r e p a r i n g a m o r e c o m p r e h e n s i v e second edition. All of this m a k e s t h e s u b j e c t a difficult one to discuss a d e q u a t e l y in a f o r t y - m i n u t e l e c t u r e so I shall h a v e to confine myself, to s o m e e x t e n t , to generalities. * A review given at the Fourth Australian Spectroscopy Conference, August 1963. [1] A. WALSH, Advances in Spectroscopy (Edited by W. H. Thompson) Vol. 2, pp. 1-22. Interscience, New York (1961). [2] J. W. ROBINSON, Analytical Chemistry (Edited by C. E. Crouthamel) Pergamon Press, London (1961). [3] J. W. ROBINSON, Progress in Nuclear Energy (Edited by C. E. Crouthamel) Series 9, Vol. 2. Pergamon Press, New York (1961). [4] J. W. ROBINSO~¢,Ind. Chemist 38, 226-30 and 362-8 (1962). [5] L. R. P. BUTLER, South African Ind. Chemist 15, 162-70 (1961). [6] N. W. POLUEKTOV, Zavodskaya Lab. 27, 830-6 (1961). [7] J. E. ALLAN, Spectrochim. Acta 18, 605-14 (1962). [8] C. E. M. FEAI~SWA,Chem. Weekblad 58, 177-83 and 189-95 (1962). [9] G. MILAZZO,Chim. Ind. (Milan) 44, 493-500 (1962). [10] W. LEITHV.,Angew. Chem. 78, 488-92 (1961). [11] W. SLAVlN, Atomic Absorption Newsletters Nos. 1-8, Perkin-Elmer Corporation, Norwalk, Conn. (1962). [12] J. B. WILLIS, Methods of Biochemical Analysis (Edited by O. Glick) Vol. 11, Interscienee, New York (1963). [13] P. T. GILBERT,JR., Anal. Chem. 34, 210R-220R (1962). [14] P. T. GILBERT, JR., Atomic Absorption Spectroscopy: A Review of Recent Develc1~ments, Proceedings of Sixth Gatlinburg Conference on Analytical Chemistry in Nuclear Reactor Technology, Office of Technical Services, Washington 25, D.C. (1963). [15] W. T. ELWELL and J. A. F. GIDLEY, Atomic Absorption Spectroscopy, Pergamon Press, Oxford (1961). 1185
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I shall discuss developments concerned with each com ponent of the a p p a r a t u s separately, th en deal generally with gains achieved in sensitivity and the widening scope of application of atomic absorption analysis over the last two years. LIGHT SOURCES To m y knowledge there are now six manufacturers of hollow cathode discharge lamps, viz. Atomic Spectral L a m ps Ltd. (Australia), Westinghouse and Miktotek (U.S.A.), Hilger and W a t t s L t d (England), Hitachi (Japan) and Quarzlampen G.m.b.H. (Germany). The range of atomic emission spectra obtainable covers forty-five or more elements. The life and quality of spectra to be expect ed from such lamps has im pr ove d considerably recently b u t at least some of the reasons for the i m p r o v e m e n t are obscure, no information having been published concerning them. One of the factors that, apparently, has suppressed emission of the metal spectrum from some tubes in the past has been the accumulation of impurities, n o t a b l y hydrogen [16], in the filler gas, b u t this difficulty has been largely overcome by at least one manufacturer. F r o m the point of view of design of lamps, a n u m b e r of workers have produced multi-cathode or alloyed-cathode lamps but have experienced difficulty with all b u t a few combinations due to the large disparity in the rates at which different metals s p u t t e r from the cathode. MASSMANN [17] has overcome this difficulty by arranging cylinders of various metals in a particular order axially in the tube and operating th e lamp using a uni-directional stream of inert filler gas. Demountable single-cathode lamps have been described by ZEEMAN and BUTLER [18], PATERSON [19] and STRASHEIM and BUTLER [20], the l at t er having both front and rear windows to p er mit simultaneous determination of several elements. The use of continua as light sources has been found v e r y useful in conjunction with photographic detection in e x p l o r a t o r y work in which a n u m b e r of elements, for which hollow cathode tubes were not available, were studied. Thus ALLAN [21] used tu n g s ten and hydrogen continua in the det erm i nat i on of atomic absorption detection limits for a n u m b e r of elements in the ai r-acet yl ene flame and FASSEL and MOSSOTTI [22] used a xenon arc in their work on V, Ti, Nb, Sc, Y t and R h in the incandescent oxy-acetylene flame. Although COOKE et al. [23] claim limited success using a continuum as a light source and photo-electric detection, such a system has not y e t been applied in analytical practice. The a d v a n t a g e to be gained is, of course, the simultaneous determination of a n u m b e r of elements using a single light source, but, as has been pointed out by WALSH [1], [16] I. J. DANZIGER,Mt. Stromlo Observatory, Canberra. Personal communication (1961). [17] H. MASSM~NN,Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 56 (1962). [18] P. B. ZEEMANand L. R. P. BUTLER,Appl. Spectroscopy 16, 120-4 (1962). [19] J. E. PATEttSON,Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 56 (1962). [20] A. STRASI~EIMand L. R. P. BUTLER, Appl. Spectroscopy 16, 109-10 (1962). [21] J. E. ALLAN,Spectrochim. Acta 18, 259-63 (1962). [22] V. A. FASSELand V. G. MOSSOTTI,Anal. Chem. 35, 252-3 (1963). [23] J. H. GIBSON, W. E. GROSS~ and W. D. COOKE, Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 47 (1962).
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the isolation of such a narrow spectral width from a continuum requires a monoc h r o m a t o r of v e r y much higher resolving power t h a n is otherwise necessary and difficulties in amplification are experienced due to the low light intensities involved. I t is n o t impossible, of course, t h a t suitable continuous sources of very high intensity will be developed in the future. The most significant recent development in light sources is t h a t devised by SULLZVAN and WALSH [24] in which an auxiliary discharge is operated across the m o u t h of the cathode of a hollow cathode lamp. This has the effect of greatly increasing the population of atoms in the upper levels of transitions ending in the ground state resulting in 100-fold gain in resonance line intensity with little or no associated increase in line width. As t h e y will soon explain to you, t h e y see great possibilities for this source in an atomic absorption i n s t r u m e n t incorporating a resonance lamp in place of the conventional monochromator. STRASHEZM [25] has described a light source consisting of a pulsed spark which is time resolved in such a way as to discriminate against ion-line emission and in f a v o u r of neutral at om emission. Although it does not give the sensitivity at t ai ned b y the use of a hollow cathode discharge tube, he claims t h a t it shows promise. VAPORISATIO:N
OF
SAMPLE
SULLIVAN and WALSR [26] have continued their work on cathodic sputtering as a means of generating atomic v a p o u r of metallic samples, having devised a system b y which an absorption m eas ur em e nt can be carried out in two minutes and having produced good calibration curves for phosphorus and silver in copper and for silicon in aluminium and steel. GOLEB and BROD¥ [27] have e x t e n d e d this technique to the de t e r m i na t i on of sodium, beryllium, magnesium, calcium and silicon at microgram levels in samples introduced into the cathode of a sputtering chamber. A new and novel approach to vaporisation of solid samples in the form of strips, wires or filaments wound on to frames of silica rod is the use of flash heating b y means of a helical capacitor-discharge lamp. Absorption spectra are observed using S W R film exposed to spectrally dispersed light from a l y m a n - t y p e discharge of 20 ;usec duration passing through the vaporised sample after a time delay of 6 #see following the vaporising flash. NELSOn, KUEBLER and KAY [28-30] have detected Pb, Au, W, Ag, A1, Ca, Cu, Fe, Mg and ten or more molecular species, often in microgram quantities, by this technique and SLAVIS [11] reports the detection of boron in a similar manner. W o r k on flames as sample vaporisers has followed three main courses, viz. the e n h a n c e m e n t of sensitivity by increasing the length of light p a t h t hrough the [24] A. WXLSH,Australian Patents pending. [25] A. STRASHEIM,Nature 196, 1194 (1962). [26] A. WALSH, Proceedings of 10th Colloquium Spectroscopicum Internationale (Edited by E. R. Lippincott and M. Margoshes) Spartan, Washington, D.C. (1963). [27] A. GOT.EB and J. K. BROD¥, Abstracts of International Conference on Spectroscopy, Applied Spectroscopy 16, 47 (1962); Anal. Chim. Acta P.~ 457-66 (1963). [28] J. G. KAY, N. A. KCEBT.ERand L. S. NELSON,Nature 194, 671 (1962). [29] L. S. NELSON, Science 186, 296-303 (1962). [30] L. S. NELSONand N. A. KVEBLER,Spectrochim. Acta 19, 781-4 (1963).
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flame, the exploration of incandescent flames as a means of generating g r o u n d state atoms of elements not detected in stoichiometric flames and continuation of the search for, and suppression of, interferences caused by the formation of refractory compounds between constituents of various types of sample. Long-path adaptors were studied by ROBINSON (reported by GILBERT [31]), by FUWA and VALLEE [32] and by a number of workers at Oak Ridge National Laboratory, viz. FELDMAN, DHUMWAD and KOITZOHANN. Two types have been used, one consisting of a T-shaped tube with the flame from a total-consumption burner directed into the vertical arm of the " T " while the horizontal arms carry the beam from the lamp, the other consisting merely of a plain tube about 1 cm in diameter and 20-90 cm long with the flame directed into one end at an angle and the beam from the lamp passed through axially. The T-tubes have generally been made of metal, but the straight tubes are usually of silica, Vycor, magnesia or similar refractory material. Gains in sensitivity between 10- and 100-fold are attained compared with those given by the use of a 10 em burner. A detection limit of 0.0006 ppm zinc in solution has been obtained by FUWA using a tube 90 em in length, for instance. Carry-over from one sample to the next has been observed with T-tubes, but straight tubes do not, apparently, suffer a n y such disability. They do, however, devitrify and crack after continued use, but are easily and inexpensively replaced. The Jarrell-Ash atomic absorption a t t a c h m e n t [33] employs three aligned Beckman oxy-hydrogen burners and five passes of the beam, effected by concave mirrors arrayed vertically front and back, to give increased path length. A detection limit of 0.06 ppm Ni in solution is claimed with this arrangement. With the aim of having each pass intersect the flame at o p t i m u m height above the burner tips, FASSEL and MOSSOTTI [22] have modified the Jarrell-Ash a t t a c h m e n t by arraying the mirrors horizontally and aligning the burners at an angle to the optic axis of the monochromator. This arrangement reduces the number of passes possible to three, but probably improves the efficiency of the apparatus. Since the finding [34] t h a t an enhancement of 500-fold or more could be achieved for molybdenum by imposing reducing (incandescent) conditions in the air-acetylene flame, enhancement by these means of the free atom concentration for a number of elements has been found to occur. GATEHOUSE and WILLIS [35] found an enhancement for Sn and ALLAN [21] for Sn, Cr and Ru in the incandescent air-acetylene flame. Use of a hotter reducing flame (incandescent oxy-acetylene) has resulted, as I have suggested [34] in the generation of free atoms of a number of elements not previously detected by atomic absorption, viz., V, Ti, Nb, Sc, Yt, Re and the rare earths by FASSEL et al. [22, 36, 37] and Be, A1, V, and Ti by [31] P. T. GXLB~.RT,JR., Anal. Chem. 34, 1848 (1962). [32] K. FvwA and B. L. V~LEE, Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 48 (1962); Anal. Chem. 85, 942-9 (1963). [33] JARRELL--AsHNewsletter No. 11. Jarrell-Ash Co., Newtonville, Mass. (1961). [34] D. J. DAVID, Nature 187, 1109 (1960). [35] B. M. GA~EHOVSEand J. B. W~LLIS,Spectrochim. Acta 17, 710 (1961). [36] V. A. FASSEL,R. H. CURRY,B. B. MYERSand R. N. KNISELEY,Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 62 (1962). [37] V. A. FASSEL,1%.H. C~RY and R. N. KNISET.Ey,Spectrochim. Acta 18, 1127-53 (1962).
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SLAVIN and MANNING [11, 38]. These findings indicate that, for these elements, it is more important to consider the volatilisation of free molecules of the oxide from a particle of the sample in the flame t h a n the volatilisation of free metal atoms. For example, taking the two extreme cases of molybdenum and aluminium, we find t h a t molybdenum, which is very refractory in the metallic state b u t which has a volatile oxide, is easily detectable in the relatively cool incandescent air-acetylene flame whereas aluminium, which is relatively volatile as the metal but has a refractory oxide, requires the higher temperature of the incandescent oxy-acetylene flame for detection. I f detection were to depend only on the volatility of the metal in each case, the reverse would hold. For these elements, then, the generation of atoms from particles of the sample in the flame must be a two-stage process, viz. the volatilisation of the oxide followed by reduction of the oxide vapour to give free atoms of the metal. (Of interest here, also, is the probable importance of the oxide in the volatilisation of molybdenum from mineral samples in arcs [39].) This two-stage process of formation of atomic vapour in flames, however, is apparently not universal and a large group of metallic elements (alkalis, alkaline earths, manganese, zinc, copper etc.) must at least partially volalitise directly from sample particles as free atoms because t h e y are detectable in oxidising flames and show little or no enhancement on the imposition of reducing (incandescent) conditions. The work on chemical interferences, which are the worst encountered in atomic absorption analysis, has consisted mainly in extending, according to existing principles, both the number of elements and the number of sample types examined. ELW~.LLand GIDLEY [15] have examined the effects of aluminium and a number of other elements on the determination of magnesium in aluminium alloys and used strontium to suppress the interferences. WALLACE [40], following the work of DEBRAS--GuEDOlg and VOINOVITCH [41] in emission, has found t h a t 8-quinolinol is as good as or better t h a n strontium in suppressing these interferences and puts forward a theory concerning the mechanism of interference suppression. HERRMA~N and LA~G [42, 43] have noted an enhancement of Mg absorption by Ca and Na and have studied the influence of spray chamber dimensions on the interference of phosphate with Ca and Mg. They avoid the effects of the interference by simulating standard to sample solutions, as also have BELCHER and BRAY [44] for the determination of Mg in iron. A~DREW and NICHOLS [45] have found t h a t nickel itself suppresses the interferences of up to 0.2% A1 and 0.15% Si on the determination of Mg in nickel and nickel alloys. STRASHEIM and WESSELS [76] add 20000 ppm Cu (as sulphate) to both sample and standard solutions to overcome the interference of the other noble metals [38] [39] [40] [41] [42] [43] [44] [45]
W. SLAVINand D. C. MANNING,Anal. Chem. 35, 253-4 (1963). D. J. DAVIDand A. C. OERTEL,Australian J. Appl. Sci. 4, 235-43 (1953). F. J. WALLACE,Analyst 88, 259-65 (1963). J. D~BRAS-GuEDONand I. VOINOVITCH,Chem. Anal. Warsaw 5, 193 (1960). R. HERRMAI~rNand W. LA~G, z. Ges. exp. Med. 135, 569-82 (1962). R. H ] ~ R R ~ and W. LA•G, Optik 19, 208--18 (1962). C. B. BELCHERand H. M. B~AY, Anal. Chim. Acta 26, 322-25 (1962). T. R. A~DREWand P. R. N. NICHOLS,Analyst 87, 25-31 (1962).
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and sodium in the determination of rhodium. I have found t h a t aluminium chloride (presumably by conversion to aluminate in the flame) and, to a lesser e x t e n t, p h o s p h a t e suppress the interferences of the alkaline earths, iron and manganese with m o l y b d e n u m absorption [46] and WILLIAMS and IISMAA [47], working in m y laboratory, have found t h a t by the addition of calcium and silicate to t h e s t a n d a r d solutions and of calcium to the sample solutions, the interferences of silicate, aluminium, calcium and magnesium on the determination of chromium in faeces could be avoided. HINSON [48] removed phosphate by anion exchange to p r e v e n t its interference in the determination of calcium in plant material as also did I [49] in the determination of strontium, it having been found t h a t calcium and p h o s p h ate interfered together, but not separately. HERRMANN and LANG [42] found t h a t th e addition of E D T A p r e v e n t e d the interference of phosphate with calcium but, in doing so, intensified the effect of phosphate on magnesium. The addition m e t h o d has been used by WILLIS in the determination of Pb, Hg, Bi and Ni in urine [50], by me to offset slight residual interferences remaining after the removal of phosphate in the determination of Sr in biological materials [49], by STRASHEIM, BUTLER and MASKEW in the determination of a n u m b e r of metals in gold [51] and b y STRASHEI~ and WESSELS [76] in the determination of rhodium. SLAVIN et al. [11] and GILBERT [13, 14] give good general discussions of interferences and their suppression, the former pointing out, as have GATEHOUSE and WILLIS [35], t h a t t h e y are m uch less severe in air-acetylene flames t h a n in those employing other combustile gases with air. This discussion m a y give the impression t h a t severe interferences in atomic absorption analysis are rife, but, in fact, t h e y are the exception r a t h e r t h a n the rule. There are m a n y examples in the literature and, doubtless, m a n y unpublished examples of freedom from interference in the d e t e r m i n a t i o n of m a n y elements in a wide v a r i e t y of materials. I n addition to Allah's t hor ough examination of the advantages to be gained from the use of organic solvents with respect to bot h concentration by extraction and direct e n h a n c e m e n t of a n u m b e r of elements [52-54], several papers have appeared on th e subject. WILLIS [55] obtained a 100-fold gain in the determination of lead in urine b y a m e t h o d similar to Allah's involving the ext ract i on of the a m m o n i u m pyrrolidine dithiocarbamate complex and has since ext ended the m e t h o d to include the det er m i nat i on o f mercury, bismuth and nickel [56]. ROBINSON and HARRIS [57] have examined 14 different solvents in the determination of nickel, finding t hat , if the feed rate were held constant, little variation in [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57]
D. J. DAVID, Analyst 86, 730-40 (1961). C. H. WILLIAMS,D. J. DAVID and O. IISM~, J. Agric. Sci. 59, 381-5 (1962). W. H. HINSON,Spectrochim. Acta 18, 427 (1962). D. J. DAVID, Analyst 87, 576-85 (1962). J. B. WILLIS, Anal. Chem. 34, 614-7 (1962). A. STRASHEIM,L. R. P. BVTLERand E. C. MASKEW,J. S. African Inst. Mining and Met. {}2, 796-806 (1962). J. E. ALLAN,Spectrochim. Acta 17, 459-66 (1961). J. E. ALLAN,Spectrochim. Acta 17, 467-73 (1961). J. E. ALLAN,Analyst 86, 530-4 (1961). J. B. WILLIS, Nature 191, 381-2 (1961). J. B. WILLIS, Anal. Chem. 84, 614-7 (1962). J. W. ROBI~SO~ and R. J. HARRIS, Anal. Chim. Acta 26, 439-45 (1962}.
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sensitivity occurred from one solvent to another (except for water and C C14 which gave relatively very low sensitivities), but t h a t there was a variation of 10-fold or more if the solutions were allowed to aspirate freely. Using isopropanol, a m a x i m u m enhancement by a factor of 1.8 for calcium and copper, after correction for flow rate, was observed by MANNING, SPRAGUE and SLAVIN[83] and t h e y found t h a t the extent of enhancement depended upon the height of observation in the flame. GIBSOn, GROSSMAN and COOKE [58] ascribe some of the enhancement produced by organic solvents to increased temperature and efficiency of evaporation. FASSEL et al. [22, 37] and SLAVIN and MANNING [38] used ethanol to enhance, in incandescent oxy-acetylene flames, the concentration of atomic vapour of elements having oxides difficult to dissociate, while SCHULER, JANSEn and JAMES [59] found ethanol beneficial, provided the sample solution was sprayed effectively, in the determination of a number of elements in high purity gold. STRASHE~M et al. [51] and BARRAS [60] studied, respectively, enhancements arising from the use of butanol and heptane. Two combinations involving sparks have been used to produce atomic vapours from samples. ROBINSOn [61], operating a spark in the stream from a totalconsumption burner, found t h a t he could not detect aluminium in absorption with the flame burning, but could without the flame, while HERRMANN and LANG [62] obtained reasonable results by operating a spark between electrodes machined from metallic samples in the barrel of a burner, the vapours so produced being carried into the flame by its gas stream. By modifying spray-chamber design, SLAWN [81] has produced a pre-mix burner t h a t gives improved stability and sensitivity compared with t h a t previously used by him. RESONANCE LINE ISOLATION A:ND PHOTOMETRY Most of the routine and research work in Australia continues to be carried out using conventional monochromators such as the Zeiss PMQ I I I , the Beckman DU and the Hilger Uvispek with electronic equipment from Techtron Appliances Pry. Ltd. for modulation of the light source, and a.c. amplificaton of the signal from the detector. Such instruments have been described by WALSH [26], by DAVE¥ [63] and by me [46]. In the United States, commercially produced complete instruments are commonly encountered. These are all based on grating monochromators. The Perkin-Elmer models 214 and 303 (the latter replacing the former) are modulated, double-beam, dual-grating instruments. The 214 employs a static half-silvered mirror to split the beam from a modulated light source and matched photomultipliers for detection, while the 303 has a moving mirror t h a t sends pulses of light from an unmodulated source alternately along the reference [58] J. H. GIBSON,W. GROSSMANand W. D. COOKE,Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 62 (1962); Anal. Chem. 85, 266-77 (1963). [59] V. C. O. SCHUL]~R,A. V. JANSE~ and G. S. JAM]~S,J. S. African Inst. Mining and Met. 6~, 807-15 (1962). [60] R. C. B~RAS, Jarrel-Ash Newsletter No. 13, 1-4 (1962). [61] J. W. ROBINSON,Anal. Chim. Acta 27, 465-9 (1962). [62] R. HERRMANNand W. LA~G, Arch. Eisenhiittenw. 33, 654-8 (1962). [63] B. G. DAVEY,Spectrochim. Acta (in press) (1963).
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and sample paths, recombining them before entry into the monochromator. Measurement of absorption is effected b y ratioing alternate signals from a single photomultiplier. Jarrell-Ash make an atomic absorption attachment for use with their #82000 grating monochromator but, since it employs d.c. amplification, it can only be used in absorption for elements having resonance lines in regions where flame emission is not excessive. Other commercial instruments, of which I have no firm details, are those produced b y Research and Control Instruments Inc., and b y Hitachi in collaboration with Perkin-Elmer. In England and Europe, Hilger, Unicam and Optica make atomic absorption attachments for existing single-beam flame emission instruments. The Hilger and Optica instruments employ d.c. amplification which does not discriminate against flame emission. The Unicam s P g 0 0 flame emission spectrophotometer, having a.c. amplification and a chopper between the flame and monochromator, is eminently suited to conversion to modulated atomic absorption operation b y attaching a spectral lamp and shifting the location of the c h o p p i n g - - I understand that this has been done. Two coincidences of non-absorbing lines with resonance lines emitted from hollow cathode tubes have been noted. These involve the nickel line at 3415/~ (which is not, in any case, the most sensitive nickel resonance line) [61, 62] and the cobalt line at 2407 A [62]. These coincidences are not very serious in t h a t all t h e y do is to produce a general slight lowering of sensitivity below that which would otherwise be attainable. Other factors, such as dilution of the sample in the flame, lower the sensitivity to a much greater extent. Overlapping of resonance lines has been found a serious factor in isotopic analysis b y ZAIDEL and KORENNOI [65, 66] and b y MANNING grid SLAVI~ [11] and must be corrected for. A very significant development in isolation of resonance lines is the finding b y Walsh and colleagues that, b y replacing the monochromator with a resonance lamp in a manner outlined b y them some years ago [64] and using an atomic line source devised b y them having at least 100 times the intensity of conventional hollow cathode tubes, they have been able to make atomic absorption measurements. Such a system has exciting possibilities with respect to the simultaneous determination of a number of elements, improvement of effective resolving power and reduction in instrumental costs, a large proportion of which arise from the cost of the monochromator in current instruments. SENSITIVITY There is little object in producing a table detailing sensitivities attained to date for all elements because the results of research, mainly in the United States over the last nine months or so, indicates that nearly all of the sensitivities listed will soon be exceeded. Since the 1961 Spectroscopy Conference, a number of publications have appeared [35, 12, 21, 26] listing sensitivities, generally below 1 ppm, for some 30 elements in flames providing 10-12 cm absorbing path-length. B y double-beam operation, which improves the stability of reading, and b y use of scale expansion, these sensitivities have, to m y knowledge, recently been [64] BARBARAJ. RUSSELLand A. WALSH,Spectrochim. Acta 15, 883-5 (1959). [65] A. N. ZAIDEL, Uspekhi Fiz Naulc 68, 123-.4 (1959). [66] A. N. ZAIDELand E. P. KORENNOI,Optika i Spektroskopiya 10, 570-6 (1961).
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improved b y at least 5-fold for the same absorbing path-length. Further, b y increasing the path-lengths using the long tube systems b y ROBINSON [31] and FuWA [32], the gains of 10-100 fold found so far b y them for platinum, zinc, cadmium, copper, nickel and cobalt are likely to be extended to other elements. Consideration of the combined effects of these advances suggests t h a t we shall soon see published lists giving detection limits for most of these 30 elements at the parts per thousand million level or better. Besides these elements, there is a group of 20 or more (those having oxides not volatilised or not dissociated in the cooler stoichiometric flames) for which the gain in sensitivity over the past two years has, effectively, been infinite since t h e y were not then detectable, b u t now have been shown to be detectable at part-per-million levels in incandescent oxy-acetylene flames. Refinement in technique in dealing with these elements should produce further gains of 100-fold or more. KNISELEY, D'SILVA and FASSEL [80] have already achieved a 10-fold gain in sensitivity for these elements b y converting a Beckman total-consumption oxy-acetylene burner to the pre-mix t y p e using a graphite tube. The gain arises from the increased stability of the modified burner. SCOPE Most analytical fields were represented in Allan's review at the 1961 conference [7] and widening of scope of applications since has been concerned, mainly, with increased activity within these fields rather than entry into new fields. In geochemistry, atomic absorption analysis has been used in the determination of Ag in lead sulphide concentrates [67] of Fe, Mn, Ni, Zn and Cu in sea water [68] and of Sr in rock phosphate [49]. In the metallurgical field, Mg has been determined in iron [44], in nickel and its alloys [45] and in aluminium and its alloys [15, 40, 62]; P b in bronze, steel and zirconium alloys [15] and in gold [51, 59]; Zn in zirconium alloys [15], gold (51, 59), bronze, gunmetal and aluminium alloys [77]; Cd in zirconium alloys [15]; Mn in steel [15, 62] and in copper and titanium alloys [15]; Cu in gold [51, 59, 69], in steel and aluminium [78, 62] and in white metal [78]; Ag in gold [51, 59, 69] and in copper [1]; Fe and P d in gold [51, 59]; A1 in zinc [1]; Si in aluminium and steel [1]; P in copper [1]; Mo in stainless steel [46]. In analyses connected with the petroleum industry, Cu, Ni, Fe, Ca, Cr, Pb, Ag, Ba and Na have been determined in oil [60, 70] and Na, Fe, Ni and Cu in catalysts [71]. In biology and agriculture, Mg has been determined in food [42, 72], blood serum [11, 42], urine [42], faeces [42, 72] and tissue [73]; [67] B. S. RAWLING,M. D. AMOSand M. C. GREAVES,Bull. Inst. Mining Met. No. 659, 15-26 (1961}. [68] B. P. ~ABRICAND, R. R. SAWYER, S. G. U N G A R and S. ADLER, Geochim. et Cosmochim. Acta 26, 1023-1027 (1962). [69] N. P. FINKELSTE1Nand D. N. LOCK,J. S. African Inst. Mining and Met. 62, 820-37 (1962). [70] S. SPRAGVEand W. SLAVIN,Atomic Absorption Newsletter No. 12, Perkin-Elmer Corp., Norwalk, Conn. (1963). [71] H. KAHN and W. SLAVIN, International Science and Technology, pp. 60-5, November (1962). [72] J. B. DAWSONand P. W. HEATON, Biochem. J. SO, 99-100 {1961). [73] D. B. CHEEK,J. E. GRAYSTONE,J. B. WILLISand A. B. HOLT,Clin. Sci. 23, 169-79 (1962).
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Ca in plants [48], serum [11] and saliva [74]; Mn and Fe in plants [75]; Sr in soil extracts, biological material and fertilisers [49]; Pb, Cu and Zn in wines [18]; Pb, Hg, Bi, Ni, Zn and Cd in urine [56]; Cr in faeces [47]. PARKER [82] has determined Mg, Ca and Zn in animal feed-stuffs, tissue and plasma. The determination of lithium isotopes has been examined b y ZAIDEL and KORENNO: [65, 66] and, lately, b y MANNING and SLAVlN [11] of Perkin-Elmer. In Australia, some 60 laboratories in such diverse fields as agriculture, medicine, ferrous and non-ferrous metallurgy, and the petroleum and electroplating industries, use atomic absorption for inorganic analysis. Consideration of the ready availability of atomic absorption apparatus in Australia and of the advantages shown b y critical comparison of atomic absorption with other methods has resulted in it being p u t forward as the standard method for magnesium in iron b y the Standards Association of Australia [79]. CONCLUSIONS
The most striking feature of developments over the past two years is the general acceptance, throughout the world, of atomic absorption as a means of analysis. I consider it safe to say now that it is firmly entrenched and will, in the future, attain importance equal to or greater than other physical methods of inorganic analysis. Despite the fact that the flame has been severely criticised as a means of vaporising samples b y several workers in the past, it is through this medium that the recent impetus in atomic absorption analysis has taken place. The most important factors now recognised to be in favour of the flame as a vaporiser are its convenience (which has always been accepted) and the fact that, b y choice of correct conditions with respect to the nature and relative composition of flame gases, b y modification of the apparatus to give long path-lengths and by the application of established methods of circumventing interferences, practically any element can be vaporised as free atoms and, provided its strongly absorbing resonance line lies above 1900 A, can be determined at the part-per-million level or better. The inefficiency of conversion of a sample into atomic vapour b y flames, which was mentioned b y ALLAN [7] in his review at the 1961 conference, still remains and it is difficult to see how it can be overcome. W h a t is required is a total-consumption burner-atomiser that is hot enough to efficiently vaporise intractable oxides, reducing enough to generate atoms of any element and, at the E. NEWBRUN, Nature 192, 1182-3 (1961). D. J. DAVID, Rev. Univ. Ind. Santander 4, 207-14 (1962). A. STRASHE~Mand G. J. WESS]SLS, Appl. Spectroscopy 17, 65-70 (1963). F. J. WALLACE, Hilger Journal 7, 39-42 (1962). F. J. WALLACE, Hilger Journal 7, 65-68 (1963). C. B. BELCHER, Proc. Royal Australian Chem. Inst. 80, 111-2 (1963). R. N. KNIS]~L~.Y, A. P. D'SILVA and V. A. FASSEL, Anal. Chem. 85, 910-1 (1963). W. SLAV::':, Atomic Absorption Newsletter No. 10, P e r k i n - E l m e r Corp., Norwalk, Conn. (1963). [82] H. E. P~_RKER, Atomic Absorption Newsletter No. 13, Perk[n-Elmer Corp., Norwalk, Conn. (1963). [83] D. C. MANNING, S. SPRAGUE and W. SL~VIN, Atomic Absorption Newsletter, Perk[n-Elmer Corp., Norwalk, Conn. (1963).
[74] [75] [76] [77] [78] [79] [80] [81]
Recent developments in atomic absorption analysis
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same time, is able to convert the sample solution completely into a spray as fine as t h a t produced b y spray-chambers of pre-mix burners used at present. Compensation for the dilution effect arising from the high temperature of flames might be achieved b y cooling the gases before atomic absorption measurement provided t h a t it can be done without loss of sensitivity due to condensation of the atomic vapour of the element or its reversion to the oxide. Flash heating b y means of a condenser discharge [30] or, perhaps, a laser shows most promise for solid samples in that instantaneous and complete vaporisation of a sample might be achieved, thus overcoming effects due to selective volatilisation. At the present stage of development of flash heating, the time involved in preparation of most types of sample would preclude its use in routine analysis, however.
Recent developments in atomic absorption analysis* D. J . DAVID C.S.I.R.O., Division of Plant Industry, Canberra, A.C.T., Australia (Received 24 September 1963)
SINCE t h e T h i r d A u s t r a l i a n S p e c t r o s c o p y Conference in A u g u s t , 1961, a large a n d w i d e s p r e a d increase in i n t e r e s t in a t o m i c a b s o r p t i o n a n a l y s i s has occurred. S o m e 130 p a p e r s h a v e b e e n p u b l i s h e d bringing t h e t o t a l since its i n t r o d u c t i o n b y W a l s h in 1955 to close on 200. F o u r t e e n e x t e n s i v e r e v i e w s [1-14] a n d a n u m b e r o f m i n o r ones h a v e a p p e a r e d a n d a t o m i c a b s o r p t i o n h a s t a k e n a m a j o r place in a t l e a s t s e v e n s p e c t r o s c o p i c a n d a n a l y t i c a l conferences. T h e n u m b e r of r e v i e w s is p r o b a b l y s o m e w h a t larger t h a n is n e c e s s a r y to c o v e r d e v e l o p m e n t s o v e r such a s h o r t p e r i o d , b u t a p a r t i a l e x p l a n a t i o n for t h e a p p e a r a n c e o f so m a n y is t h a t t h e r a p i d r a t e of a d v a n c e h a s r e s u l t e d in s o m e of t h e m being o u t o f d a t e p r a c t i c a l l y as soon as t h e y were p u b l i s h e d . T h e b e s t of t h e s e r e v i e w s are t h o s e of WALSI~ [1] for t h e t h e o r e t i c a l a s p e c t s , WILLIS [12] for t h e biological a p p l i c a t i o n s a n d GILBERT [14] for a n e x h a u s t i v e a n d r e a s o n a b l y r e c e n t c o v e r a g e o f all w o r k in t h e field. One b o o k on a t o m i c a b s o r p t i o n a n a l y s i s [15] h a s a p p e a r e d a n d I u n d e r s t a n d t h a t t h e a u t h o r s of it are a t p r e s e n t p r e p a r i n g a m o r e c o m p r e h e n s i v e second edition. All of this m a k e s t h e s u b j e c t a difficult one to discuss a d e q u a t e l y in a f o r t y - m i n u t e l e c t u r e so I shall h a v e to confine myself, to s o m e e x t e n t , to generalities. * A review given at the Fourth Australian Spectroscopy Conference, August 1963. [1] A. WALSH, Advances in Spectroscopy (Edited by W. H. Thompson) Vol. 2, pp. 1-22. Interscience, New York (1961). [2] J. W. ROBINSON, Analytical Chemistry (Edited by C. E. Crouthamel) Pergamon Press, London (1961). [3] J. W. ROBINSON, Progress in Nuclear Energy (Edited by C. E. Crouthamel) Series 9, Vol. 2. Pergamon Press, New York (1961). [4] J. W. ROBINSO~¢,Ind. Chemist 38, 226-30 and 362-8 (1962). [5] L. R. P. BUTLER, South African Ind. Chemist 15, 162-70 (1961). [6] N. W. POLUEKTOV, Zavodskaya Lab. 27, 830-6 (1961). [7] J. E. ALLAN, Spectrochim. Acta 18, 605-14 (1962). [8] C. E. M. FEAI~SWA,Chem. Weekblad 58, 177-83 and 189-95 (1962). [9] G. MILAZZO,Chim. Ind. (Milan) 44, 493-500 (1962). [10] W. LEITHV.,Angew. Chem. 78, 488-92 (1961). [11] W. SLAVlN, Atomic Absorption Newsletters Nos. 1-8, Perkin-Elmer Corporation, Norwalk, Conn. (1962). [12] J. B. WILLIS, Methods of Biochemical Analysis (Edited by O. Glick) Vol. 11, Interscienee, New York (1963). [13] P. T. GILBERT,JR., Anal. Chem. 34, 210R-220R (1962). [14] P. T. GILBERT, JR., Atomic Absorption Spectroscopy: A Review of Recent Develc1~ments, Proceedings of Sixth Gatlinburg Conference on Analytical Chemistry in Nuclear Reactor Technology, Office of Technical Services, Washington 25, D.C. (1963). [15] W. T. ELWELL and J. A. F. GIDLEY, Atomic Absorption Spectroscopy, Pergamon Press, Oxford (1961). 1185
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D . J . DAVID
I shall discuss developments concerned with each com ponent of the a p p a r a t u s separately, th en deal generally with gains achieved in sensitivity and the widening scope of application of atomic absorption analysis over the last two years. LIGHT SOURCES To m y knowledge there are now six manufacturers of hollow cathode discharge lamps, viz. Atomic Spectral L a m ps Ltd. (Australia), Westinghouse and Miktotek (U.S.A.), Hilger and W a t t s L t d (England), Hitachi (Japan) and Quarzlampen G.m.b.H. (Germany). The range of atomic emission spectra obtainable covers forty-five or more elements. The life and quality of spectra to be expect ed from such lamps has im pr ove d considerably recently b u t at least some of the reasons for the i m p r o v e m e n t are obscure, no information having been published concerning them. One of the factors that, apparently, has suppressed emission of the metal spectrum from some tubes in the past has been the accumulation of impurities, n o t a b l y hydrogen [16], in the filler gas, b u t this difficulty has been largely overcome by at least one manufacturer. F r o m the point of view of design of lamps, a n u m b e r of workers have produced multi-cathode or alloyed-cathode lamps but have experienced difficulty with all b u t a few combinations due to the large disparity in the rates at which different metals s p u t t e r from the cathode. MASSMANN [17] has overcome this difficulty by arranging cylinders of various metals in a particular order axially in the tube and operating th e lamp using a uni-directional stream of inert filler gas. Demountable single-cathode lamps have been described by ZEEMAN and BUTLER [18], PATERSON [19] and STRASHEIM and BUTLER [20], the l at t er having both front and rear windows to p er mit simultaneous determination of several elements. The use of continua as light sources has been found v e r y useful in conjunction with photographic detection in e x p l o r a t o r y work in which a n u m b e r of elements, for which hollow cathode tubes were not available, were studied. Thus ALLAN [21] used tu n g s ten and hydrogen continua in the det erm i nat i on of atomic absorption detection limits for a n u m b e r of elements in the ai r-acet yl ene flame and FASSEL and MOSSOTTI [22] used a xenon arc in their work on V, Ti, Nb, Sc, Y t and R h in the incandescent oxy-acetylene flame. Although COOKE et al. [23] claim limited success using a continuum as a light source and photo-electric detection, such a system has not y e t been applied in analytical practice. The a d v a n t a g e to be gained is, of course, the simultaneous determination of a n u m b e r of elements using a single light source, but, as has been pointed out by WALSH [1], [16] I. J. DANZIGER,Mt. Stromlo Observatory, Canberra. Personal communication (1961). [17] H. MASSM~NN,Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 56 (1962). [18] P. B. ZEEMANand L. R. P. BUTLER,Appl. Spectroscopy 16, 120-4 (1962). [19] J. E. PATEttSON,Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 56 (1962). [20] A. STRASI~EIMand L. R. P. BUTLER, Appl. Spectroscopy 16, 109-10 (1962). [21] J. E. ALLAN,Spectrochim. Acta 18, 259-63 (1962). [22] V. A. FASSELand V. G. MOSSOTTI,Anal. Chem. 35, 252-3 (1963). [23] J. H. GIBSON, W. E. GROSS~ and W. D. COOKE, Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 47 (1962).
Recent developments in atomic absorption analysis
1187
the isolation of such a narrow spectral width from a continuum requires a monoc h r o m a t o r of v e r y much higher resolving power t h a n is otherwise necessary and difficulties in amplification are experienced due to the low light intensities involved. I t is n o t impossible, of course, t h a t suitable continuous sources of very high intensity will be developed in the future. The most significant recent development in light sources is t h a t devised by SULLZVAN and WALSH [24] in which an auxiliary discharge is operated across the m o u t h of the cathode of a hollow cathode lamp. This has the effect of greatly increasing the population of atoms in the upper levels of transitions ending in the ground state resulting in 100-fold gain in resonance line intensity with little or no associated increase in line width. As t h e y will soon explain to you, t h e y see great possibilities for this source in an atomic absorption i n s t r u m e n t incorporating a resonance lamp in place of the conventional monochromator. STRASHEZM [25] has described a light source consisting of a pulsed spark which is time resolved in such a way as to discriminate against ion-line emission and in f a v o u r of neutral at om emission. Although it does not give the sensitivity at t ai ned b y the use of a hollow cathode discharge tube, he claims t h a t it shows promise. VAPORISATIO:N
OF
SAMPLE
SULLIVAN and WALSR [26] have continued their work on cathodic sputtering as a means of generating atomic v a p o u r of metallic samples, having devised a system b y which an absorption m eas ur em e nt can be carried out in two minutes and having produced good calibration curves for phosphorus and silver in copper and for silicon in aluminium and steel. GOLEB and BROD¥ [27] have e x t e n d e d this technique to the de t e r m i na t i on of sodium, beryllium, magnesium, calcium and silicon at microgram levels in samples introduced into the cathode of a sputtering chamber. A new and novel approach to vaporisation of solid samples in the form of strips, wires or filaments wound on to frames of silica rod is the use of flash heating b y means of a helical capacitor-discharge lamp. Absorption spectra are observed using S W R film exposed to spectrally dispersed light from a l y m a n - t y p e discharge of 20 ;usec duration passing through the vaporised sample after a time delay of 6 #see following the vaporising flash. NELSOn, KUEBLER and KAY [28-30] have detected Pb, Au, W, Ag, A1, Ca, Cu, Fe, Mg and ten or more molecular species, often in microgram quantities, by this technique and SLAVIS [11] reports the detection of boron in a similar manner. W o r k on flames as sample vaporisers has followed three main courses, viz. the e n h a n c e m e n t of sensitivity by increasing the length of light p a t h t hrough the [24] A. WXLSH,Australian Patents pending. [25] A. STRASHEIM,Nature 196, 1194 (1962). [26] A. WALSH, Proceedings of 10th Colloquium Spectroscopicum Internationale (Edited by E. R. Lippincott and M. Margoshes) Spartan, Washington, D.C. (1963). [27] A. GOT.EB and J. K. BROD¥, Abstracts of International Conference on Spectroscopy, Applied Spectroscopy 16, 47 (1962); Anal. Chim. Acta P.~ 457-66 (1963). [28] J. G. KAY, N. A. KCEBT.ERand L. S. NELSON,Nature 194, 671 (1962). [29] L. S. NELSON, Science 186, 296-303 (1962). [30] L. S. NELSONand N. A. KVEBLER,Spectrochim. Acta 19, 781-4 (1963).
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D . J . DAVID
flame, the exploration of incandescent flames as a means of generating g r o u n d state atoms of elements not detected in stoichiometric flames and continuation of the search for, and suppression of, interferences caused by the formation of refractory compounds between constituents of various types of sample. Long-path adaptors were studied by ROBINSON (reported by GILBERT [31]), by FUWA and VALLEE [32] and by a number of workers at Oak Ridge National Laboratory, viz. FELDMAN, DHUMWAD and KOITZOHANN. Two types have been used, one consisting of a T-shaped tube with the flame from a total-consumption burner directed into the vertical arm of the " T " while the horizontal arms carry the beam from the lamp, the other consisting merely of a plain tube about 1 cm in diameter and 20-90 cm long with the flame directed into one end at an angle and the beam from the lamp passed through axially. The T-tubes have generally been made of metal, but the straight tubes are usually of silica, Vycor, magnesia or similar refractory material. Gains in sensitivity between 10- and 100-fold are attained compared with those given by the use of a 10 em burner. A detection limit of 0.0006 ppm zinc in solution has been obtained by FUWA using a tube 90 em in length, for instance. Carry-over from one sample to the next has been observed with T-tubes, but straight tubes do not, apparently, suffer a n y such disability. They do, however, devitrify and crack after continued use, but are easily and inexpensively replaced. The Jarrell-Ash atomic absorption a t t a c h m e n t [33] employs three aligned Beckman oxy-hydrogen burners and five passes of the beam, effected by concave mirrors arrayed vertically front and back, to give increased path length. A detection limit of 0.06 ppm Ni in solution is claimed with this arrangement. With the aim of having each pass intersect the flame at o p t i m u m height above the burner tips, FASSEL and MOSSOTTI [22] have modified the Jarrell-Ash a t t a c h m e n t by arraying the mirrors horizontally and aligning the burners at an angle to the optic axis of the monochromator. This arrangement reduces the number of passes possible to three, but probably improves the efficiency of the apparatus. Since the finding [34] t h a t an enhancement of 500-fold or more could be achieved for molybdenum by imposing reducing (incandescent) conditions in the air-acetylene flame, enhancement by these means of the free atom concentration for a number of elements has been found to occur. GATEHOUSE and WILLIS [35] found an enhancement for Sn and ALLAN [21] for Sn, Cr and Ru in the incandescent air-acetylene flame. Use of a hotter reducing flame (incandescent oxy-acetylene) has resulted, as I have suggested [34] in the generation of free atoms of a number of elements not previously detected by atomic absorption, viz., V, Ti, Nb, Sc, Yt, Re and the rare earths by FASSEL et al. [22, 36, 37] and Be, A1, V, and Ti by [31] P. T. GXLB~.RT,JR., Anal. Chem. 34, 1848 (1962). [32] K. FvwA and B. L. V~LEE, Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 48 (1962); Anal. Chem. 85, 942-9 (1963). [33] JARRELL--AsHNewsletter No. 11. Jarrell-Ash Co., Newtonville, Mass. (1961). [34] D. J. DAVID, Nature 187, 1109 (1960). [35] B. M. GA~EHOVSEand J. B. W~LLIS,Spectrochim. Acta 17, 710 (1961). [36] V. A. FASSEL,R. H. CURRY,B. B. MYERSand R. N. KNISELEY,Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 62 (1962). [37] V. A. FASSEL,1%.H. C~RY and R. N. KNISET.Ey,Spectrochim. Acta 18, 1127-53 (1962).
Recent developments in atomic absorption analysis
1189
SLAVIN and MANNING [11, 38]. These findings indicate that, for these elements, it is more important to consider the volatilisation of free molecules of the oxide from a particle of the sample in the flame t h a n the volatilisation of free metal atoms. For example, taking the two extreme cases of molybdenum and aluminium, we find t h a t molybdenum, which is very refractory in the metallic state b u t which has a volatile oxide, is easily detectable in the relatively cool incandescent air-acetylene flame whereas aluminium, which is relatively volatile as the metal but has a refractory oxide, requires the higher temperature of the incandescent oxy-acetylene flame for detection. I f detection were to depend only on the volatility of the metal in each case, the reverse would hold. For these elements, then, the generation of atoms from particles of the sample in the flame must be a two-stage process, viz. the volatilisation of the oxide followed by reduction of the oxide vapour to give free atoms of the metal. (Of interest here, also, is the probable importance of the oxide in the volatilisation of molybdenum from mineral samples in arcs [39].) This two-stage process of formation of atomic vapour in flames, however, is apparently not universal and a large group of metallic elements (alkalis, alkaline earths, manganese, zinc, copper etc.) must at least partially volalitise directly from sample particles as free atoms because t h e y are detectable in oxidising flames and show little or no enhancement on the imposition of reducing (incandescent) conditions. The work on chemical interferences, which are the worst encountered in atomic absorption analysis, has consisted mainly in extending, according to existing principles, both the number of elements and the number of sample types examined. ELW~.LLand GIDLEY [15] have examined the effects of aluminium and a number of other elements on the determination of magnesium in aluminium alloys and used strontium to suppress the interferences. WALLACE [40], following the work of DEBRAS--GuEDOlg and VOINOVITCH [41] in emission, has found t h a t 8-quinolinol is as good as or better t h a n strontium in suppressing these interferences and puts forward a theory concerning the mechanism of interference suppression. HERRMA~N and LA~G [42, 43] have noted an enhancement of Mg absorption by Ca and Na and have studied the influence of spray chamber dimensions on the interference of phosphate with Ca and Mg. They avoid the effects of the interference by simulating standard to sample solutions, as also have BELCHER and BRAY [44] for the determination of Mg in iron. A~DREW and NICHOLS [45] have found t h a t nickel itself suppresses the interferences of up to 0.2% A1 and 0.15% Si on the determination of Mg in nickel and nickel alloys. STRASHEIM and WESSELS [76] add 20000 ppm Cu (as sulphate) to both sample and standard solutions to overcome the interference of the other noble metals [38] [39] [40] [41] [42] [43] [44] [45]
W. SLAVINand D. C. MANNING,Anal. Chem. 35, 253-4 (1963). D. J. DAVIDand A. C. OERTEL,Australian J. Appl. Sci. 4, 235-43 (1953). F. J. WALLACE,Analyst 88, 259-65 (1963). J. D~BRAS-GuEDONand I. VOINOVITCH,Chem. Anal. Warsaw 5, 193 (1960). R. HERRMAI~rNand W. LA~G, z. Ges. exp. Med. 135, 569-82 (1962). R. H ] ~ R R ~ and W. LA•G, Optik 19, 208--18 (1962). C. B. BELCHERand H. M. B~AY, Anal. Chim. Acta 26, 322-25 (1962). T. R. A~DREWand P. R. N. NICHOLS,Analyst 87, 25-31 (1962).
1190
D . J . DAVID
and sodium in the determination of rhodium. I have found t h a t aluminium chloride (presumably by conversion to aluminate in the flame) and, to a lesser e x t e n t, p h o s p h a t e suppress the interferences of the alkaline earths, iron and manganese with m o l y b d e n u m absorption [46] and WILLIAMS and IISMAA [47], working in m y laboratory, have found t h a t by the addition of calcium and silicate to t h e s t a n d a r d solutions and of calcium to the sample solutions, the interferences of silicate, aluminium, calcium and magnesium on the determination of chromium in faeces could be avoided. HINSON [48] removed phosphate by anion exchange to p r e v e n t its interference in the determination of calcium in plant material as also did I [49] in the determination of strontium, it having been found t h a t calcium and p h o s p h ate interfered together, but not separately. HERRMANN and LANG [42] found t h a t th e addition of E D T A p r e v e n t e d the interference of phosphate with calcium but, in doing so, intensified the effect of phosphate on magnesium. The addition m e t h o d has been used by WILLIS in the determination of Pb, Hg, Bi and Ni in urine [50], by me to offset slight residual interferences remaining after the removal of phosphate in the determination of Sr in biological materials [49], by STRASHEIM, BUTLER and MASKEW in the determination of a n u m b e r of metals in gold [51] and b y STRASHEI~ and WESSELS [76] in the determination of rhodium. SLAVIN et al. [11] and GILBERT [13, 14] give good general discussions of interferences and their suppression, the former pointing out, as have GATEHOUSE and WILLIS [35], t h a t t h e y are m uch less severe in air-acetylene flames t h a n in those employing other combustile gases with air. This discussion m a y give the impression t h a t severe interferences in atomic absorption analysis are rife, but, in fact, t h e y are the exception r a t h e r t h a n the rule. There are m a n y examples in the literature and, doubtless, m a n y unpublished examples of freedom from interference in the d e t e r m i n a t i o n of m a n y elements in a wide v a r i e t y of materials. I n addition to Allah's t hor ough examination of the advantages to be gained from the use of organic solvents with respect to bot h concentration by extraction and direct e n h a n c e m e n t of a n u m b e r of elements [52-54], several papers have appeared on th e subject. WILLIS [55] obtained a 100-fold gain in the determination of lead in urine b y a m e t h o d similar to Allah's involving the ext ract i on of the a m m o n i u m pyrrolidine dithiocarbamate complex and has since ext ended the m e t h o d to include the det er m i nat i on o f mercury, bismuth and nickel [56]. ROBINSON and HARRIS [57] have examined 14 different solvents in the determination of nickel, finding t hat , if the feed rate were held constant, little variation in [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57]
D. J. DAVID, Analyst 86, 730-40 (1961). C. H. WILLIAMS,D. J. DAVID and O. IISM~, J. Agric. Sci. 59, 381-5 (1962). W. H. HINSON,Spectrochim. Acta 18, 427 (1962). D. J. DAVID, Analyst 87, 576-85 (1962). J. B. WILLIS, Anal. Chem. 34, 614-7 (1962). A. STRASHEIM,L. R. P. BVTLERand E. C. MASKEW,J. S. African Inst. Mining and Met. {}2, 796-806 (1962). J. E. ALLAN,Spectrochim. Acta 17, 459-66 (1961). J. E. ALLAN,Spectrochim. Acta 17, 467-73 (1961). J. E. ALLAN,Analyst 86, 530-4 (1961). J. B. WILLIS, Nature 191, 381-2 (1961). J. B. WILLIS, Anal. Chem. 84, 614-7 (1962). J. W. ROBI~SO~ and R. J. HARRIS, Anal. Chim. Acta 26, 439-45 (1962}.
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sensitivity occurred from one solvent to another (except for water and C C14 which gave relatively very low sensitivities), but t h a t there was a variation of 10-fold or more if the solutions were allowed to aspirate freely. Using isopropanol, a m a x i m u m enhancement by a factor of 1.8 for calcium and copper, after correction for flow rate, was observed by MANNING, SPRAGUE and SLAVIN[83] and t h e y found t h a t the extent of enhancement depended upon the height of observation in the flame. GIBSOn, GROSSMAN and COOKE [58] ascribe some of the enhancement produced by organic solvents to increased temperature and efficiency of evaporation. FASSEL et al. [22, 37] and SLAVIN and MANNING [38] used ethanol to enhance, in incandescent oxy-acetylene flames, the concentration of atomic vapour of elements having oxides difficult to dissociate, while SCHULER, JANSEn and JAMES [59] found ethanol beneficial, provided the sample solution was sprayed effectively, in the determination of a number of elements in high purity gold. STRASHE~M et al. [51] and BARRAS [60] studied, respectively, enhancements arising from the use of butanol and heptane. Two combinations involving sparks have been used to produce atomic vapours from samples. ROBINSOn [61], operating a spark in the stream from a totalconsumption burner, found t h a t he could not detect aluminium in absorption with the flame burning, but could without the flame, while HERRMANN and LANG [62] obtained reasonable results by operating a spark between electrodes machined from metallic samples in the barrel of a burner, the vapours so produced being carried into the flame by its gas stream. By modifying spray-chamber design, SLAWN [81] has produced a pre-mix burner t h a t gives improved stability and sensitivity compared with t h a t previously used by him. RESONANCE LINE ISOLATION A:ND PHOTOMETRY Most of the routine and research work in Australia continues to be carried out using conventional monochromators such as the Zeiss PMQ I I I , the Beckman DU and the Hilger Uvispek with electronic equipment from Techtron Appliances Pry. Ltd. for modulation of the light source, and a.c. amplificaton of the signal from the detector. Such instruments have been described by WALSH [26], by DAVE¥ [63] and by me [46]. In the United States, commercially produced complete instruments are commonly encountered. These are all based on grating monochromators. The Perkin-Elmer models 214 and 303 (the latter replacing the former) are modulated, double-beam, dual-grating instruments. The 214 employs a static half-silvered mirror to split the beam from a modulated light source and matched photomultipliers for detection, while the 303 has a moving mirror t h a t sends pulses of light from an unmodulated source alternately along the reference [58] J. H. GIBSON,W. GROSSMANand W. D. COOKE,Abstracts of International Conference on Spectroscopy, Appl. Spectroscopy 16, 62 (1962); Anal. Chem. 85, 266-77 (1963). [59] V. C. O. SCHUL]~R,A. V. JANSE~ and G. S. JAM]~S,J. S. African Inst. Mining and Met. 6~, 807-15 (1962). [60] R. C. B~RAS, Jarrel-Ash Newsletter No. 13, 1-4 (1962). [61] J. W. ROBINSON,Anal. Chim. Acta 27, 465-9 (1962). [62] R. HERRMANNand W. LA~G, Arch. Eisenhiittenw. 33, 654-8 (1962). [63] B. G. DAVEY,Spectrochim. Acta (in press) (1963).
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and sample paths, recombining them before entry into the monochromator. Measurement of absorption is effected b y ratioing alternate signals from a single photomultiplier. Jarrell-Ash make an atomic absorption attachment for use with their #82000 grating monochromator but, since it employs d.c. amplification, it can only be used in absorption for elements having resonance lines in regions where flame emission is not excessive. Other commercial instruments, of which I have no firm details, are those produced b y Research and Control Instruments Inc., and b y Hitachi in collaboration with Perkin-Elmer. In England and Europe, Hilger, Unicam and Optica make atomic absorption attachments for existing single-beam flame emission instruments. The Hilger and Optica instruments employ d.c. amplification which does not discriminate against flame emission. The Unicam s P g 0 0 flame emission spectrophotometer, having a.c. amplification and a chopper between the flame and monochromator, is eminently suited to conversion to modulated atomic absorption operation b y attaching a spectral lamp and shifting the location of the c h o p p i n g - - I understand that this has been done. Two coincidences of non-absorbing lines with resonance lines emitted from hollow cathode tubes have been noted. These involve the nickel line at 3415/~ (which is not, in any case, the most sensitive nickel resonance line) [61, 62] and the cobalt line at 2407 A [62]. These coincidences are not very serious in t h a t all t h e y do is to produce a general slight lowering of sensitivity below that which would otherwise be attainable. Other factors, such as dilution of the sample in the flame, lower the sensitivity to a much greater extent. Overlapping of resonance lines has been found a serious factor in isotopic analysis b y ZAIDEL and KORENNOI [65, 66] and b y MANNING grid SLAVI~ [11] and must be corrected for. A very significant development in isolation of resonance lines is the finding b y Walsh and colleagues that, b y replacing the monochromator with a resonance lamp in a manner outlined b y them some years ago [64] and using an atomic line source devised b y them having at least 100 times the intensity of conventional hollow cathode tubes, they have been able to make atomic absorption measurements. Such a system has exciting possibilities with respect to the simultaneous determination of a number of elements, improvement of effective resolving power and reduction in instrumental costs, a large proportion of which arise from the cost of the monochromator in current instruments. SENSITIVITY There is little object in producing a table detailing sensitivities attained to date for all elements because the results of research, mainly in the United States over the last nine months or so, indicates that nearly all of the sensitivities listed will soon be exceeded. Since the 1961 Spectroscopy Conference, a number of publications have appeared [35, 12, 21, 26] listing sensitivities, generally below 1 ppm, for some 30 elements in flames providing 10-12 cm absorbing path-length. B y double-beam operation, which improves the stability of reading, and b y use of scale expansion, these sensitivities have, to m y knowledge, recently been [64] BARBARAJ. RUSSELLand A. WALSH,Spectrochim. Acta 15, 883-5 (1959). [65] A. N. ZAIDEL, Uspekhi Fiz Naulc 68, 123-.4 (1959). [66] A. N. ZAIDELand E. P. KORENNOI,Optika i Spektroskopiya 10, 570-6 (1961).
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improved b y at least 5-fold for the same absorbing path-length. Further, b y increasing the path-lengths using the long tube systems b y ROBINSON [31] and FuWA [32], the gains of 10-100 fold found so far b y them for platinum, zinc, cadmium, copper, nickel and cobalt are likely to be extended to other elements. Consideration of the combined effects of these advances suggests t h a t we shall soon see published lists giving detection limits for most of these 30 elements at the parts per thousand million level or better. Besides these elements, there is a group of 20 or more (those having oxides not volatilised or not dissociated in the cooler stoichiometric flames) for which the gain in sensitivity over the past two years has, effectively, been infinite since t h e y were not then detectable, b u t now have been shown to be detectable at part-per-million levels in incandescent oxy-acetylene flames. Refinement in technique in dealing with these elements should produce further gains of 100-fold or more. KNISELEY, D'SILVA and FASSEL [80] have already achieved a 10-fold gain in sensitivity for these elements b y converting a Beckman total-consumption oxy-acetylene burner to the pre-mix t y p e using a graphite tube. The gain arises from the increased stability of the modified burner. SCOPE Most analytical fields were represented in Allan's review at the 1961 conference [7] and widening of scope of applications since has been concerned, mainly, with increased activity within these fields rather than entry into new fields. In geochemistry, atomic absorption analysis has been used in the determination of Ag in lead sulphide concentrates [67] of Fe, Mn, Ni, Zn and Cu in sea water [68] and of Sr in rock phosphate [49]. In the metallurgical field, Mg has been determined in iron [44], in nickel and its alloys [45] and in aluminium and its alloys [15, 40, 62]; P b in bronze, steel and zirconium alloys [15] and in gold [51, 59]; Zn in zirconium alloys [15], gold (51, 59), bronze, gunmetal and aluminium alloys [77]; Cd in zirconium alloys [15]; Mn in steel [15, 62] and in copper and titanium alloys [15]; Cu in gold [51, 59, 69], in steel and aluminium [78, 62] and in white metal [78]; Ag in gold [51, 59, 69] and in copper [1]; Fe and P d in gold [51, 59]; A1 in zinc [1]; Si in aluminium and steel [1]; P in copper [1]; Mo in stainless steel [46]. In analyses connected with the petroleum industry, Cu, Ni, Fe, Ca, Cr, Pb, Ag, Ba and Na have been determined in oil [60, 70] and Na, Fe, Ni and Cu in catalysts [71]. In biology and agriculture, Mg has been determined in food [42, 72], blood serum [11, 42], urine [42], faeces [42, 72] and tissue [73]; [67] B. S. RAWLING,M. D. AMOSand M. C. GREAVES,Bull. Inst. Mining Met. No. 659, 15-26 (1961}. [68] B. P. ~ABRICAND, R. R. SAWYER, S. G. U N G A R and S. ADLER, Geochim. et Cosmochim. Acta 26, 1023-1027 (1962). [69] N. P. FINKELSTE1Nand D. N. LOCK,J. S. African Inst. Mining and Met. 62, 820-37 (1962). [70] S. SPRAGVEand W. SLAVIN,Atomic Absorption Newsletter No. 12, Perkin-Elmer Corp., Norwalk, Conn. (1963). [71] H. KAHN and W. SLAVIN, International Science and Technology, pp. 60-5, November (1962). [72] J. B. DAWSONand P. W. HEATON, Biochem. J. SO, 99-100 {1961). [73] D. B. CHEEK,J. E. GRAYSTONE,J. B. WILLISand A. B. HOLT,Clin. Sci. 23, 169-79 (1962).
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Ca in plants [48], serum [11] and saliva [74]; Mn and Fe in plants [75]; Sr in soil extracts, biological material and fertilisers [49]; Pb, Cu and Zn in wines [18]; Pb, Hg, Bi, Ni, Zn and Cd in urine [56]; Cr in faeces [47]. PARKER [82] has determined Mg, Ca and Zn in animal feed-stuffs, tissue and plasma. The determination of lithium isotopes has been examined b y ZAIDEL and KORENNO: [65, 66] and, lately, b y MANNING and SLAVlN [11] of Perkin-Elmer. In Australia, some 60 laboratories in such diverse fields as agriculture, medicine, ferrous and non-ferrous metallurgy, and the petroleum and electroplating industries, use atomic absorption for inorganic analysis. Consideration of the ready availability of atomic absorption apparatus in Australia and of the advantages shown b y critical comparison of atomic absorption with other methods has resulted in it being p u t forward as the standard method for magnesium in iron b y the Standards Association of Australia [79]. CONCLUSIONS
The most striking feature of developments over the past two years is the general acceptance, throughout the world, of atomic absorption as a means of analysis. I consider it safe to say now that it is firmly entrenched and will, in the future, attain importance equal to or greater than other physical methods of inorganic analysis. Despite the fact that the flame has been severely criticised as a means of vaporising samples b y several workers in the past, it is through this medium that the recent impetus in atomic absorption analysis has taken place. The most important factors now recognised to be in favour of the flame as a vaporiser are its convenience (which has always been accepted) and the fact that, b y choice of correct conditions with respect to the nature and relative composition of flame gases, b y modification of the apparatus to give long path-lengths and by the application of established methods of circumventing interferences, practically any element can be vaporised as free atoms and, provided its strongly absorbing resonance line lies above 1900 A, can be determined at the part-per-million level or better. The inefficiency of conversion of a sample into atomic vapour b y flames, which was mentioned b y ALLAN [7] in his review at the 1961 conference, still remains and it is difficult to see how it can be overcome. W h a t is required is a total-consumption burner-atomiser that is hot enough to efficiently vaporise intractable oxides, reducing enough to generate atoms of any element and, at the E. NEWBRUN, Nature 192, 1182-3 (1961). D. J. DAVID, Rev. Univ. Ind. Santander 4, 207-14 (1962). A. STRASHE~Mand G. J. WESS]SLS, Appl. Spectroscopy 17, 65-70 (1963). F. J. WALLACE, Hilger Journal 7, 39-42 (1962). F. J. WALLACE, Hilger Journal 7, 65-68 (1963). C. B. BELCHER, Proc. Royal Australian Chem. Inst. 80, 111-2 (1963). R. N. KNIS]~L~.Y, A. P. D'SILVA and V. A. FASSEL, Anal. Chem. 85, 910-1 (1963). W. SLAV::':, Atomic Absorption Newsletter No. 10, P e r k i n - E l m e r Corp., Norwalk, Conn. (1963). [82] H. E. P~_RKER, Atomic Absorption Newsletter No. 13, Perk[n-Elmer Corp., Norwalk, Conn. (1963). [83] D. C. MANNING, S. SPRAGUE and W. SL~VIN, Atomic Absorption Newsletter, Perk[n-Elmer Corp., Norwalk, Conn. (1963).
[74] [75] [76] [77] [78] [79] [80] [81]
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same time, is able to convert the sample solution completely into a spray as fine as t h a t produced b y spray-chambers of pre-mix burners used at present. Compensation for the dilution effect arising from the high temperature of flames might be achieved b y cooling the gases before atomic absorption measurement provided t h a t it can be done without loss of sensitivity due to condensation of the atomic vapour of the element or its reversion to the oxide. Flash heating b y means of a condenser discharge [30] or, perhaps, a laser shows most promise for solid samples in that instantaneous and complete vaporisation of a sample might be achieved, thus overcoming effects due to selective volatilisation. At the present stage of development of flash heating, the time involved in preparation of most types of sample would preclude its use in routine analysis, however.