(175F) Seim, H. J., Calkins, R. C., Macksey. J. A,, Anal. Chem., 47, 12813 (1975). (176F) Shapunov, L. A,, Vil'dt. N. G., Okhrimeta, S. D.. Vii'dt. A. L.. Zavod, Lab., 39, 959 (1973). (177F) Shemet, V. V., Burkova. L. V., Zaitseva. T. M., Kulikov, F. S., Luft, B. D., Novikov. V. B., Khusid, L. B., Zh. Anal. Khim., 28, 1806 (1973). (178F) Sim, P. G., Appl. Spectrosc., 28, 23 (1974). (179F) Skotnikov, S.A,, Zavod. Lab., 40, 405 (1974). (18OF) Sloman, K. G., Foltz, A. K.. Yeransian, J. A.. Anal. Chem., 47, 56R (1975). (181F) Sokolowska, W., Pr. Nauk lnst. Chem. Nleorg. Met. Plerwiastkow Rzadklch Politech. Wroclaw., 17, 405 (1973); Chem. Abstr., 80, 152435k (1974). (182F) Soldano, B. A,, Kwan, P. W. O., Appl. Spectrosc., 29, 271 (1975). (183F) Sonobe, T., Asakura, Y., Satoh, H., Imai, S., Yamanouchi, H.. Shimomura, T.. Tokai Jigyo-sho, Doryokudo, Kaku-nenryo Kaihatsu Jigyodan. (Rep.) 7972, 99 (1973): Chem. Abstr.. 80, 32958m (1974). (184F) Spectroscopy Division Report 1973-4, lndia AEC Bhabha At. Res. Cent. (Rep.), BARC783, 127 pp (1974). (185F) Sreeramamurty, P., Marathe, S.M., Kapoor, S. K., Saraswathy. M.. lndla AEC Bhabha At. Res. Cent. (Rep.), BARC-680, 5 pp (1973). (186F) Straub, W. A,, Hurwitz, J. K., Anal. Chem., 47, 112R (1975). (187F) Stroganova, N. S.. Ryabukhin, V. A,, Galkina, I. P., Ermakov, A. N., Zh. Anal. Khim., 28, 1313 (1973). (188F) Sudo, E., Saito, M., Tetsu To Hagane, 60, 1805 (1974); Chem. Abstr., 82, 799281.1 (1975). (189F) Sugimae, A., Anal. Chem., 46, 1123 (1974). (19OF) Sugimae, A,, Anal. Chem., 47, 1840 (1975). (191F) Sugimae, A., Appl. Spectrosc., 28, 458 (1974). (192F) Sugimae, A., Bunseki Kagaku, 22, 1350 (1973). (193F) Takao. Z.,Ed., "Standard Methods for Gas Determination in Metallic Materials" (Kinzoku
Zairyo no Hyoyun Gasu Bunseki Hoho) Maruzen, Tokyo, 1974; Chem. Abstr., 81, 163058m (1974). (194F) Tanaka, I., Sato, K., Matsumoto. R., Bunsekl Kagaku, 24,423 (1975). (195F) Tarasova, I. I., Dudenkova. L. S.. Khitrov, V. G., Belousov. G. E., Zh. Anal. Khim., 29, 2147 (1974). (196F) Temma. T., Miwa, S., Bunsekl Kagaku, 23, 863 (1974). (197F) Temma, T., Miwa, S., Bunseki Kagaku, 23, 1475 (1974). (198F) Tiptsova-Yakovleva, V. G., Dvortsan, A. G.. Semenova, I. E., Zh. Anal. Khim., 30, 1577 (1975). (199F) Toelg. G., Talanta, 21, 327 (1974). (200F) Toelle, H., Mikrochim. Acta, 771 ( 1973). (201F) Trokhachenkova, 0.P., Gradskova, N. A , , Zhinkin, D. Ya., Makulov, N. A., Zb. Anal. Khim., 30, 1380 (1975). (202F) Truksa, A,, Elektronika, 15, 112 (1974). (203F) Tumanov, A. A,, Shakhverdi, N. M., Tr. Khim. Khim. Tekhnol., No. 3, 43 (1973). (204F) Turina, N., Weber, K., Kem. lnd., 22, 595 (1973): Chem. Abstr., 82, 46105) (1975). (205F) Turulina, 0. P., Zakhariya, N. F., Zh. Prikl. Spektrosk., 21, 203 (1974). (206F) Ungureanu, C., isotopenpraxis, 11, 237 (1975). (207F) Ungureanu, C., Pascalau, M., Stud. Cercet. Fiz,, 25, 663 (1973): Chem. Abstr., 80, 22285a (1974). (208F) Vall, G. A,, Shatakaya, S. S., Yudeievich, I. G., Torgov, V. G., lzv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk. No. 5, 88 (1973). (209F) Van Raaphorst, J. G., Ordelman, J., Tolk, A,, Rep., RCN-215, 13 pp (1974). (210F) Varga, Gy., lzotoptechnika, 17, 80 (1974). (211F) Vassilaros, G. L., Talanta, 21, 803 (1974). (212F) Vengsarkar, B. R., Saksena, M. D., Curr. Sci., 44, 461 (1975). (213F) Venkatasubramanian, R., Dixit, R. M., Saranathan, T. R., Fresenius' Z. Anal. Chem., 271, 357 (1974). (214F) Vlasov. V. S.,Popova, G. D., Krasnikova, G. V., Yudelevich. I. G., Zavod. Lab., 39, 1048 (1973).
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Flame Emission, Atomic Absorption, and Atomic Fluorescence Spectrometry Gary M. Hieftje" Department of Chemistry, Indiana University, Bloomington, Ind. 4740 7
Thomas I?.Copeland Department of Chemistry, Northeastern University, Boston, Mass. 02 7 75
Dorys R. de Olivares' Department of Chemistry, Indiana University, Bloomington, lnd. 4 740 I
This is the first biennial review to be prepared by the present authors, the previous three having been written by Professors James D. Winefordner of the University of Florida and Thomas J. Vickers of Florida State University. We all owe a deep debt of gratitude to both these men for their efforts on behalf of the analytical community. The title of this review section has been changed from "Flame Spectrometry" to the present one. This change rePresent address, F a c u l t a d de Ciencias, Universidad de Andes, M e r i d a , Venezuela.
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flects the increasingly common usage of nonflame devices as atom cells in atomic absorption and atomic fluorescence spectrometry. Retention of the term flame emission" distinguishes this section from the review entitled "Emission Spectrometry", where emission from sources other than flames is covered. The present review covers books, articles, and chapters which appeared in the time period between January 1, 1974, and November 15, 1975. Unfortunately, any publication appearing between October 1973, and January 1, 1974 will not have been included either in the previous review
Gary M. Hleftje received his A.B. degree from Hope College in 1964, after working three years at Miles Laboratories in Zeeland. Michigan. From 1964 to 1965, he served as a research assistant in physical qhemistry under J. Thomas, Jr., of the Illinois State Geological Survey. in 1965, he entered graduate school at the University of Illinois, where he received the Ph.D. degree under the direction of H. V. Malmstadt in 1969. In September 1969, he was appointed assistant professor of chemistry at indiana University and was promoted to associate professor in September 1973. During the period from 1969-1971, he served as a director of research for the ill-fated corporation, Precision Lasers. His research interests include the investigation of basic mechanisms in atomic emission, absorption and fluorescence spectrometric analysis, the development of atomic methods of analysis, on-line computer control of chemical instrumentation and experiments, the use of time-resolved luminescence processes for analysis, application of information theory to analytical chemistry, and the use of stochastic processes to extract basic and kinetic chemical information. He has presented over 60 invited talks on these subjects at national and international conferences and meetings and at universities, colleges, industries, and government laboratories. He is a member of the American Chemical Society, the Society for Applied Spectroscopy, the Optical Society of America, Sigma Xi, and Phi Mu Alpha. He currently serves on the editorial board of Spectrocbimica Acta, Parf B.
Dorys R. de Ollvares performed undergraduate work in chemistry at the Central University of Venezuela, Caracas, Venezuela. From 1970 to 1971, she served as research assistant in natural products under the direction of T. Nakano at lVlC (Venezuelan Research Institute). She undertook graduate studies at Indiana University in 1972 under the direction of Gary M. Hieftje and completed requirements for the Ph.D. degree in 1976. Currently, she holds a position as assistant professor of chemistry at the University of the Andes, Merida, Venezuela. She is a member of the Optical Society of America and the Society for Applied Spectroscopy.
Thomas R. Copeland is an assistant professor of chemistry at Northeastern University He received his A B degree from Cornell University in 1969 and his Ph D from Colorado State University in 1973 working in R K Skogerboe’s group From July 1973 through July 1974, he was a Postdoctoral Research Associate with Dr Hieftje s group at Indiana University He loined the Northeastern faculty in September 1974 His research interests in analytical chemistry include the study of spectroscopic methods of trace element analysis and the study of electrochemical methods of trace analysis. He is a member of the American Chemical Society, the Soclety for Applied Spectroscopy, and Sigma Xi He is chairman-elect of the analytical group of the American Chemlcai Society’s Northeastern Section.
(37A) or here. Our a ologies go to the authors whose work has been overlooked$ecause of this. As in the previous review, it has been necessary to limit the number of articles being reviewed, in order to provide a meaningful coverage. Over 2000 publications dealing with the reviewed subject have appeared in the past two years, and we have attempted to be selective rather than encyclopedic in our treatment. Approximately 1400 papers were initially selected for examination; from that group, the smaller number being reviewed here was chosen.
The criteria employed in choosing articles to be reviewed are similar to those set down by Winefordner and Vickers (37A). These criteria are: 1) no paper was reviewed which largely duplicated another or incorporated only minor changes or innovations compared to an earlier publication. Included in this category are articles published in more than one language. For journals which appear in more than one language, reference is made, wherever possible, to the English version of the journal, despite its ordinarily delayed date of appearance. When it has been necessary to cite an article appearing in a language other than English, every effort has been made to secure the Chemical Abstracts reference and include that along with the English translation in the Literature Cited. 2) Most papers published in the following analytical and physical journals have been reviewed-Advances i n High Temperature Chemist r y , Analyst, Analytica Chimica Acta, Analytical Chemist r y , Analytical Letters, Applied Spectroscopy, Applied Spectroscopy Reviews, Journal of t h e Association of O f f i cial Analytical Chemists, Critical Reviews in Analytical Chemistry, Critical Reviews i n Environmental Control, Chemical Instrumentation, Combustion and Flame, High Temperature Science, Instruments and Experimental Techniques, Journal of Aerosol Science, Journal of A n a lytical Chemistry ( U S S R ) ,Journal of Applied Spectroscopy ( U S S R ) ,Journal of Chemical Education, Journal of t h e Optical Society of America, Optics and Spectroscopy ( U S S R ) , Optics Communication, O p t i k , Progress in High Temperature Physics and Chemistry, Review of Scientific Instruments, Spectrochimica Acta Part B, Spectroscopy Letters, Talanta, and Fresenius’ Zeitschrift f u r Analytische Chemie. 3 ) No papers published in unreviewed journals have been considered. Included in this list are Americ a n Laboratory, Research and Development, Industrial Research, Laboratory Practice, Atomic Absorption Newsletter, S p e x Speaker, Flame Notes, Laser Focus, Optical Spectra, and other trade journals, free journals primarily used for advertisement, and magazines intended for consumption by the general public. 4) Only a few selected applications or articles from journals other than those listed above in (2) have been reviewed. However, particularly innovative publications or those of fundamental importance have been reviewed, whatever their source. 5) As stated earlier, the emphasis in this review will be on flame spectrometry and on studies which deal with devices intended t o be direct replacements for a chemical flame as an atom cell. Thus, applications involving carbon or metal atomization systems will be reported while those utilizing electrical discharges or plasmas will not. By agreement, dc, radiofrequency, and microwave discharge sources will be reviewed in the section entitled Emission Spectrometry, authored by R. M. Barnes. Clearly, the foregoing criteria have limited the number of papers to be included in this review. The authors sincerely regret any bruised egos that might result from this selection process, but strongly feel that the resulting coverage will be of greatest utility to practicing atomic spectroscopists. This review has been organized somewhat differently than before. References have been organized into six major sections, which have been divided along lines we feel will be most useful. These sections are: A. Reviews, Books, and Bibliographies; B. Fundamental Studies; C. Automation and Advances in Instrumentation; D. Developments in Technique and Procedure; E. Analytical Comparisons and Figures of Merit, and F. Applications. References pertaining to each section have been listed in alphabetical order within each section and listed under Literature Cited. Flame emission, atomic absorption, and atomic fluorescence spectrometry constitute a maturing subdisci line of analytical chemistry. Consequently, changes in t Be past two years have been evolutionary but have produced some discernible trends. Most notable is the rowth of applications involving nonflame atom cells and t e conclusion that such devices are unlikely to ever replace the chemical flame entirely. The hi h incidence of matrix interferences, the necessity of deafing with transient atom opulations, and the difficulty of handling small samples a1 combine to render these atom cells useful in only those particular analytical situations where a flame will not serve. Perhaps surprisingly, interest in atomic fluorescence spectrometry seems
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to have recently leveled off. The absence of commercially available atomic fluorescence instrumentation is no doubt partially responsible for this fact; however, the potential utility of atomic fluorescence in multielement analysis might generate renewed emphasis in the future. The subject of multielement analysis has itself become a subject worthy of examination. In recognition, we have collected a number of papers in a small subsection dealing with multielement analysis.
REVIEWS, BOOKS, AND BIBLIOGRAPHIES I t has been the custom in recent reviews to report briefly on the important International Conferences on Atomic Spectroscopy, the most recent of which was held in Melbourne, Australia, from August 25-29, 1975. Because this year commemorates the 20th anniversary of the introduction of atomic absorption as an analytical technique, the location of the conference seemed particularly appropriate. The meeting was extraordinarily well-planned and was conducted in three parallel sessions, containing a total of 101 papers. Eight plenary lectures were presented and an opening address was delivered by conference chairman Alan Walsh; a copy of this address has recently appeared in print (32A). Walsh stressed the potential importance and utility of sputtering discharges and described instrumental configurations in which a sputtering chamber could be employed alternatively as a source of radiation, a sample atomizer, and resonance monochromator. Considerable work toward applying sputtering sources to atomic analysis appears to be underway a t CSIRO in Melbourne and reports of interesting applications can be expected in the near future. Plenary speakers a t the Melbourne conference and their subjects were the following: D. A. Segar stressed the importance of nonflame atomizers in environmental investigations; K. Laqua spoke on one kind of sputtering system, the glow discharge, and pointed out that such a source can exhibit minimal matrix interferences while enabling the simultaneous determination of most elements in the periodic table. The more fundamental aspects of glow discharges were considered by N. H. Tolk in his talk on low-energy ion-atom and ion-surface collisions and how such collisions can generate optical radiation. A. G. Gaydon gave an authoritative presentation on the state of equilibrium in flame gases and the importance of such a state to analytical measurements. G. M. Hieftje reviewed studies performed with an isolated-droplet sample introduction system, and stressed the utility of the device in elucidating mechanisms of atom formation in analytical flames. N. Omenetto described a number of clever experiments which enable the local sensing of physical parameters in flames. He showed how right-angle optical systems, employing atomic fluorescence, can yield correct measurements of local temperatures in flames; in other experiments, laser sources were shown to be useful in isolating chosen regions of the flame for characterization and observation. P. Hannaford spoke on the influence of spectral line profiles in atomic absorption spectroscopy and described many of his experiments on that subject. K. Yasuda described the use of Zeeman modulation of spectral lines to perform background subtraction in atomic absorption spectroscopy. Although limited to only a few elements a t the present time, Zeeman modulation of either the source or atom cell appears to have some potential importance in routine analytical work. Unfortunately, the authors of this review do not believe that any of the plenary lectures or general papers given a t this conference will be published collectively. However, a more extensive review of the conference has been published ( 8 A ) and a brief conference report appeared in the December 1975 issue of Applied Optics. Several books pertaining to or dealing with flame emission, atomic absorption, or atomic fluorescence spectrometry have appeared in the past two years. Especially welcome is the second edition of the book by A. G. Gaydon (13A) dealing with the spectroscopy of flames. The second edition has not been extensively altered but is updated somewhat from the earlier version. A major new text dealing with atomic absorption and fluorescence spectroscopy by Kirkbright and Sargent (19A) deals in detail with most facets of atomic spectrometry and should be read by all 144R
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those seriously interested in the field. Other texts dealing specifically with atomic absorption appearing in the last two years are those by Pinta (25A, 26A) and the second edition of the book by Robinson (27A).A book dealing specifically with atomic fluorescence spectroscopy by Sychra (28A) has recently appeared but the authors of this review have not yet been able to secure a copy for examination. The brief treatment (in German) of atomic methods of analysis by Moenke (23A) should be of interest to some practical users. Parsons (24A) has collected in a handbook a large number of figures of merit, analytical parameters, and physical constants of use to both the theorist and the practicing analytical atomic spectroscopist. Perhaps the most useful reviews in atomic spectroscopy to appear in the last biennium are the “Annual Reports on Analytical Atomic Spectroscopy” (39A, 40A) published by the Society for Analytical Chemistry in London. The report is not only clear, but is critical as well and is written in an easy-to-read narrative style. An interesting journal review on atomic absorption spectrometry was published by Brooks and Smythe ( 4 A ) , who outlined trends in the field during the period 1955-1971. They noted the “coming of age” of atomic absorption, as revealed by changes in activity from instrument development to application and by numbers of papers being published per year. Interesting historical notes in the early development of atomic absorption were provided by Walsh ( 3 I A ) in his review. Other journal reviews dealing specifically with atomic absorption were those by Brown and Sapio ( 3 A ) , Brandenberger ( 2 A ) , and Massmann (22A). Other authors dealt with the application of atomic absorption to specific areas. Fell (IOA) gave an overview of the importance of atomic absorption to clinical chemistry, Fishkova ( I I A ) reviewed the determination of precious metals, Berman ( I d )discussed biochemical applications of flame emission and atomic absorption, and Kharlamov and Eremina (18A) considered methods for analyzing steels and alloys. A few excellent reviews appeared which discussed alternative atomization systems to the chemical flame. Woodriff (38A) compared several carbon rod and furnace systems and discussed their strengths and weaknesses. de Galan (9A) presented a brief comparison of furnace-type atomizers with microwave and radiofrequency plasmas. Methods for multielement atomic spectroscopic analysis were categorized and evaluated by Winefordner, Fitzgerald, and Omenetto (35A). From this latter review and the obvious growing interest in the literature, it is becoming increasingly clear that an inductively coupled plasma might become the source of choice for most multielement determinations, despite its cost and the large amount of radiofrequency interference it generates. In their opinion (35A), the only technique in serious contention for multielement applications is atomic fluorescence spectrometry employing a high-intensity xenon arc lamp of the EIMAC design. Multiplex techniques, although they provide multielement capabilities, suffer from a “multiplex disadvantage”. A correlation method is also proposed and advocated but appears to the authors of the present review very much like a multiplex technique. An excellent and critical review of atomic fluorescence spectrometry as an analytical technique was prepared by Browner ( 5 A ) . Brief evaluative comments were also published by Winefordner (36A). West (33A) critically reviewed atomic fluorescence and atomic absorption spectrometry while other authors reviewed flame spectrometric or atomic spectrometric techniques in a more general way (6A, 15A, 16A, 20A, 34A). Flame photometry reviews were prepared by Mason (21A) and Frei (12A). A thorough and highly useful discussion of methods of sample injection and atomization used in atomic spectrometry was prepared by Syty (29A). Syty’s review points out both the strengths and weaknesses of most techniques which have been employed or proposed for atom production. Cresser, Keliher, and Kirkbright ( 7 A ) reviewed past studies on separated flames and discussed their utility in analytical atomic spectrometry. The importance of chemiluminescence to flame emission spectrometry was evaluated by Glover (14A). A document which should be of interest and find a place
on the shelf of every analytical atomic spectroscopist is that published by IUPAC on nomenclature, symbols, units, and their usage in analytical flame spectroscopy ( I 7 A ) .Adherence to the IUPAC suggestions would help clarify and improve most publications in this field; extensive use of the document is recommended by the authors of this review.
FUNDAMENTAL STUDIES Most of the publications discussed in this section pertain directly to flame emission, atomic absorption, or atomic fluorescence spectrometry. However, others gleaned from physics or engineering literature have also been included when they contain information of fundamental importance or having potential practical application to analytical atomic spectrometry. Nebulization, Desolvation, and Solute Vaporization. In a stud involving several different pneumatic nebulizer designs, Eicht (73B) found the droplet sizes followed an upper-limit distribution pattern, with the largest drops being produced having a diameter approximately three times the average droplet diameter. However, Fall’kova and Fishkova (36B) discovered that a simple empirical equation cannot be used to predict the average droplet size in analytical burnerhebulizer systems because of the disruptive effects of desolvation. A review of Chigier (25B) dealt with the influence of the aerodynamic environment on trajectories and velocities of droplets in a spray. Particularly important were the effects of fuel-to-oxidant ratio and the temperature distribution of the medium. The influence of temperature gradients on droplets containing a dissolved solute was studied in more detail by Gardner (45B). El Golli, Bricard, Turpin, and Treiner (34B) showed theoretically and experimentally that the presence of dissolved solute can affect droplet evaporation rates. The combustion of fuel droplets a t high pressures was considered by Rush and Krier (I06B). Saad and Antonides (207B) calculated the temperature distribution within an evaporating droplet having a source of internal heat, a situation which could arise in the flame spectrometric analysis of reactive solutes. Clampitt and Hieftje (27B) showed that the thermal conductivity of flame gases is an important factor governing the evaporation rate of droplets. Applying this information to a practical analytical situation, they were able to obtain a fourfold increase in signal from a conventional nebulizer-burner system merely by adding a high thermal-conductivity gas such as helium to an acetylene flame, Grigor’ev, Lisienko, and Muzgin (49B) studied the effect of organic solvents in flame spectrometry and found that, while droplet size was not strongly solvent-dependent, sample delivery rate to the flame and the resulting flame temperature were. Lunde and Paus (77B) obtained a four- to fivefold signal increase in atomic absorption through use of ethanol as a solvent rather than water. GomiiEek and Sphn (46B) found chlorinated organic solvents to have a marked effect on atom formation, but offered no explanation of the observation. In an attempt to elucidate the mechanism of vaporization of solute particles in an analytical flame, Bastiaans and Hieftje (10B) utilized a discrete droplet generator and measured vaporization rates of individual solute particles. With these rates and from times required to vaporize particles of known size, alternative vaporization models were compared. Complications during vaporization were also noted, including possible particle explosion or disintegration, fractional crystallization in the drying of the particle, fractional volatilization of a particle and the effects of chemically reactive, highly volatile or relatively involatile matrices. Further investigations on the mechanism of vaporization were reported by Clampitt (26B), along with details of studies aimed at understanding the evaporation of aerosol droplets in a flame. Montaser (87B) used a computer-controlled flame spectrometer to examine the fundamentals of atom formation in both flame and nonflame sources. Peleg and Alcock (94B) found that the vaporization rate was not the same from different faces of single crystals of alumina and magnesia held in a high temperature environment. Law and Williams ( 7 I B ) used a laser to ignite small particles of pure magnesium and found the resulting burning kinetics to be controlled by the rate of heat transfer to the particle surface. Presumably, thermal con-
trol prevails in this situation because of the relatively low boiling point (1378 “C) of magnesium. Transferred to practical flame spectrometry, this finding implies that vaporization of low-boiling solute particles would also be thermally controlled and would be enhanced by increased flame-gas thermal conductivity. Pleskach (99B)studied the effect of phosphate on calcium atomization and ascribed his observations to the formation of apatite [Ca&l(P04)3]. Bakhir and his co-workers (8B, 9 B ) determined refractive indices and scattering coefficients for polydisperse aerosols of alumina and considered errors which might arise in determining particle sizes through measurement of scattering. Urusov (119B) developed a semi-empirical equation for calculating the atomization energy of a large number of different inorganic crystals. Hamano and Asaga (50B) described the preparation of spherical alumina particles in a high-current plasma. Atom Formation and Distribution. An excellent series of articles by L’Vov, Kruglikova, Polzik, and Katskov (79B-81B) considered the production and distribution of atoms along flames supported on slot burners. Models were proposed and developed that predict the cross-sectional distribution of particle aerosols in a long-path flame. h e cause this distribution is dependent on the particle size, the authors point out that all factors should be held constant which affect particle size in conventional atomic absorption spectrometry. In particular, standards and samples should both have volatile matrices whenever possible and should not contain species that form complexes with the analyte. In an extension of their model, the authors also show that final atom concentrations at any point in a slot flame depend on matrix volatility, the diffusion coefficient of the atoms, and the flame gas velocity. This model is used to explain “lateral diffusion interference” effects and can be employed in a simple scheme for measuring atomic diffusion coefficients in hot gases. Penkin, Redko and Kryukov (95B) also described a method for measuring diffusion coefficients. Fujiwara, Haraguchi, and Fuwa (40B-42B) studied atom formation in nitrous oxide-acetylene and air-acetylene flames by measuring the spatial distribution of atoms as a function of fuel-to-oxidant ratio. They found that elements could be grouped according to where in the flame they absorbed most strongly and the fuel-to-oxidant ratio that produced the greatest absorption; these findings were correlated with the tendency of many elements to form stable oxides. Rubeska (105B)measured the atomization efficiency of tin in hydrogen flames and found the lowest temperature flame to yield the greatest atom concentration. The . controllin reaction in the production of tin atoms was hypothesize! to be the reduction of SnO by hydrogen atoms. Using low pressure flames, Boukaert, D’Olieslager, and DeJaegere (16B) determined rates of atom formation for several elements. Finding that atom formation rates were far too fast to be controlled by simple thermal dissociation, they suggested a mechanism involving reduction of a metal monochloride molecule by hydrogen atoms to produce a free atom and hydrogen chloride. Farber, Harris, and Srivastava (37B) used mass spectrometry to show that seeding a flame with potassium atoms altered the nature of iron and rhenium molecular species that are formed. Equilibria between atoms and molecular species were studied by a number of workers. Poluektov and Meshkova (IOOB)investigated the recombination kinetics of alkalimetal ions in an air-acetylene flame while Kitagawa, Yanagisawa, and Takeuchi (68B)described a new direct method of measuring the fraction of copper atoms formed during dissociation of CuCl in an air-hydrogen flame. In this latter study, concentrations of copper atoms and CuCl molecules were determined separately using absorption measurements, so that calculations did not require the assumption of chemical equilibrium between the species. Tomkins and Frank (116B)measured the vaporization of solid copper by a hot gas containing chlorine atoms and proposed a mechanism operative in the well-known Beilstein test for halogens. Van der Hurk, Hollander, and Alkemade (120B-122B) examined several spectral band systems commonly observed in flame spectrometry and discussed their origin. Excitation energies and energy-level diagrams were preANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
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sented for several alkaline-earth bands. Human and Zee ers (60B)showed that alkaline-earth bands can be o served by fluorescence as well as by emission and suggest an analytical technique which employs molecular fluorescence in flames using tunable laser excitation. Ionization a n d Radical Formation and, Recombination. One of the simplest and most common methods for measurement of ionization in flames utilizes electrical robes. An evaluation of the probe method was presented y Bogoslovskii, Zaichikov, and Samoilov (15B), who considered the effects of environmental turbulence, probe design, and probe temperature on the resulting measurements. A new method for the measurement of electron densities in flames utilizing microwave interferometry was proposed by Coghe, Ghezzi, and Gottardi ( 2 8 B ) . Many studies on ionization in flames are performed on hydrogen-oxygen-nitrogen flames, because of that flame’s low concentration of naturally occurring charged species. Most of the studies cited in this section (except those noted) have employed this flame. Hayhurst and Kittelson (54B, 5 5 B ) used mass spectroscopic probes to investigate the ionization of alkaline earths; the difficulties and uncertainties involved in this technique are explored in detail. They present ionization rate constants for strontium and calcium and suggest possible mechanisms; hydration energies of the resulting ions are also given. Williams and Sheinson (127B) investigated the effect of added oxygen on ionization produced in low temperature hydrocarbon flames. Page and Woolley ( 9 1 B ) used a microwave cavity to measure electron concentrations produced by ionization of solid particles in a flame. The number of electrons released in the flame is proportional to the radius of the particle for low-volatility elements such as uranium and titanium. For nickel and iron, which atomize appreciably in the Hz-Nz0 2 flame, electrons are produced primarily through atomic rather than through particulate ionization. Gould and Miller ( 4 7 B ) found that the addition of rhenium to e1ectro.nrich flames results in high concentrations of negative rhenium-containing ions and a corresponding decrease in the flame electron concentration. Recombination rates of electrons with naturally occurring flame ions were determined by Hayhurst and Telford ( 5 6 B ) and by Karachevtsev (66B). Hayhurst and Telford (56B) measured the recombination rate of free electrons with hydronium ions, one of the fastest reactions to occur in the gas phase and probably the principal ionic reaction occurring in burnt gases of hydrocarbon flames. Anthony, Bulewicz, Kelly, and Padley ( 6 B ) used a rotating single probe to study recombination rates of electrons and metal ions and showed that flame temperature, flame composition, and total ion concentration all markedly affect the measured rates. Lambert and Van Tiggelen ( 7 0 B ) seeded a H2-02-N2 flame with low molecular weight alkanes to determine formation rates of CH and OH radicals and ionized species. Radical recombination rates and mechanisms were investigated by Jensen and Jones ( 6 4 B ) by means of mass spectrometric tracer and photometric methods. The effect on these rates of added tin, tungsten, or molybdenum atoms (acting as catalysts) was ascertained. Zaitsev and Tverdokhlebova (128B) explained the large voltage jump which occurs in cylindrical platinum probe measurements as the probe is moved from the cold to the hot part of the reaction zone in an oxy-acetylene flame. Atom Formation in F u r n a c e s a n d Reduction Cells. Several elegant treatments have appeared describing rates and mechanisms of atom formation in carbon furnaces. It is interesting to note the similarity between many of the mechanisms now being roposed and those which years ago had been shown to ho d in carbon-arc emission sources. Many workers seekin to understand atom formation in carbon furnaces woulcf do well to consult this rich body of earlier literature. An attractive approach to treating atom liberation and distribution from carbon furnaces has been employed by Tessari and his co-workers ( 9 3 4 115B, 117B). In this approach, an effective transfer function for the atom observation region is developed, and the influence of experimental parameters on the transfer function is subsequently determined. Their model considers the importance of atom lib-
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eration rate and mass transfer rate to the observation region, and loss of atoms by diffusion into the furnace material. Agreement between the theory and experimental findings was obtained. In another treatment, Fuller (43B, 4 4 B ) proposed a two-step atomization process, one step involving liberation of atoms from the sample and the other the loss of atoms from the furnace. The best fit of this model to experimental results required the assumption that metal salts were reduced to free atoms by the carbon furnace. The model was able to predict the importance of several occurrences, including the premature loss of atoms during a furnace ashing cycle. Campbell and Ottaway (22B, 2 3 B ) also support a model attributing atom formation to reduction by carbon a t the furnace wall. Johnson, Sharp, West, and Dagnall (65B)have calculated furnace temperatures a t which atom vaporization first begins. Maessen and Posma ( 8 3 B ) , and Pinta and Riandey ( 9 8 B ) have considered the importance of various factors in determining the shape of absorption signals measured with carbon atomizers. In the first paper of a series, Massmann and Guecer ( 8 5 B ) discussed factors which must be taken into account in developing furnace procedures in atomic absorption. These factors include the emission and absorption by atoms (other than the analyte), molecules, and radicals, and thermochemical reactions between the sample and the furnace material. Findlay, Zdrogewsky, and Quickert (38B) explored the use of a thermocouple to measure furnace temperatures during heating and cooling. In a study on the production of mercury from reduction cells, Shimomura, Hayashi, Morita, and Onishi (IIOB) proposed a reaction mechanism involving oxidation followed by disproportionation of mercurous ion. The proposed mechanism accounts for the observed interference by complexing ions, such as halides, sulfides, and cyanides. Radiational Excitation a n d Radiational or Nonradiational Deactivation in Flames. A series of excellent publications by Lijnse (74B-76B) explored various facets of electronic excitation-transfer collisions in flames. Cross sections for quenching and doublet-mixing between alkalimetal atoms and inert gases or molecular gases (such as nitrogen, oxygen, hydrogen, and water) were measured. In the quenching of alkali-metals by nitrogen, a model was proposed which involved an ionic, van der Waals-bound intermediate state. Chemiluminescence produced by alkalihalogen reactions was studied by Oldenburg, Gole, and Zare ( 8 9 B ) ;particular emphasis on potassium iodide chemiluminescence was given by Kaufmann, Kinsey, and Palmer ( 6 7 B ) . Edlestein, Eckstrom, Perry, and Benson ( 3 3 B ) measured photon yields for several metal-NzO reactions which might be important in flames. Excited state lifetimes and collisional quenching rates of electronically excited atoms were determined in a series of investigations by Husain and his co-workers (12B, 13B, 17B-19B). These measurements utilized time-resolved atomic fluorescence and probed relatively long-lived or metastable states. Ewing, Trainor, and Yatsiv ( 3 5 B ) examined the collisional relaxation of electronically excited lead atoms, while Zakharov, Myasoedov, Ozhegov, and Karyakin (129B) investigated the quenching of excited metal atoms in an uncooled hollow cathode. Several investigators published new or improved values for excited-state lifetimes or atomic oscillator strengths. Abjean and Johannin-Gilles ( 2 B , 3 B ) used an atomic beam to measure the oscillator strength of a zinc transition whereas Siomos, Figger, and Walther (111B) used timeresolved atomic fluorescence to measure the lifetime of excited iron atoms. McDermott and Nash (86B)reported oscillator strengths for copper lines involving autoionization levels, Le Toulouzan, Darrigo, and Valentin ( 7 2 B ) measured the oscillator strength of potassium transitions. Fundamental studies on atomic spectroscopy by physicists often eventually find their way into the analytical laboratory. Some of these studies which we feel would be of interest, or perhaps importance, to analytical chemists follow, Quantum beats between close-lying atomic levels can be produced by rapid population of an excited state; quantum beats from the sodium D lines are discussed by Haroche, Gross, and Silverman ( 5 I B ) .A new method for detection of optical resonances employing lasers has been sug-
gested by Huber and his co-workers ( 5 9 B ) . Hyman and von Rosenberg ( 6 2 B ) were able to measure infrared radiation from sodium atoms as they radiationally deactivated from the P3/2 state to the Pl/z state. The effects of photon trapping (radiational transfer) on line intensities in atomic fluorescence and emission spectroscopy was discussed by Held and Stephens ( 5 7 B ) , who developed equations relating atomic fluorescence to atom concentration a t low atomic partial pressures. Level-crossing (Hanle effect) experiments using laser excitation were performed by Rasmussen, Schieder, and Walther (101B). Nonresonance fluorescence produced by interaction between magnesium atoms and their solid rare-gas matrix was measured by Knight, Brittain, Starr, and Joyner ( 6 9 B ) . Induced atomic fluorescence was investigated by Callender, Gersten, Leigh, and Yang ( 2 1 B ) . Induced atomic fluorescence can occur when radiation not resonant with an atomic transition is absorbed by a molecular gas. Photodissociation of the molecules can then yield atomic species in excited states, which then fluoresce. Like conventional atomic fluorescence, the induced fluorescence increases linearly with incident source power but also with temperature of the observed atomic vapor. In a more analytically oriented study, Urbain and Desquesnes (118B) explained the background spectral continuum produced by alkali-metals as being due to recombination of atoms and hydroxyl radicals (visible continuum) or between alkali ions and free electrons (ultraviolet continuum). S p e c t r a l Line Profiles and Their Effect on Working Curves. Izotova, Preobrazhenskii, Tambovtsev, and Frish ( 6 3 B ) emphasized the importance of employing deconvolution techniques to eliminate from measured spectral line profiles the unwanted contributions from measuring-instrument response and distortions produced by the scanning-recording system. Wagenaar, Pickford, and de Galan (125B) used a pressure-scanning Fabry-Perot interferometer to measure atomic absorption line profiles in flames. L'Vov, Polzik, Katskov, and Kruglikova ( 7 8 B ) employed similar techniques to measure the shift in spectral lines which occurs in flames compared to those emitted by hollow cathode lamps. The shifts so measured gave excellent agreement with theory. The spectral profile of the iron 372.00-nm line emitted by a hollow cathode lamp was measured by Zechev, Dyulgerova, and Pacheva (130B). The importance of collisional broadening and optical density of metal vapors on their atomic absorption profiles was investigated by Nemets. Nikolaev, and Flisyuk (88B). The extreme broadening of spectral lines which occurs a t high atomic vapor pressures was investigated by Douda and Bair (30B),in their studies on pyrotechnic flares. The effect of hollow cathode lamp spectral line profiles on analytical curves in atomic absorption spectrometry was examined by Wagenaar, Novotny, and de Galan (126B). These workers found that even infinitely narrow hollow cathode lamp spectral lines produce some curvature in atomic absorbance plots because of a line shift. A tunable laser could be used to overcome this limitation. The authors found very little effect of hollow cathode lamp line broadening on either sensitivity or curvature of analytical plots. A theoretical evaluation of errors produced in absorption measurements by the use of nonmonochromatic light was published by Agterdenbos, Vlogtman, and van Broekhoven (4B, 5 B ) . Loss of metal atoms during irradiation of mercury vapor can cause unexpected curvature of absorbance plots according to Slevin, Busch, and Vickers (114B). Apparently, mercury atoms are lost by the formation of dimers via a metastable excited state. Characteristics of Flames. Determining the composition of combustion flames continues to be an active area of research. Hayhurst (53B) reviewed a number of aspects of investigating flames with quadrupole mass spectrometry, including ion identification, ionization of alkali and alkaline earth additives, ion-molecule reactions, ion recombination, and ion hydration. Hastie ( 5 2 B ) also reviewed mass spectrometric flame sampling but paid particular attention to probe-related perturbations on the validity of experimental data. A chapter by Palmer ( 9 2 B ) deals in detail with chemical kinetics and the question of equilibrium in flames. Becker, Haaks, and Tatarczyk ( 1 I B )used a tunable dye laser t o measure Cs radicals in flames. Oxygen and hy-
drogen atom concentration profiles were traced in a premixed propane-oxygen flame by Downs and Simmons ( 3 1 B ) . The distribution of other gases and solid components in hydrocarbon diffusion flames was measured spectroscopically by Fissan and Boltendahl ( 3 9 B ) . Abdel-Khalik, Tamaru, and El-Wakil ( I B ) studied heat and mass transfer in a diffusion flame and found radiation to be responsible for approximately 40% of the total heat transferred. Ashby ( 7 B ) pointed out that the chemical flame is an ideal subject for undergraduate study of high temperature chemical reactions and devised a corresponding laboratory experiment. The structure of Bunsen-type flames was treated in detail by Sivashinsky ( 1 1 2 B ) ,who employed a thermal-diffusion flame model and assumed a strong temperature dependence of the reaction rate in the flame. A model for premixed flames proposed by Brown, Fristrom, and Sawyer (20B) was able to accurately predict flame velocities and reaction zone properties. Burning velocities for somewhat unusual flames were measured by Vanpee, Vidaud, and Cashin ( 1 2 3 B ) , in their study of the premixed cyanogenfluorine flame and by Magnus, Chintapalli, and Vanpee (84B)on the nitric oxide-hydrogen flame. Hieftje and Bystroff (58B) measured noise spectra produced by hydroxyl radical and sodium atom emission in an air-acetylene flame. When a flowing inert-gas sheath surrounded the flame, stability a t the flame base was enhanced. However, when a quartz tube was placed around the flame, fluctuations a t the flame base increased while those in upper regions of the flame decreased. The authors also noted discrete-frequency peaks in some noise spectra, and attributed them to turbulent eddies a t the edge of the flame. Natural pulsations in diffusion flames were studied by Grant and Jones (48B),who noted well-defined oscillations near 10 Hz; the frequency of these fluctuations was not affected by the kind or dimensions of the burner nor by fuel flow rate. Flame pulsation was also discussed by Remenyi (1043) and by Sklyarov and Furletov (113B). Durao and Whitelaw ( 3 2 B ) employed a thermocouple and laser doppler velocimetry to obtain instantaneous velocity and temperature measurements in oscillating diffusion flames. The measured fluctuations were nearly sinusoidal (again near 10 Hz), except near the reaction zone, where double frequencies and spiky signals were observed. A laser schlieren method for investigating flames was described by Petrak and Rudolph ( 9 7 B ) . An overview of methods for temperature determination in flames and plasmas using spectroscopic measurements in the infrared was prepared by Penzias ( 9 6 B ) . Huster ( 6 1 B ) described a method for flame temperature measurements based on the use of a xenon high-pressure lamp as a comparison radiator. With the xenon lamp, temperatures in excess of 5000 K could be measured. Ostroumenko, Eremenko, and Tsikora ( 9 0 B ) have shown that the measurement of resonance and nonresonance atomic absorption lines of nickel can yield reliable flame-temperature measurements. By correcting for conductive and convective heat losses, Sato, Hashiba, Hasatani, Sugiyama, and Kimura (108B) were able to obtain valid flame temperature values with a thermocouple. Craig, Carlton, and Schoonmaker ( 2 9 B ) have developed a physical chemistry laboratory experiment *involving the sodium line-reversal method for measuring flame temperatures. Reif, Fassel, and Kniseley have continued their in-depth treatment of spectroscopic flame-temperature measurements with one report (102B) on variations in temperatures measured by different methods and another (103B) on the observation of isothermal zones in some laboratory flames. In the first study, typical laboratory flames were theoretically modelled; temperatures of these flames which would have been measured by different techniques were then compared. Unless the thermometric species being employed resides in an isothermal region, and unless Abel inversion techniques are employed, calculated temperatures might differ by as much as 800 "C from the true average temperature. In their second report, these authors show that titanium and iron reside in an effectively isothermal environment in the interzonal region of a nitrous oxideacetylene slot flame. Abel inversion techniques are explored in more detail by Blair (14B) and by Chao and GouANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
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lard (24B). Vasilieva, Deputatova, and Nefedov (124B) have found that, a t high metal atom concentrations, the inhomogeneity of a flame does not materially affect the measured flame temperature or the maximum intensity of a self-reversed line. L’Vov, Kruglikova, Polzik, and Katskov (82B) performed elaborate calculations to determine temperatures and equilibrium composition of nitrous oxideacetylene and air-acetylene flames. One of their interesting findings is that neither flame should contain ultravioletabsorbing species when they are fuel rich. Users of the term “coo1 flame” were taken to task by Sheinson and Williams (109B),who point out how the term should be accurately employed. ADVANCES I N INSTRUMENTATION In this section, publications will be reviewed which deal primarily with new instrumentation designed for or applicable to flame emission, atomic absorption, and atomic fluorescence spectrometry. We have attempted to divide the papers into categories dealing with specific instrumental components; however, because many papers deal with several aspects of a spectrometric system, some overlap is expected. Therefore, readers interested in a thorough coverage of any specific component are urged to examine all categories. Nebulizers a n d Burners. A few innovations in pneumatic nebulizers were recently reported. Kniseley, Amenson, Butler, and Fassel (118C) modified a Valente-Schrenk low-flow nebulizer to enable its use with corrosive (acidcontaining) solutions. The modified nebulizer operated a t gas (argon) flows as low as 0.8 l./min and demonstrated an efficiency of approximately 5%. Rohleder, Dietl, and Sansoni (187C) considered the merits of controlled solution delivery to a total-consumption nebulizer/burner. By varying the solution delivery rate, the authors were able to extend the analytically useful concentration range of their instrument by over 1 order of magnitude, since optimum burner operating parameters changed very little with solution uptake rate. An infrared heater was used (5C) in a nebulizer spray chamber to increase the efficiency of sample delivery to a flame. Aidarov and Sadykov found this approach to be much more effective than one employing preheated nebulizer gases. The most common alternative to pneumatic nebulization employs ultrasonic waves to generate the sample aerosol. A batch-type ultrasonic nebulizer, found to be relatively insensitive to fluid level, was discussed by Denton and Swartz (50C). The new nebulizer generated droplets in the size range 1-6 pm and employed sample flow rates of approximately 1 ml/min. Application of the device to flame emission spectrometry with a premixed oxygen-hydrogen flame was described by Gutzler and Denton (83C). Although the authors found lower detection limits for many elements with this burnerhebulizer combination, interferences were increased markedly over those experienced with the nitrous oxide-acetylene flame. A series of papers by Issaq and Morgenthaler (99C-101C) described the combination of a commercial “Ultramist” nebulizer with a temperature-controlled desolvation chamber. The importance of various operating parameters was investigated; chamber temperature was especially important and had to be selected on the basis of the chemical element being determined. Application of the nebulizer/heated-chamber system to the analysis of synthetic ocean water was quite successful; deposition of charged droplets on internal surfaces was minimized by plating the surfaces with platinum and electrically grounding them. Ultrasonic nebulization was also employed by Prager and Seitz (175C) in a filter photometer designed to measure HPO emission. The device was applied to the determination of phosphorus in air and natural waters. Electrostatic nebulization was found by Kozhenkov, Kirsh, and Fuks (120C) to be capable of producing nearly uniform droplets in the size range 1.5-15 pm. A review of several kinds of flames, burners, and nebulizers in common use in flame spectrometry was prepared by Dresser, Mooney, Heithmar, and Plankey (57C). Although the review does not present an accurately balanced picture of those atomization systems which are in most popular use, it is detailed and well-written. A study by Murphy and Veillon (155C) compared several total-consumption and 148R
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premix burners, flame-gas mixtures, and a carbon-rod atomizer. Although interelement interferences were not examined, the authors found surprisingly similar detection limits for all flames and burners investigated, although the carbon furnace system provided better sensitivities. Suddendorf and Denton (209C) determined burning parameters for oxygen-hydrogen and oxygen-acetylene flames with the aid of a clever temperature-controlled single-hole burner, and set down safe operating limits for each flame. A comparison between circular (Meker) and slot-type burners for use in atomic fluorescence spectrometry was published by Murugaiyan and Natarajan (156C). Slot flames were found to produce the least scattering and the greatest sensitivity when they were aligned lengthwise along the path of source radiation; sheathing the flames with inert gas did not materially affect the amount of scattering of source radiation. One wonders whether pre-filter effects might not be more severe with this arrangement. A comparison between single- and triple-slot air-acetylene burners was made by Agemian, Aspila, and Chau (3C). Elements with high oxide-dissociation energies show enhancement by atomic absorption in the triple-slot burner; for other elements, the triple slot burner can decrease sensitivity because of additional dilution of atoms by the flame gases. Less variation in absorption with flame height was also found when the triple-slot burner was used. A precaution concerning the dangers of possible phosgene production during aspiration of trichloroacetic acid into analytical flames was voiced by Legg (134C). Uchida and Iida (221C) quantitatively measured the effect of heating a long-path atomic absorption quartz tube and of exhausting vapors from the tube a t a known rate. Optimal values for tube temperature and exhaust rate were found to differ from one element to another; the absorption tube memory effect was also found to differ and varied from 3 to 45 seconds, depending on the element. Nakahara and Musha (157C) applied a premixed inert-gas (entrained air)hydrogen flame to the determination of gallium. A sensitivity of 0.20 pg/ml was obtained but depressive interferences by acids and other elements were found. These interferences could be largely eliminated by addition of magnesium to sample solutions. Sacks and Rentner (191C) designed a water-cooled burner useful for operation with either nitrous oxide-acetylene or cyanogen-acetylene gas mixtures. The resulting low burning-velocity flames were used for the size determination of airborne manganese- and magnesium-containing particulates. Curtis, Stevenson, and Stephens ( 4 5 C ) shielded an air-acetylene flame with flowing oxygen on a slot burner. The shielded flame gave higher sensitivity than a nitrous oxide-acetylene flame for copper and thallium, produced comparable sensitivities for many other elements, and poorer sensitivities for those elements which form refractory oxides. Presumably, interelement interferences would be greater for the shielded flame than for nitrous oxide-acetylene flames. Electrothermal a n d Cold-Vapor Atom-Formation Devices. The design and application of devices intended to serve as alternatives to the chemical flame remain active areas of research. Overall, these devices (often negatively termed nonflame atomizers) appear to expand the area of application of atomic absorption and atomic fluorescence spectrometry, but are unlikely to ever replace the chemical flame entirely. This view was supported by Thomerson and Thompson (216C) in their brief but informative review. Another review, by Dresser, Mooney, Heithmar, and Plankey (58C), also discussed and compared furnaces, tubes, filaments, and other atomizers. Atomization efficiencies for a number of elements in a graphite cuvette were measured by Nikolaev and Podgornaya (160C); the extremely low atomization efficiencies found for chromium, vanadium, and titanium were explained on the basis of their formation of refractory carbide compounds. Theses on the subject of electrothermal and reductive atom formation techniques are those by Stone (206C) and Siemer (198C).
Carbon and graphite remain the materials of choice for most electrothermal atomization devices and new configurations using these materials continue to appear. In the version of Issaq and Zielinski (102C), holes were cut in a graphite tube to more easily allow vaporized material to es-
cape during the sample charring step. If allowed to accumulate, this matrix material would revolatilize during the presumed atomization step and cause a false absorption signal. Apparently, the perforated tube also produced more intense localized heating and enhanced reliability of sample atomization. Katskov, Kruglikova, L'Vov, and Polzik (113C) developed a graphite furnace consisting of two coaxial cylinders. When the cell is heated to 2900 K, atomic vapor passes from the outer chamber, through the walls of the inner cylinder, to the center of the system where absorption takes place. The device is capable of handling samples as large as 0.1 g and exhibits detection limits between 10-8 to 10-%0. The useful lifetime of the coaxial furnace is approximately 100 cycles. The modified carbon cup described by Belyaev, Koveshnikova, and Kostin ( 1 4 C ) is taller than similar systems and presumably confines atomic vapor for a longer period of time. A careful signalto-noise analysis of the device indicated that a t furnace temperatures above 1700 "C, flicker noise caused by air turbulence above the atomizer limits precision. Below 1700 "C, primary source (hollow cathode lamp) flicker noise is limiting. A two-chamber furnace for atomic absorption spectrometry, developed by Church, Hadeishi, Leong, McLaughlin, and Zak (35C), reportedly provides more complete sample combustion and obviates the need for prior chemical treatment of samples or for drying and ashing cycles. In this arrangement, an oxidizing gas carries sample material from a platinum cup enclosed by a furnace to an absorption tube which is also heated. Although applied only to mercury determination, the system can analyze solid, liquid, or gaseous samples. According to Kundu and Prevot (123C),such an oxygen-rich atmosphere helps prevent deposition of volatilized organic matrix material on the walls of the graphite tube. In addition, the oxygen-rich atmosphere destroys most organic matter a t temperatures below 500 "C, so that loss of volatile metals is less likely. Similar to the twochamber furnace is the hollow-T carbon atomizer of Robinson and Wolcott (184C, 185C). However, the hollow-?' atomizer enables continuous nebulization of the sample solution to be employed and is claimed to suffer few interferences. Application of the device to samples of various kinds is described by Robinson, Wolcott, and Rhodes (186C). Montaser, Goode, and Crouch (152C) explored the use of graphite braid as an atomizer for atomic absorption and atomic fluorescence. The braid was found to require less power (to reach 2500 OC requires only 350 watts), and responded more quickly than do the more massive graphite supports, and needed no cooling water. At an atomizer temperature of 2000 "C, the lifetime of a single braid is about 60 determinations. Applications of the graphite braid were discussed in more detail by Montaser and Crouch (150C). Chapman, Dale, and Kelly (28C) introduced and tested a "mini-Massmann"-like carbon-rod atomizer which employs a transverse hole or tube into which samples are placed. Kantor, Clyburn, and Veillon (110C) used a graphite atomizer into which pre-dried sample aerosol was continuously introduced. Their thorough study of the device revealed substantial sample matrix effects. Molnar and Winefordner (148C) employed a vitreous carbon furnace into which sample solution spray was continously fed. A specially designed, high-efficiency nebulizer was used to produce an extremely fine spray which, after being passed through a desolvation unit, could be introduced directly into the furnace. Compared to flame atomic fluorescence, the vitreous furnace yielded lower detection limits for T1, Pb, Bi, and Sn, but higher detection limits for T e and Ag. Absolute detection limits were lower in the vitreous furnace for all elements by a factor of a t least 25. Molnar and Winefordner (149C) used the same vitreous atomizer in a discrete sampling mode. In this procedure, small quantities of sample solution were injected into the nebulizer with a microsyringe; atoms so produced were detected by atomic fluorescence, as before. Detection limits comparable to other flame and nonflame atomizers were obtained; however, no interference studies or applications to real samples were made. Molnar, Chuang, and Winefordner (147C) observed emission from atomic vapor in the vitreous carbon furnace, but found the process inefficient and impractical compared to competitive techniques. This view
is not shared by Ottaway and Shaw (164C), who claimed outstanding detection limits for sodium and potassium when measured by emission from a carbon furnace. A vitreous (glassy) carbon strip atomizer was employed by Yangisawa and Takeuchi (231C) and a number of interferences were noted. In an effort to reduce mercury losses during volatilization of sample matrix in a carbon furnace, Lech, Siemer, and Woodriff (133C) plated the inside of the furnace with gold which served as an amalgamation medium. As long as furnace temperatures were maintained below the melting point of gold, little loss of mercury occurred. Baird and Gabrielian ( I O C ) explored the advantages of lining a graphite tube with tantalum foil. The foil lining increased graphite tube lifetime, prevented atoms from diffusing through the walls of the tube (thereby providing increased signal amplitude), and enabled the atomizer to be used a t lower atomization temperatures. Runnels, Merryfield, and Fisher (189C) found that coating the inside of a graphite furnace with a carbide-forming element (e.g., La, Zr) precludes carbide formation by other elements and enhances their detection. Apparently, the carbide coating, once applied, lasts the lifetime of the furnace. Ortner and Kantuscher (163C) similarly prevented carbide formation during silicon determination by impregnating their graphite tubes with salts of Ta, Ti, or W. A means of restoring decrepit graphite atomizers or maintaining new ones was developed by Clyburn, Kantor, and Veillon (39C). Their technique consisted of incorporating methane into the purge gas surrounding the atomizer during its operation. Thermal decomposition of the methane then continuously applies a new pyrolytic surface and greatly extends the atomizer lifetime. However, care must be taken to properly regulate the methane flow to prevent excessive buildup of pyrolytic carbon. In a similar procedure, introduced by Siemer, Woodriff, and Watne (201C), items to be coated with pyrolytic carbon are inserted into a tube furnace through which methane is passed. In this way, layers of carbon can be deposited on the items a t rates between 0.01-0.1 mm/h. Inexpensive carbon frits could also be made in the furnace by sintering 80-150 mesh graphite powder which had been packed into a quartz tube. Thompson, Godden, and Thomerson (217C) observed that propane, ethylene, and acetylene work as well as methane for the deposition of pyrolytic carbon. They also postulated that the signal enhancement which occurs for certain metals when Ca is added to samples is due to the preferential formation of calcium carbide. A simple, but clever, device for renewing graphite support-rod surfaces in a commercial graphite furnace was described by Johnson and Skogerboe (108C). Some of the pitfalls inherent in carbon-cup aging were pointed out by Chooi, Todd, and Boyd (32C). Lundgren, Lundmark, and Johansson (138C) used an infrared radiation detector to closely monitor and regulate the temperature of a carbon tube atomizer. Such temperature regulation provided more accurate control over ashing and atomization steps, and resulted in reduced analyte losses and in the ability to volatilize an element of interest before matrix-generated molecular species appear. The authors applied their device to the detection of 0.03 ng/ml Cd in sea water. Montaser and Crouch (151C) also employed radiation programming and found it to be superior to other methods of heating regulation except in its influence on atomizer lifetime. For longest useful lifetime, the atomizer's input power rather than its temperature should be controlled. Cresser and Mullins (44C) calculated theoretical temperature-time curves for electrically heated tungsten and graphite filaments in an effort to predict the best operating characteristics. They propose the future use of smaller filaments to reduce power supply requirements; for example, a sufficiently tiny atomizer might be able to be raised to a high temperature merely by a current pulse from a bank of charged capacitors. Siemer and Stone (200C) pointed out that the reduced sensitivity of nonresonance spectral lines is sometimes useful in the furnace atomic absorption determination of lead, to prevent overload of readout systems. Noncarbon electrothermal atomizers also continue to receive attention. Grushko, Ivanov, and Chupakhin ( S I C ) briefly explored the use of a resistively heated tantalum ribbon as an atomizer. A simple tantalum strip atomizer ANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
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was described by Sensmeier, Wagner, and Christian (196C). Tantalum and molybdenum ribbon evaporators were compared by McIntyre, Cook, and Boase (143C). Although the molybdenum support could not be used a t as high a temperature as tantalum, it did not become brittle and had an extended lifetime (1200 cycles). In the determination of Co, Ni, and Cu, interelement effects were less for the molybdenum filament than for either tantalum or carbon. The authors also note that background radiation from any metal atomizer will be less than from carbon a t the same temperature, because of the lower emissivity of the metal. Molybdenum was also employed in the micro-tube atomizer of Ohta and Suzuki (162C). The micro-tube atomizer demonstrated high sensitivity while requiring little operating power (15 W). A tungsten-rhenium wire loop (which is more ductile than pure tungsten) was employed by Newton and Davis (159C). Samples were electrolytically deposited on the loop and subsequently determined by atomic absorption. Interestingly, an appreciable amount of metal could be extracted from solution merely by dipping the wire loop into it; even without the passage of the electrolytic current, the authors found readily detectable and reproducible amounts of plated or “ion exchanged” metal on the loop. A low-current, atmospheric-pressure pulsating-dc discharge in argon was employed by Layman and Hieftje (132C) to atomize samples of various kinds into a microwave plasma. High sensitivities were found with minimal matrix interferences. Application of the device to atomic absorption and fluorescence is likely. Layman and Hieftje (131C) also developed a method for the automated electronic measurement of microliter or submicroliter sample volumes from low-mass electrothermal atomizers. The technique is based on measurement of the time required to evaporate the solvent from an unknown volume of a sample. In a courageous display of auditory fortitude, Sacks and Holcombe (19OC) explored the generation of atomic vapor from exploding electroplated-silver wires. Jensen, Dolezal, and Langmyhr (105C) electrodeposited cadmium, lead, and zinc into a hanging mercury drop, and transferred it to a graphite boat. Following low-temperature evaporation of the mercury, the remaining metals were atomized without interference from their previous matrix. Adams, Kirkbright, and West ( I C ) employed a graphite-tube atomizer and an electrodeless discharge lamp source for the vacuum-ultraviolet determination of iodine. A similar system was used by Kirkbright, West, and Wilson (116C), except that a cathode sputtering cell was utilized for sample atomization. Malykh, Men’shikov, Morozov, and Shipitsyn (141C) examined the use of an ac arc discharge as a source of atomic vapor for absorption measurements. A mechanical chopper was used to view the atomic vapor only during the current break in the arc, so that little background radiation was detected. However, one would expect the system to exhibit all the matrix interferences characteristic of an ac arc and would not expect it to find wide acceptance in atomic absorption. Several improvements in reduction-cell atom generation have been reported in the past two years. Clinton ( 3 7 C ) described a method for eliminating the reduction cell mist that is usually carried into the absorption chamber during mercury determination. The reduction cell is merely stirred vigorously to produce an equilibrium mercury-vapor concentration above the reaction mixture. The airborne mercury vapor is then displaced into the absorption cell by added tap water. Hawley and Ingle ( 8 8 C ) discussed improvements in methodology and instrumentation and various means of optimizing operating parameters in the cold vapor atomic absorption and fluorescence determination of mercury; these modifications were shown to reduce analysis time while improving sensitivity. Clifton ( 3 6 C ) showed that solution temperature in a mercury-vapor reduction cell affects the rate a t which mercury atoms are liberated; if precautions are not taken, temperature changes can cause erroneous results. Toffaletti and Savory (218C) discussed the use of sodium borohydride as a reducing agent for liberating mercury from solution. In their system, volatilized mercury is passed into a heated absorption tube which serves to decompose residual organomercury compounds and thereby equalize their response. Rathje, Marcero, and Dattilo (183C) used an absorbent-filled tube for remote sam150R
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pling of mercury vapors. Inexpensive attachments for coldvapor mercury determination with existing atomic absorption spectrophotometers were described by Lieu, Cannon, and Huddleston (135C), and by Thistlethwaite and Trease (215‘2). For some time, opposing factions have argued whether peak height measurement or peak area measurement provides the greatest accuracy for the determination of transient atom populations. Schramel (193C), Sarbeck and Landgraf (192C), and Sturgeon, Chakrabarti, and Bertels (207C) agree that integration gives more linear calibration curves and can aid in eliminating some kinds of matrix interference, when compared to peak height measurement. A more thorough treatment, which considered instrument response-time and band-width, was presented by Posma, Smit, and Rooze (174C). A drift-free integrator designed for the measurement of transient atomic absorption signals was designed by Cox ( 4 2 C ) . Solid or Novel Sample-Introduction Techniques. The most common non-nebulization method for introducing samples into an analytical flame involves the formation of volatile hydrides from such elements as arsenic and selenium. Several workers have explored modifications to this standard procedure. Siemer and Hagemann (199C) used sodium borohydride to reduce selenium to its hydride. The hydride was subsequently swept by a flowing hydrogen stream into a long-path quartz tube where oxygen was injected; the resulting combustion decomposed the hydride to form selenium atoms. Tsujii and Kuga (219C, 220C) generated arsine and stibene from their sample and directed it into a hydrogen-argon-entrained air flame. Nondispersive atomic-fluorescence detection was accomplished with an electrodeless discharge lamp source and a solar-blind photomultiplier tube. Maruta and Sudoh (142C) introduced generated arsine into an air-hydrogen flame and found several interferences. An inexpensive hydride generator for the determination of arsenic and antimony was described by van Loon and Brooker (222C).Skogerboe, Dick, Pavlica, and Lichte (203C) discovered that elements other than the hydride-formers could be liberated and sent into a flame in the form of their volatile chlorides. Samples were placed in a resistance furnace and hydrogen chloride gas passed over them and into a flame; precision of the technique was &lo%.
Shapkina and Prudnikov (197C) deposited samples on a platinum wire loop (0.5 mm) which was inserted into an air-acetylene flame. Interferences displayed by the technique could be reduced by resistively heating the platinum wire. Mozhaev and Volkhonovich (154C) employed a platinum wire spiral to introduce solutions into a flame. A resistively heated tantalum filament was used to vaporize sample into air-hydrogen and air-acetylene flames by Grime and Vickers (80C). A detection limit below 10 pg was obtained for lithium but interferences required the use of artificial blood serum standards. Grime and Vickers ( 7 9 C ) also explored the use of a Delves-type cup for the emission and fluorescence determination of T1 in an air-hydrogen flame. Emission and fluorescence produced no better detection limits than standard absorption procedures, although precision was improved. Ward, Mitchell, and Aldous (227C) determined that interferences were reduced in the Delves-cup method through use of a nitrous oxideacetylene flame. Mitchell, Ward, and Kahl (145C) tested microsampling cups made of different metals and found a platinum-rhodium cup to give the lowest detection limits and best precision, when coupled with a nitrous oxideacetylene flame. Platinum and graphite micro-crucibles were used by Prudnikov ( I 76C) to vaporize impurities from a potassium chloride sample into an air-acetylene flame. Low detection limits for lithium and cadmium were reported but the presence or absence of interferences was not. Prudnikov (277C, I 7 8 C ) also employed electrically heated platinum and graphite microfilaments to vaporize samples into an airacetylene flame. Atoms produced in the flame were sent into a heated, T-shaped quartz tube to maximize sensitivity. Katskov, Kruglikova, and L’Vov ( 1 1 1 C ) enclosed solid samples in porous graphite capsules to avoid loss of material during flash vaporization. When the capsule was resistively heated, atomic vapor diffused through the walls of
the capsule into an air-acetylene or nitrous oxide-acetylene flame. With this technique, even refractory metals present in the solid sample could be determined directly, although elements that form stable carbides (Hf, Nb, W) were not amenable to determination. Langmyhr and his COworkers published a series of papers (125C-129C) describing the application of an inductively-heated graphite tube atomizer. Samples could be dried, ashed, and atomized from the tube and sent directly into a flame. Willis (229C) explored the possibility of directly introducing into a flame, suspensions of powdered geological samples, He found that only those particles havin an initial diameter less than 12 pm contribute significantfy to the observed absorption signal. With suspensions ground to less than -325 mesh (44-pm particle diameter), the atomization efficiency of a given metal varies only by a factor of 2 between rocks of different types. Govindaraju, Mevelle, and Chouard (74C, 75C) used a threaded iron rod to introduce solid samples into an air-acetylene flame. In this approach, the threaded rod is inserted repeatedly into the powdered sample to be measured, some of which (approximately 7 5 mg) clings to the threads. The method requires little or no sample preparation and produces adequate precision (5%). Matrix effects were checked by mixing pure powdered standards with powdered samples of suspected interferents; it is therefore still not established that species present in the same particle would not affect each other. Bogden and Joselow ( 1 7 C ) described a rapid micromethod for the determination of trace elements in blood. A drop of blood was merely soaked into a filter paper which was subsequently introduced and burned in an air-acetylene flame. Husar, Husar, and Stubits (98C) gathered airborne particulates on a filter paper, extracted them into water, and flash-vaporized them in a tungsten boat using a capacitor discharge. Sulfur volatilized from the particulates was then carried into a commercial flame photometric detection system. Berndt and Jackwerth ( 1 6 C ) injected small volumes of solutions directly into a conventional nebulizerburner system and found that injection flow rates were quite important. Laser volatilization of solid samples was studied by Vul’fson, Karyakin, and Shidlovskii (223C, 224C). An “electrocontact” atomizer was used by Gadaev (7OC) for determination of impurity elements in rocks. P r i m a r y Sources f o r Atomic Absorption a n d Atomic Fluorescence. Hollow Cathode L a m p s . The profiles of spectral lines emitted by continuously operating, modulated, and pulsed hollow cathode lamps have been measured by several workers. Keliher and Wohlers (115C) used an Echelle-grating spectrometer to determine line profiles from calcium, silver, and aluminum hollow cathode lamps operated in a dc mode. As lamp current increased, line widths usually increased as well, except for the aluminum lamp, which showed a surprising minimum in the linewidth vs. current curve. De Jong and Piepmeier ( 4 8 C ) employed a piezoelectrically driven interferometer to measure copper and silver line profiles from hollow cathode lamps pulsed at currents up to 400 mA. Self-reversal of copper spectral lines was found to increase rapidly after the first 21 ps of a current pulse, and became extreme after 100 ws. Silver line profiles, however, exhibited extreme self-reversal even during the first 21 ps of the pulse. Self-reversal and line broadening were greater at the edge of the discharge than at its center, indicating that atoms are probably produced earliest at the hollow cathode edge. When pulse current was increased, self-reversal increased more rapidly during the pulse. Piepmeier and de Galan (170C, 171 C) studied pulsed and modulated copper and calcium hollow cathode lamps and obtained results similar to the previous workers. For lamps modulated a t frequencies up to 1600 Hz (square wave, 50% duty factor), spectral lines were narrower than those produced during dc operation of the lamp at the same average power input. Spectral lines emitted by pulsed lamps broaden and exhibit increasing self-reversal as the pulse frequency increases. A slight wavelength shift of calcium spectral lines from pulsed lamps was noted and attributed to a doppler effect caused by the rapidly expanding atomic vapor during application of a current pulse. A compact, multicathode lamp was constructed by Flinn and Stephens ( 6 6 C ) using wire electrodes. Although some
deposition of one cathode material on another (particularly for cadmium) occurred, performance was not seriously degraded. Human, Zeegers, and Van Elst (97C) compared emission intensities and line widths from hollow cathode lamps boosted with dc or microwave secondary dischar es. Both booster methods performed similarly and tende to more greatly increase the intensity of ultraviolet lines than visible spectral lines. Overall, the authors prefer the use of a dc booster discharge because of its greater ease of operation and increased stability. Dobrosavljivic and Marinkovic (51C) studied the effect of gas pressure and operating current on excitation characteristics of a “hot” hollow cathode discharge operated at 0.4 A. Cooled hollow cathode discharges were examined by Gri ok’eva, Zhiglinskii, and Turkin (78C). Kirkbright and bilson (117C) also employed a cooled, demountable hollow cathode as a primary source of radiation for volatile elements such as S, I, As, Se, and Hg. In this lamp, a continuous flow of Ar purge gas across the cathode prevented the buildu of high atom concentrations and minimized line broa ening. Zhiglinskii, Zaretskaya, and Turkin (233C) developed a light source which employed hollow cathode-type excitation but a graphite crucible for elemental evaporation. De Jong and Piepmeier (49C) described a capillary-bore hollow cathode lamp. When viewed across the top of the open capillary, this source produced spectral lines which were less self-reversed and had greater radiance than those from a conventional hollow cathode lamp. Norris and West ( 1 6 l C ) explored the practical utilization of overlap between spectral lines emitted by a hollow cathode lamp and those of a different element present in a flame. They found 11 analytically useful cases of overlap which gave sensitivities greater than 250 pg/ml of the element investigated. Sufficient overlap for this purpose can occur even when lines are as far apart as 0.05 nm. The strong overlap which occurs between the rhenium 364.046nm line and the neon 364.053-nm line was used for analysis by Lovett and Parsons (136C, 137C). Working curves obtained with this combination were linear and detection limits were only tenfold worse than those obtained with a rhenium hollow cathode lamp. A new kind of current regulator for use with pulsed (10 ms) hollow cathode lamps, described by Defreese, Woodruff, and Malmstadt ( 4 7 C ) , regulates current to within 0.01% between 20 and 210 mA and employs a digital feedback system. Electrodeless Discharge L a m p s . An excellent review on the construction and operation of electrodeless discharge lamps, prepared by Haarsma, de Jong, and Agterdenbos ( 8 4 C ) ,clearly summarizes the present status of the sources: while potentially extremely useful, electrodeless lamps are affected by a large number of poorly understood constructional and operating variables. Until these factors are systematically characterized, widespread use of the sources is unlikely. However, the following general conclusions can be drawn: quartz is the best envelope material and a 5- to 10-mm inside diameter is convenient; a pure metal or its iodide or chloride appear to be the best sources of atomic vapor for the lamps; the most useful fill gas is argon; modulation of the lamps is desirable, both because it is easy and it increases lamp lifetime. Avni and Winefordner (9C) measured electric fields, temperatures, and electron and ion current densities, mobilities, and velocities in a microwave-excitated electrodeless discharge. While such a discharge appears to exist in a steady state, it is not in local thermodynamic equilibrium. Also, electron densities are less than ion densities, presumably because of electron affinity processes. Knapp, Molnar, and Winefordner (121C) showed that a power divider will allow two thermostated electrodeless discharge lamps to be driven by the same microwave power supply. Jansen, Hollander, and Franken (104C) constructed small electrodeless discharge lamps for strontium and found them to emit spectral lines of comparable width but the intensity of those from hollow cathode lamps. The authors were able to eliminate a cyclic drift problem that plagues many users of such lamps, merely by inserting a three-port circulator (with a dummy load in one port) between their microwave power supply and the lamp. Bentley and Parsons ( 1 5 C ) showed that arsine could be used as a source of metal atoms for arsenic electrodeless lamps. The resulting
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sources are easy to make, long-lived, stable, and intense. Spectral Continua. Johnson, Plankey, and Winefordner (106C) compared a pulsed xenon arc lamp (over 100-kW peak power, 250-W average power) to a 150-W EIMAC continuous-wave xenon arc as an excitation source for atomic fluorescence spectrometry. Surprisingly, the continuous-wave source gave 10 times better detection limits, even though gated detection was employed for the pulsed source. Presumably, the EIMAC source proved better because of its far greater efficiency in focusing radiation onto a small atom-containing volume. An important observation made by the authors is that radiation scattering was not observed with either source when an inert-gas-separated air-acetylene or nitrous oxide-acetylene flame was used and supported on a capillary burner head. The superiority of the EIMAC source led Chuang and Winefordner (33C) to explore its use with a graphite filament atomizer. Atomic fluorescence detection limits obtained with this source were comparable to those derived through use of electrodeless discharge lamps. Fowler, Knapp, and Winefordner (67C) employed for atomic fluorescence a continuum source with a double modulation technique involving optical chopping (to eliminate contributions from flame emission) and refractor-plate wavelength modulation (to eliminate contributions from radiation scattering). Brinkmann and Sacks ( 2 0 C ) explored the use of exploding wires as intense ultraviolet continuum excitation sources for atomic fluorescence. At a wavelength of 200 nm, the peak radiance produced by an exploding wire is approximately lo5 that from a continuous-wave 1600-W xenon arc lamp. However, the practicality of such a source must be questioned because of its necessarily low repetition rate and inconvenience in operation. Tunable Lasers. Although tunable lasers are not now being widely used in analytical atomic spectrometry, they have great potential as sources for the future, provided their cost can be reduced and their ease of operation improved. In this section, some analytical studies which employed tunable lasers will be reviewed. Also, some physical studies will be briefly outlined that are of potential or current importance to analytical atomic spectroscopists. Review articles discussing the present state of development of tunable lasers and providing an overview of their application to atomic spectroscopy include those by Lange, Luther, and Steudel (124C), Walther (225C), and Allkins ( 7 C ) . A thorough bibliography on dye lasers was prepared by Magyar (140C) and covered the years 1966-1972. A very inexpensive tunable dye laser, developed by Harrington and Malmstadt (86C), consists of a readily assembled nitrogen laser and a commercial programmable monochromator modified with a dye cell and mirror attachments. The tunability range of this combination laser is from 358-641 nm with a spectral band-pass of about 1 nm. Godard (71C) developed an inexpensive traveling-wave nitrogen laser which would be useful for pumping some tunable dye lasers and should decrease greatly the cost of such a system. The high degree of sensitivity achievable by the use of tunable dye lasers in atomic spectrometry was demonstrated in several recent articles. Fairbank, Hansch, and Schawlow ( 6 3 C ) reported a detection limit for Na of 100 atoms/cm3, by means of optical resonance fluorescence in a heated vapor cell. Neumann and Kriese (158C) achieved sub-picogram detection of P b by atomic fluorescence, using furnace atomization. They also re orted that a laser excitation power density of 8 kW/cm Lp is necessary to produce saturation of the atomic transition of P b a t 283.3 nm. The sensitivity of atomic absorption spectroscopy has been found to be greatly increased by placing the absorption cell inside the optical cavity of a broad-band or tunable laser system. Burakov, Misakov, Nechaev, and Yankovskii (23C) used this intracavity method to determine trace amounts of K, Rb, and La salts, placed on carbon electrodes as aqueous solutions. They found a two- to threefold increase in sensitivity compared to conventional methods. Childs, Fred, and Goodman (31C) obtained a 2OOOx enhancement in the detection of Cs vapor by intracavity laser quenching. Horlick and Codding (93C) used a photodiode array for acquisition of spectral data on laser intracavity absorption of rare earth solutions. They dis152R
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cussed the effect of the cavity length on sensitivity. Green and Latz (77C) observed a somewhat surprising linear relationship between the atom concentration of Na, Ba, and Eu in an air-acetylene flame, and the amount of quenching due to intracavity atomic absorption. Maeda, Ishitsuka, and Miyazoe ( 139C) investigated intracavity absorption of atomic lines of Na, Li, Sr, Ba, and Cs in an air-acetylene flame. They reported enhancements in sensitivity and detection limits by factors of IO3 and 10, respectively, compared to conventional atomic absorption flame spectrometry. Brunner and Paul (22C) have developed a theoretical model to explain the enhancement in sensitivity produced by the intracavity absorption method. Applications of tunable dye lasers to high resolution atomic spectroscopy have been reported in several papers. Figger, Siomos, and Walther (65C) determined radiative lifetimes of single fine-structure energy levels and multiplets in the Fe I spectrum by excitation of each level with a short-pulse tunable dye laser. Svanberg and Belin (211C) used a continuous-wave tunable dye laser in conjunction with a radiofrequency lamp to produce multiple excitation steps in the determination of the hyperfine structure and GJ factors of highly excited states in alkali atoms. Rasmussen, Schieder, and Walther (182C) found a 45% reduction of the line width (compared to broad-band excitation) of the Hanle signal observed in a level-crossing experiment on Ba, performed with a narrow-band tunable dye laser. Chen and Fried (30C) determined collisional energy-transfer rates between Na atoms at 5890 and 5896 A, taking advantage of the slow effective radiation loss at high Na vapor concentrations, where trapping of radiation occurs. Burakov, Naumenkov, and Kolosovskii (24C) used a ruby-laserpumped dye to investigate spectral absorption coefficients of a plasma. The technique of multiphoton spectroscopy has become widely used since the development of powerful tunable dye lasers. Hansch, Harvey, Meisel, and Schawlow (8%) observed the optically forbidden 3s-4d transition of Na, using two-photon excitation. They obtained high resolution spectra without Doppler broadening and a line-width of less than 30 MHz, using two counter-propagating laser beams whose Doppler shifts cancel. Pepescu, Collins, Johnson, and Popescu (169C) investigated multiphoton excitation and ionization of atomic Cs. Harris and Bloom (87C) developed a resonantly two-photon-pumped frequency converter which can generate tunable ultraviolet and vacuum-ultraviolet radiation. Using a Hg-vapor cell as a nonlinear frequency-doubling medium, they observed output wavelengths from 195-200 nm. The importance to atomic spectroscopy of tunable radiation in this spectral region is obvious. Atomic vapors of Zn, Cd, and Hg have also been suggested as possible frequency-doubling media for coherent outputs in the wavelength range 138-400 nm. Hodgson, Sorokin, and Wynne (92C) also reported the generation of tunable, coherent, vacuum-ultraviolet radiation by frequency doubling in Sr vapor. Wang and Davis (226C) produced saturation of a twophoton transition in T1. They observed broadening of the transition line width and a deviation from the low-lightlevel intensity dependence for both third-harmonic generation and the induced fluorescence; this evidence indicated saturation of the transition. Ward and Smith (228C) investi ated the saturation of a two-photon process in Cs vapor. T i e y observed a greater increase in broadening of the third harmonic emission than of the fundamental fluorescence emission as laser source power was increased. Spectral Sorting Devices a n d Nondispersive Detection. Goode and Crouch (72C) described a method for measuring stray radiation in atomic absorption spectrometers. Because detected stray radiation increases as the square of the slit width, while narrow line radiation increases linearly, the relative contribution of stray light to a spectrophotometer output can be determined from a simple relation evaluated at different slit settings. Hodges and Belcher (91C) used the grating of a grazing-incidence vacuum spectrometer as a mirror by setting it to zero order. A small second monochromator placed at the exit slit of the large spectrometer then served to isolate the visible lines of analytical interest. Rubinovich and Kurochkina (188C) evaluated the effect of slit width and other parameters on the sensi-
tivity of atomic analysis. However, because their considerations did not include spectral interference, the findings are of limited utility. In flame emission spectrometry, it is usually necessary to correct for a background spectrum; this correction can often be accomplished through simple but clever means. The repetitive optical scanning (wavelength modulation) technique for background correction was applied by Rains and Menis (180C) to the determination of aluminum in a nitrous oxide-acetylene flame. With this method, it was possible to overcome spectral interference from C2, CH, CN, and OH bands. The refractor-plate method for repetitive scanning was discussed in more detail by Epstein and O’Haver (61C), who showed how to optimize and set up the spectrometric system. They also demonstrated that an overlapping spectral line can be eliminated through proper choice of the modulated wavelength interval or of the slit width. A rotating acrylic refractor plate was utilized by Katskov, Kruglikova, L’Vov, Orlov, and Polzik (112C) for wavelength modulation. A clever method for sending two wavelengths through the top and bottom of a spectrometer exit slit was described by Brinkmann and Sacks (20C). The method uses a quartz plate to refract one of the beams and divert it from its normal path to a path through the exit slit. The high resolution of an Echelle spectrometer enables high sensitivity atomic absorption determination of many elements with a continuous primary source, according to Keliher and Wohlers (114C). With this combination, sensitivity was only slightly less than that obtainable with a hollow cathode lamp, and calibration curves were nearly as linear. Although the authors were not able to work below 300 nm because of limitations in the spectrometer and in their primary source, the limitations are not fundamental and can be overcome, as a publication now in press will show. The thesis by Bartschmid (12C) and the papers by Tsujii, Kuga, and Sugaya (219C, 220C) and by Chupakhin, Dorofeev, and Aidarov (34C) testify to the utility of nondispersive atomic fluorescence photometry, even in practical situations. The excellent paper by Larkins and Willis (130C), however, points out that scattering can be a serious problem in nondispersive systems, especially during the analysis of materials which form refractory oxides in the atom cell. Ordinarily, scattering in nondispersive systems is no more than fivefold worse than that encountered in monochromator-based systems. Because the scattering is largely of Mie origin, polarizers are inefficient and impractical in eliminating it. Methods for minimizing scattering in atomic fluorescence will be discussed in a later section of this review. Dispersionless photometers employed in atomic absorption were discussed and applied by Dorofeev and his co-workers (55C, 56C) and by Talalaev, Miranova, and Brevnova (212C). Aidarov, Dorofeev, and Ivanov (4C) designed an inexpensive non-dispersive atomic fluorescence system employing solar-blind photoresistors. Photodetectors and Detector Arrays.. A high level of activity continues in the development and exploitation of photodiode arrays and television-type readout systems. Two types of photodiode arrays are in common use: linear arrays and two-dimensional matrices. Most of the work performed with linear detectors has been by Horlick and Codding (94C). These authors recently explored multielement and multiline atomic absorption analysis and showed how several hollow-cathode lines of a given element could be detected simultaneously t o increase signal-to-noise ratio. However, even with this advantage, linear array detection produced rather poor signal-to-noise ratios, despite the use of relatively concentrated analytical solutions. An excellent and critical review of two-dimensional diode arrays and television-type multichannel detectors was prepared by Talmi (213C, 214C). Talmi explained the operating characteristics of each commonly used kind of detector and illustrated where each kind would provide the greatest advantage in its application to atomic spectrometry; this material should be required reading for all those interested in utilizing detector arrays. Milano, Pardue, Cook, Santini, Margerum, and Raycheba (144C) designed an extremely flexible vidicon scanning spectrometer and applied it to atomic emission spectrometry. The spectrometer re-
sponded linearly over a range of four orders of magnitude but was resolution-limited by its optical system to approximately 1 nm. A thorough evaluation of the device was presented and a comparison made with linear diode arrays. The same device was applied by Cook, Milano, and Pardue (41C) to the simultaneous flame emission determination of sodium and potassium in blood serum. Knapp, Omenetto, Hart, Plankey, and Winefordner (122C) employed a commercial ultraviolet-enhanced vidicon tube for the measurement of emission from an argonseparated nitrous oxide-acetylene flame. Detection limits were presented for 11 elements and the existence of some spectral interferences was noted. Busch, Howell, and Morrison (26C) employed a similar silicon vidicon tube to monitor a 20-nm spectral window a t approximately 1.4-8, resolution. They listed a number of 20-nm spectral windows, each of which contains spectral lines of several elements which could be simultaneously determined. Busch, Howell, and Morrison (27C) also employed a silicon intensified-target tube to simultaneously measure emission from calcium, sodium, and potassium in a nitrous oxide-acetylene flame. Small quantities of serum solution were injected into the flame and the resulting emission signal integrated by the commercial vidicon system. One of the most generally useful applications of television-type detectors was that described by Wood, Dargis, and Nash (230C). In their system, a prism-Echelle spectrograph provided two-dimensional spectral dispersion; the dispersed spectrum was then amplified by an image intensifier tube and recorded by a random-access television camera having a secondary-electron-conduction target. The camera could be made to select any spectral line imaged on its face and to determine the line magnitude. Up to 400 such wavelength and intensity measurements could be made each second and a t any point within the 230-860 nm spectral range. A resolution of 0.3 8, was obtained although the authors made no mention of linearity or dynamic range of their device. Jackson, Aldous, and Mitchell (103C) applied a silicon vidicon tube to the atomic absorption determination of wear metals in used lubricating oils. Sensitivity was found to be somewhat lower than obtainable with conventional single-channel instruments; some spectral interferences were also noted. In addition, because their commercial vidicon readout system responded quite slowly, primary source modulation could not be used and flame emission interference occurred. The same device was applied to the determination of trace metals in water. The authors of the present review believe that photodiode arrays and television-type detectors offer great promise in analytical spectrometry. However, a t the present time, their limited size dictates a compromise between coverage of a broad spectral interval at poor resolution or the examination of fewer spectral lines a t greater resolution. Although future detectors might be produced in a larger format, cost would likely increase proportionately. For this reason, the devices probably offer their greatest promise as two-dimensional detectors for recording Echelle spectra in the manner of Wood, Dargis, and Nash (230C). Panichev and his co-workers (166C, 167C) used a scintillation recording technique to measure the concentrations of elements in powdered samples fed into a flame. Extremely low detection limits were obtained, but precision was not high. Bower and Ingle (18C) described an inexpensive device for the accurate determination of photomultiplier current gains. Systems for Multielement Analysis. An excellent review on multielement methods in atomic spectrometry, prepared by Winefordner, Fitzgerald, and Omenetto ( 3 5 A ) , was discussed in the present review under the section entitled “Reviews, Books, and Bibliographies.” Consistent with the views of that publication, Johnson, Plankey, and Winefordner (107C) designed a computer-controlled rapid-scan atomic fluorescence spectrometer which employed a spectral continuum source. Wavelengths at which the fluorescence of each element was to be measured were selected via keyboard commands to the computer. The controlled spectrometer would then rapidly slew to the desired wavelength where fluorescence would be measured by a photon-counting system until the desired signal-to-noise ratio was obtained. The spectrometer would then be automatically ANALYTICAL CHEMISTRY, VOL. 48,’ NO. 5, APRIL 1976
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slewed to the next wavelength of interest, etc. The system yielded excellent detection limits, seemed simple to operate, and would not be outrageously expensive. Another approach to multielement atomic fluorescence analysis was taken by Palermo, Montaser, and Crouch (165C). In their system, hollow cathode lamps of the elements of interest were sequentially and rapidly pulsed. A nondispersive detection system em loying a solar-blind photomultiplier tube then responde! to the fluorescence of each element in turn and produced a signal which was decoded by an online computer. Noise levels in this instrument prevented detection limits from being equal to those obtainable by sequential atomic fluorescence measurements; also, scattering of radiation by nonvolatile matrices will likely be more of a problem in a nondispersive system. Clyburn, Bartechmid, and Veillon (38C)employed a continuum source and photon counting for atomic fluorescence spectrometry from a graphite atomizer, but did not explore in detail multielement applications of the device. The relatively straightforward but somewhat expensive approach of Falchuk, Evanson, Vallee, and Matthews (64C) used a multielement hollow cathode lamp, a polychromator, and a number of detection channels to record atomic absorption from several elements simultaneously. The system was applied to the determination of zinc, copper, and cadmium in biological materials. A dispute over the applicability of multiplex techniques to atomic spectrometry has arisen in the literature, as pointed out earlier. Plankey, Glenn, Hart, and Winefordner (173C) constructed a Hadamard-transform spectrometer for use in atomic spectrometry but obtained extremely lowquality signals. Because such a spectrometer measures all incoming radiation a t once, strong background emission or radiation from one or two intense spectral lines will produce shot noise from a photon detector (such as a photomultiplier tube) that can obscure the signal from a weak atomic line. However, Horlick and Yuen (95C) feel this situation is unlikely to be important in atomic spectrometry, where strong, narrow lines are ordinarily measured against a low background. The authors of the present review do not agree with this latter opinion, since even one strong radiation line or band could produce sufficient shot noise to limit the detectability of a weak line. Horlick and Yuen (95C) also show, however, that a Fourier-transform multiplex spectrometer can be useful in atomic spectrometry in providing a larger optical aperture and in enabling the convenient .detection of a large number of spectral lines through “aliasing”. Fourier transform techniques were also applied to multielement atomic fluorescence spectrometry by Fuller (69C). Automation a n d Optimization of Flame Spectrometers. The study by Skene, Stuart, Fritze, and Kennett (202C) illustrates an excellent approach to computerization of instrumentation. The authors began by determining the response time of the instrumental system, and defined data rates, necessary inputs, outputs, and desirable precision, linearity, and accuracy. From these criteria, necessary interfacing hardware and software were developed and a completely automated spectrometer was implemented. The thesis by Kaley (109C) describes an atom reservoir useful in automated flame atomic fluorescence spectrometry. Pierce, Brown, and Fraser (172C) combined a commercial atomic absorption spectrophotometer with standard components from an automatic analyzer in developing a technique for the totally automated determination of inorganic iron. Houle (96C) described an interface between an atomic absorption instrument and a programmable calculator while Bankston and Goldsmith ( I I C ) presented a flexible computer program useful for treating atomic absorption data. Surskii and Avdeenko (210C) described a clever arrangement for the automated calibration of a graphite-furnace atomic absorption spectrophotometer. In the method, a second graphite furnace is used to repetitively vary the atom concentration in the absorbing light beam by a known amount. The amount of absorption produced by this second vapor cell can then be related to the slope of a calibration curve, from which the concentration of a sample present in the first graphite furnace can be determined. 154R
ANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
Green and Nixon (76C) statistically examined a number of parameters in atomic absorption spectrophotometry and discussed ways of evaluating the importance of each. Ramirez-MuAoz (181C) published a table which enables the performance of a specific atomic absorption instrument to be evaluated in light of the average performance of similar devices. Parker, Morgan, and Deming (168C) optimized a number of experimental factors in atomic absorption usin a simplex algorithm, while Adrienssens and Knoop (2C7 determined optimal conditions for the nonflame atomic absorption determination of Ir, Pt, and Rh. O t h e r Instrumental Innovations a n d Evaluations, Several novel methods have been recently pro osed to overcome the problem of radiation scattering anznonspecific absorption which sometimes occurs in atomic absorption. Koizumi and Yasuda (119C) used the Zeeman effect in the determination of traces of Hg (2537 A) in a hightemperature furnace. In this method, the intensities of the x and u* components are measured by using two linear polarizers; the x component serves as an atomic absorption line for Hg in the sample and the cr* components as internal standard (background) lines. The advantages of the Zeeman approach are its sensitivity, freedom from chemical pretreatment of the sample, speed, and convenience of use. Unfortunately, practically useful Zeeman splitting occurs only for a small number of chemical elements, thereby limiting the range of application of “Zeeman modulation”. Stephens and Ryan (204C) designed three dc discharge lamps which maintain a stable plasma in a magnetic field and are consequently useful for application of the Zeeman effect to analytical atomic spectrometry. They discuss the behavior of the lamps a t varying magnetic field strengths, filler gas pressures, and operating currents. Stephens and Ryan (205C) also reported a double-beam atomic absorption spectrometer in which the sample and reference beams are generated by Zeeman splitting of the source emissionline. Donnelly, Eccleston, and Gully (52C) developed an automatic background-correcting system suitable for use with transient ( 0 . 5 s duration) atomic vapor populations. The device consists of two alternating pulsed light sources: a normal hollow cathode lamp as the analyte spectral lamp and a modified hydrogen-arc lamp as the continuum source. The practical advantages of the device were demonstrated by Donnelly and Eccleston ( 5 3 C ) in the determination of tin present in rock samples. They reported a detection limit of 1 pg of Sn for a 1-g starting sample atomized in a graphite furnace. Donnelly, Ferguson, and Eccleston (54C) used the same method with a carbon-rod atomizer for the determination of Pb, Zn, Fe, Cu, Ni, Co, and Cd in sea water. Automatic systems for correction of light scattering in atomic fluorescence spectrometry have also been reported. Rains, Epstein, and Menis (179C) developed a system in which radiation from a scattering source (xenon lamp) and a line source (electrodeless discharge lamp) were alternately passed through the flame gases by a rotating-sector mirror. The applicability of the method was demonstrated by the determination of 0.11 and 0.26 pg Cd per gram of Standard Reference Orchard Leaves and Liver, respectively. Epstein, Rains, and M6nis (62C) also employed the automatic scatter-correction method to the determination of traces of Cd and Zn in coal, bovine liver, orchard leaves, and fly-ash Standard Reference Materials without prior separation or preconcentration. They evaluated the effects of scattering and chemical interferences, for premixed argon(entrained air)-hydrogen and air-acetylene flames. Precision and accuracy of the results obtained with this method were compared to data from analysis by atomic absorption. Dujmovic, Adrian, and Knoezinger (59C) designed a new double-beam instrument which utilizes both a line and a continuum spectral source to compensate for nonspecific losses of light in atomic absorption. Compensation of quickly varying, large nonspecific light losses (up to a t least 1.0 absorbance unit) is possible. The same authors (60C) evaluated the use of true integration in atomic absorption. The advantages of this method over analog or digital signal averaging were demonstrated in the trace determination of Hg in water (by cold vapor generation) and Cu in urine (using an air-acetylene flame). The reported
detection limits are 2 x 1O-I3 g/ml Hg and 1.3 X g/ml cu. Morren (153C) showed how the relative contribution of narrow and broad spectral features to a given measurement could be ascertained without resorting to wavelength-scanning. Because the broad-band feature will generate a measured signal proportional to the square of the spectral bandwidth while the narrow-band feature will contribute in linear proportion, the amount of each can be determined from the recorded variation in signal with spectrometer slit width. Becker-Ross and Falk (13C) used a specially designed hollow cathode lamp and a detector with a CsI photocathode to measure absorption by bromine atoms in the vacuum-ultraviolet spectral region. Because the CsI photocathode does not respond to wavelengths longer than 190 nm, interference from extraneous sources of radiation is minimized and instrumentation can be simplified. Application of the approach to other vacuum ultraviolet elements (F, C1, Br, I, H , S, Se, P , As) should be feasible. Although the combination of chromatographic techniques with flame emission spectrometry is quite well established, the use of on-line atomic absorption with separations methods is becoming increasingly popular. Segar (195C) fed the effluent from a gas chromatographic column directly into a graphite furnace for the determination of Hg and Pb. Although Cr determination was attempted, it was unsuccessful because of excessive heat build-up in the column. Chau, Wong, and Goulden (29C) employed a silica furnace to decompose gas chromatographically separated selenides into atomic Se, which was then determined by absorption with a hollow cathode lamp source. Coker (40C) developed a simple, rapid technique for the determination of individual and total lead alkyls in gasoline using a combination of gas chromatography and atomic absorption. Herrmann and his co-workers (82C, 9OC) developed an indirect method for measuring fluorine eluted from a gas chromatograph. When passed through an absorption oven heated to 680 OC, fluorine increased the volatility and atom concentration of sodium located there; atomic absorption detection of the sodium then provided an indirect detection limit for fluorine of 11 ng. Yoza, Kouchiyama, Miyajima, and Ohashi (232C) connected an atomic absorption unit to a gel chromatographic column and were able to detect condensed phosphates by measurement of atomic absorption of Mg complexed to them. A conventional totalconsumption burner (air-hydrogen) was used as a photometric detector for high-pressure liquid chromatography by Freed (68C). Burgett and Green (25C) discovered that hyperventilating a hydrogen diffusion flame with oxygen enabled the flame to handle up to 50 gl of solvent without causing flame-out; Cr, S, and P were determined with the new detector. Micromethods for use in atomic absorption spectrophotometry were developed by Antonetti, Ducros, and Olivie (8C) and by Danielson and Oberg (46C). The first of these papers describes the use of a tiny graphite crucible under partial vacuum for sample vaporization. With the device, 50 pg/ml could be determined in as little as 7 nl of solution. In the second study, a Pt-Ir filament was employed to introduce microvolumes of sample solution into a hydrogenair flame. Sample volumes as small as 1 nl yielded detection limits in the range of lo-" g, provided the signal was integrated. Seeley and Skogerboe (194C) described a method in which porous spectroscopic graphite electrodes were used as filters for atmospheric particulates. Although the particulate samples so gathered were analyzed by dc arc spectroscopy, atomic absorption or fluorescence could also have been used. DEVELOPMENT IN T E C H N I Q U E A N D PROCEDURE Elimination, Discussion, a n d Discovery of I n t e r f e r ences. The increased use of nonflame atomizers for trace analysis has led to the discovery of a wide variety of interference effects. Numerous effects found for graphite atomizers including metal oxide reduction and vapor phase reactions were discussed by Aggett and Sprott ( 3 0 ) . Interferences encountered using a tantalum boat were studied by Schrenk and Everson (1750). The aging of carbon cups vastly reduced the precision of plasma zinc determinations
as reported by Chooi, Todd, and Boyd (380).Matrix problems were also encountered which were not totally obviated by use of the standard additions procedure. Findlay, Zdrojewski, and Quickert ( 5 3 0 ) emphasize the need for precise control of drying and ashing temperatures to avoid pre-atomization volatilization of the element of interest. Leucke, Eschermann, Lennartz, and Papastamataki (1090) studied interferences encountered in nonflame AA analyses of geological samples. Large quantities of inorganic ions, acids used in the decomposition steps, and high concentrations of dissolved silicate demand careful thermal decomposition of the sample to reduce interferences as much as possible. Gomiscek, Lengar, Cernetic, and Hudnik ( 6 3 0 ) discuss problems associated with the nonflame atorpization of volatile chelates (tetramethylenedithiocarbamates). Tominaga, Kimura, Miyazaki, and Umezaki (1970) studied the determination of cadmium in detail. Acids did not seriously interfere, nor did 21 metal ions. However, Mg, Cu, Fe, Co, and Ni chlorides seriously impaired the analysis. Flameless AA determination of Hg (by reduction/Hg evolution techniques) also suffers from several interference effects. Clifton ( 3 9 0 ) has shown that the temperature of the Hg reducing solution greatly affects the rate of Hg evolution and must therefore be controlled or the signal must be integrated. Vitkun, Zelyukova, and Poluektov (2060) demonstrated that Hg forms compounds with Se and T e (HgSe, HgTe) and recommend using formaldehyde, which does not reduce Se or Te, as the reducing solution. Jonasson ( 8 1 0 ) has found that interferences on the absorption signal of Hg by Cu(I1) and Ag(1) (which are severe) can be overcome by eliminating C1- from the reducing solution. The discovery of interelement interferences in flame techniques and some procedures for minimizing certain effects are still topics of considerable interest. Chemical interferences have received most attention. Bradfield ( 2 7 0 ) has noted a suppression of the atomic absorption of Mn in plant materials if Ca, Mg, and S04-2 are present. Carter ( 3 4 0 ) has found that aluminum strongly suppresses the AA signal due to Sr in the nitrous oxideCPHSflame. In addition, Ca and Mg act as releasing agents. Varying concentrations of these elements in silicate rocks lead to widely varying results unless excess releasing agent, e.g., La, is added to all samples. Begak ( 1 1 0 ) has found that Ca, Ba, and La, all suppress the AA signal of Mo while individual acids can cause enhancement or suppression. Childs and Gafke ( 3 7 0 ) found that although numerous concomitants of Cd and P b in fish tissue cause interferences, the composite effect is (38C) Clyburn, S. A., Bartschmid, B. R., Veilion, C., Anal. Chem., 48, 2201 (1974). (39C) Clyburn. S. A., Kantor. T., Veillon, C., Anal. Chem., 48, 2213 (1974). (40C) Coker, D. T., Anal. Chem., 47, 386 (1975). 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171 R
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Analytical Comparlsons and Figures 01 Merlt
Appllcatlons
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172R
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
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