Anal. Chem. 1980, 52,2 9 0 R - 3 0 5 R
Flame Emission, Atomic Absorption and Fluorescence Spectrometry Gary Horlick Department of Chemistry, University of Alberta, Edmonton, Alberta‘ T6G 2G2, Canada
INTRODUCTION-BOOKS, REVIEWS, A N D BIBLIOGRAPHIES
Spectroscopy’’ reviewing 19’77 and 1978 are now available through The Chemical Society, Distribution Center, Blackhorse Road, Letchworth, Herts. SG6 I H N , England. Each of these volumes are about 300 pages long and contain a total of about 3200 references. T h e large number results, in part, from the extensive listing of conference proceedings listed along with literature references. In addition excellent semiannual bibliographies are published in the Atomic Absorption Newsletter (13A, 19A-2IA). These bibliographies are not limited to atomic absorption but also cover, for example, papers dealing with ICP’s and dc plasmas. Each entry has the complete title and each bibliography contains a complete applications category and element index. Over the last two years 1262 papers have been listed in these bibliographies. Some papers of general interest that have appeared in the last couple of years include one by Alan Walsh entitled “Atomic Spectroscopy-What Next” (24A). In this paper he notes that within the last 25 years the two major advances that occurred in atomic absorption spectrometry were the development of the N20-C2H2 flame and electrothermal atomization, both developments in atomization systems. He feels major innovations are still required in this part of the atomic spectroscopy experiment. Plasma systems and direct solids analysis are areas of the future. Another general review was published by Pinta (15A) entitled “Present Tendencies in Atomic Spectroscopy” which includes 76 references. Willis (25A) reviewed 24 years of analytical atomic spectroscopy at the CSIRO Division of Chemical Physics in Clayton, Victoria (Australia). Also, while not specifically dealing with flame methods, the three issues of Spectrochimica Acta designated the Kaiser Memorial Issues I, 11, and I11 ( I A ) contain several papers of interest to atomic spectroscopists and analytical chemists. Several books of interest to analytical atomic and flame spectroscopists have appeared in the last two years. A new addition of “Flames, Their Structure, Radiation and Temperature” by Gaydon and Wolflard was published (IOA). A new edition of another old standby also appeared in 1979. “Fundamentals of Analytical Flame Spectroscopy” by Alkemade and Herrmann ( 4 A ) has been published by Adam Hilger. An excellent book within its scope (Le., flame) it suffers only in that electrothermal methods are not covered to any extent. However, the flame emission spectral tracings, an excellent feature of the first edition, have been retained. Other books devoted t o AA a n d flame methods include “Spectrochemical Analysis by Atomic Absorption” by Price ( 1 6 A ) , a n d second editions of “Atomic Absorption Spectroscopy” by Slavin ( I 7A) and “Atomic Absorption, Fluorescence and Flame Emission Spectroscopy. A Practical Approach“ by Thompson and Reynolds (23A). Solvent extraction is often a critical part of atomic analytical methods. “Solvent Extraction in Flame Spectroscopic Analysis” by Cresser (6A) and “Separation and Concentration Techniques for Atomic Absorption: A Guide to the Literature” by Wilson (26A) review and document the field. Wilson‘s article tabulates the literature from 1970 to 1978 and contains 71 references. A couple of related books include “Applied Atomic Spectroscopy” edited by Grove ( I IA)and “Analytical Laser Spectroscopy“ edited by Omenetto (13A). Among the new journals a quarterly journal entitled Progress in Analytical Atomic Spectroscop?, has appeared. The first issue contained a major review entitled “Recent advances in electrothermal atomization in graphite furnace atomic absorption spectrometry“ by Sturgeon and Chakrabarti (3A). Finally, Spectrochimica Acta devoted an entire issue ( 2 A ) to “Nomenclature, symbols, units and their usage in spectro-
The passage of a decade frequently becomes a time to take stock of a field of endeavor. As flame based analytical techniques move from the 70’s to the ~ O ’ S ,major changes and developments are occurring in the field. From the point of view of the published literature of the last two years the flame has diminished somewhat in importance as a system for generating, studying, and utilizing atomic vapor. Over the last two decades steady growth has occurred in the so-called “nonflame” methods of atomization and the literature of the field is now clearly dominated by electrothermal atomization atomic absorption spectrometry and related techniques. Along with this trend, as predicted in the last review (12A)plasma systems in general and, in particular, the inductively coupled plasma (ICP) are currently undergoing exceptional growth in analytical utilization. The general area of plasmas is covered by P. N. Keliher in his review on “Emission Spectroscopy” in this issue. ICP systems are now beginning to compete head-on with atomic absorption spectrometry as emphasized and illustrated in Boumans et al. ( 5 A ) . In addition over the last couple of years traditional manufacturers of atomic absorption instrumentation have developed and are beginning to market inductively coupled plasma systems (9A.22A). As a final comment in this area I would like to quote, in part, from the recent editorial of Sabina Slavin in the last Atomic Absorption Neusletter of the decade (i8A): “To provide our readers with a more comprehensive picture of atomic spectroscopy today, we will now accept papers submitted in the related fields of atomic fluorescence and atomic emission, particularly plasma emission, as well as atomic absorption. T o reflect this expanded coverage more accurately the journal will become Atomic Spectroscop3 as of January 1980.” Despite t h e above comments, considerable activity continues in flame research and related areas. The flame remains and will continue to be one of the simplest, most effective, and inexpensive sj.stems available for the generation and utilization of atomic vapor for analytical purposes As will be seen in this review considerable work at both fundamental and practical levels is being carried out. The topics covered in this review are grouped into 1 2 major areas: Books, Reviews, and Bibliographies; Fundamental Studies in Flames; Developments in Instrumentation, Measurement Techniques and Procedure; Flame Emission Spectrometry; Flame Atomic Absorption Spectrometry; Flame Molecular Absorption Spectrometry; Electrothermal Atomization Atomic Absorption Spectroscopy; Hg Cold Vapor Atomic Absorption Spectrometry; Hydride Generation Techniques; Graphite Furnace Atomic Emission Spectrometry; Atomic Fluorescence Spectrometry; and Analytical Comparisons. T h e cited literature is also grouped under these headings and, where applicable, appropriate subheadings. It is not within the size scope of the ANALYTICAL CHEMISTRY Fundamental Review issue for the reviews to be all encompassing and, in fact. reviewers are being encouraged to be as succinct as possible. The aim of this reviewer was to be reasonably complete in dealing with what might be called the fundamental aspects of the field and representative with the applications aspects of the field always keeping the mind that the focal point was the utilization of flames and electrothermal atomizers as sources of atomic vapor for analytical measurements. The Chemical Society, London, continues to publish excellent comprehensive reviews (7A, 8 A ) . Volumes VI1 and VI11 of t h e “Annual Reports in Analytical Atomic 290 R
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1980 American Chemical Society
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Gary Horllck is professor of chemistry at the University of Alberta. He received his B.Sc. degree from the University of Alberta in 1965 and his Ph.D. degree from the University of Illinois in 1969 under the direction of H. V. Malmstadt. He joined the faculty at the University of Alberta in December 1969. Research interests are in the general area of analytical spectroscopy with emphasis on the development and application of atomic spectrochemical measurement systems. Particular research interests include experimental and theoretical characterization of analyte emission in inductively coupled plasmas, development of new sample introduction systems for inductively coupled plasmas, development of new sample introduction systems of inductively coupled plasmas, the application of integrated circuit detector arrays to spectrochemical measurements, the development and utilization of Michelson interferometers and Fourier transform spectroscopy for spectrochemical measurements, the development of correlation-based measurement and data handling techniques applicable to chemical systems, and minicomputer and microprocess-based acquisition and processing of chemical data. He is currently the North American editor of Spectrochimica Acta, Part 6 , and is a member of the editorial boards of Applied Spectroscopy, Mikrochimica Acta, and Canadian Journal of Spectroscopy.
chemical analysis”. This issue contains articles dealing with IIJPAC recommendations on: I. General Atomic Emission Spectroscopy, 11. Data Interpretation, 111. Analytical Flame Spectroscopy and Associated NonFlame Procedures.
FUNDAMENTAL STUDIES IN FLAMES This section deals with developments in flames, aerosol measurements, basic studies of processes in flames directly related to analytical techniques, noise characterization of analytical flame systems, applications of Raman spectroscopy to flames, and acoustic measurements in flames. The sections on flame emission spectrometry, flame atomic absorption spectrometry, flame molecular absorption spectrometry, and in particular atomic fluorescence spectrometry also document some papers of fundamental interest. Developments in Flames. A methane-oxygen--air flame has been characterized by Cordos and his colleagues (6B, 7B). A maximum flame temperature of 2253 K was obtained and both atomic emission and fluorescence measurements were carried out. Lower temperature flames for atomic absorption measurements (air-propane) have been used by Tsujino et al. (23B) and Razumov (20B). A nitrous oxideehydrogen flame has been used for atomic absorption and emission spectrometry of organic solvent systems (15B). Direct aspiration of volatile solvents at high uptake rates without carbon formation was possible. Air-organic solvent (toluene, benzene, octane, or hexane) flames have been studied for use in atomic absorption analysis by Morozov et al. (18R). Several articles of potential interest to flame spectroscopists appear in the journal Combustion and Flume. An extensive list of reaction rate coefficients for flame calculations has been listed by Jensen and Jones (12B). Inhibition of air-H, flames by HBr has been discussed by Dixon-Lewis (8B). The emission spectra of premixed CO-F, (25B)and CH,-F2 flames (24B) have been presented and low pressure H 2 / 0 2(26B)and CS2/02 ( I B ) flame; have been studied. The mechanism of formation of nitrogen oxide in hydrocarbon-air flames has been studied spectroscopically ( I 7 B ) . The ion chemistry of CH4-02 flames has been studied in detail by Goodings et al. (9B,IOB) and the ion composition of oxyacetylene flames has been studied by Hayhurst and Kittelson ( I I B ) . Ions in flames have also been studied using mass spectrometry ( 4 B ) ,electrostatic probes (22B),and a Langmuir probe (16R). Temperature measurements and OH concentration in a lean CH,-air flame have been obtained using high resolution laser absorption spectroscopy ( 1 4 B ) ,and a three wavelength pyrometer for measuring flame temperatures has been presented ( 5 B ) . Reif et al. (21B) discussed spectroscopic flame temperature measurements and their physical significance in the fifth article of a series. More specifically, experimental relative transition probabilities for 43 FeI lines in the 3450-3950 A region were presented. In two articles Brown and Parsuns ( 2 B , 3B) measured relative atomic transition probabilities using both line and/or continuum source atomic absorption in an air-C2H2 flame. Problems associated with the line source
approach when hyperfine structure exists were discussed. Results were presented for Cu, Ag, Mn, Mo, Na, Sc, T i , Cr, Fe, Co, and Ni. Transition probabilities for P b I (13B) and AgI ( I 9 B ) have also been presented in the literature. Aerosol Measurements. Over the last few years there has been considerable renewed interest in the aerosol generation step of atomic spectrochemical methods. This has been spurred by several factors including the development of the ICP and the requirement of aerosol generation at relatively low gas flows, the realization that subtile interference effects can occur at the aerosol generation and transport step (39B), and the fact that the limiting noise source for a spectrochemical determination may occur a t the aerosol generation step. Developments in ICP aerosol generation (nebulization) systems are discussed by P. N. Keliher in his review on “Emission Spectroscopy” in this issue, and developments in nebulizers for flame systems are discussed later on in this review under the heading “Sample Introduction into Flames”. In this section developments in aerosol measurements will be discussed. For those interested in obtaining a background in this area the book edited by Shaw (38B entitled “Fundamentals of Aerosol Science” is a good starting point. The Journal of Aerosol Science contains several papers dealing with aerosol characterization. Hinds et al. (29B)discussed the application of polarization measurements to size measurement of aerosols. McComb and Salih (32B)and Ruso et al. (37B) utilized laser doppler velocimetry to characterize aerosols. The utilization of inertial impactors was presented by Rao and Whitby (36B) and Prodi et al. (35B). Norden and Van As (33B)presented a method for generating indium aerosols for atmospheric tracer studies. Nottrodt et al. (34B) compared atomic absorption spectrometry and proton induced X-ray emission (PIXE) as methods for the measurement of absolute element concentrations in aerosols. Particle and droplet size measurement systems have been presented and discussed by Ho et al. (30B), based on laser scattering measurements, by Jones et al. ( 3 I B ) based on holography, and by Curoon and Borman (27B)based on Fraunhofer diffraction. Driscoll et al. (28B)have carried out submicron particle size measurements in a C2H2-O2flame. Basic Studies of Processes i n Flames Directly Related to Analytical Techniques. The single droplet technique pioneered several years ago by Hieftje and Malmstadt (45B) continues to provide some of the most clear-cut fundamental data on flame spectrometric processes. In two excellent papers Boss and Hieftje (42B, 43B) presented theoretical and experimental studies of the spatial dis-ribution of atoms surrounding an individual solute particle vaporizing in an analytical flame. The models described the synergic effects of analyte vaporization and diffusion rates on the spatial distribution of atoms surrounding an individual solute particle. In their experimental study the radial analyte distribution of individual atomic clouds was determined and used to verify the theoretical model. Diffusion coefficients of analyte atoms were determined and the lateral diffusion interference of phosphate on calcium in N20-C2H2flames was studied. It was found that the calcium vaporization rate and not its diffusion coefficient changes with addition of phosphate, causing the observed alteration in spatial distribution. Boss and Hieftje (41R) also presented a new accurate method for the measurement of rise velocities in laminar flames based on the single droplet sample introduction technique. Droplet generator-based techniques are also being developed and utilized in several other laboratories. Joshi and Sacks (47B) described a unique circular slot burner-droplet generator system in which sample solution in the form of equally spaced, uniform size droplets produced by a piezoelectrically driven pinhole droplet generator is introduced along the flame axis through a hole in the burner. T h e burner is designed to support a premixed laminar flow N20-C2H2 flame. Radial vapor transport of Ca from CaC1, particles appeared to be diffusion controlled in a region of the flame that had a fairly uniform temperature of 2850 f 50 K. Tn a following paper Joshi and Sacks (48B) used this system to indicate that ionization of individual solute vapor clouds in this flame was mass-action controlled. Holcombe et al. (46R) used a droplet generator combined with a desolvation chamber to study vaporization and atomization of large particles in an acetylene air flame. The Cl-, N O i , SO4’ salts of Cu and Zn were studied and spatial effects shown to be significant. Fernandez ANALYTICAL CHEMISTRY, VOL. 5 2 , NO 5, APRIL 1980
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and Bastiaans (44B) used a “drop machine” to obtain spacially resolved flame temperature measurements with Co as the thermometric species. Overall the foregoing seven papers contain a wealth of information and results about basic flame processes of interest to analytical spectroscopists. They attest to the power of the single droplet technique and should be consulted for more details. Several other studies of basic flame processes have also been presented. A detailed theory of atom vapor transport and distribution of an analyte in an analytical flame has been developed by Li (49B) using a moving point source model. Roos (50B) discussed particle volatilization and calibration curvature in flame spectrometry. Curvilinearity in calibration graphs was confirmed a t high concentrations of Si(NO&. Schlierin studies were used by Szivos et al. (52B)to study the correlation between flame geometry and atomic absorption sensitivity of Ag, Cu, Fe, and P b in H 2 0 and organic solvents. Bellan and Summerfield (40B)examined, theoretically, the assumptions commonly used for the gas phase surrounding a burning droplet. She et al. ( 5 I B ) described a method of measuring the velocity of atoms in real time based on a laser time-of-flight velocimeter operating on the principle of laser resonance fluorescence. Free atom fractions ( p ) and equilibrium compositions in flame spectrometry were calculated by Wittenberg et al. (53B). Noise Characterization of Analytical Flame Systems. Several very interesting papers dealing with signal-to-noise ratios and noise characterization in analytical spectrometry have appeared in the last two years. In a major two-part article, Alkemade et al. (55B) and Boutilier et al. (57B) presented a review and tutorial discussion of noise and signal-to-noise ratios in analytical spectrometry. Boumans (3%) presented a tutorial review of some elementary concepts in the statistical evaluation of trace element measurements with a good discussion of detection limits. Alkemade et al. (54B), in an excellent paper, presented a detailed discussion of emission noise spectra from premixed sheathed C2H2-air flames. Noise power spectra from 0.02 Hz to 20 kHz are reported for the emission of sodium and background and detection limits are discussed in the context of the noise frequency distribution results. Noise power spectra of flame atomic absorption spectrometric measurements were presented by Bower and Ingle (58B). Noise power spectra from 0 to 5 Hz are presented for the 100% T, 0% T, and analyte signals in atomic absorption spectrometry. A l / f limiting noise component is shown to exist in lamp, flame transmission and analyte absorption signals, limiting effective integration times to 10 s or less. Bower and Ingle (59B, 60B) have also carried out detailed studies of the optimization of instrumental variables in flame atomic absorption spectrometry. Fujiwara et al. (62B) evaluated the spectral (Le., as a function of wavelength) noise distribution in analytical flames. Total, shot, and flicker noises were measured for both emission and fluorescence (Eimac excitation) signals in air-C2H2, N,O-C,H,, N20-propane, air H,, and an isooctane liquid fuel flames from 200 to 600 nm. inally Verbeek and Ure (62B) were able to improve precision in the determination of Ba and A1 by atomic emission spectrometry by servomechanical stabilization of CN band emission in a N20-C2H,. T h e CN band head a t 387.2 nm was monitored with one channel of a two channel instrument and the signal used to control the flow rate of acetylene to the flame. Applications of Raman Spectroscopy to Flames. A number of applications of Raman spectroscopy to fundamental studies of flames are now appearing. These are briefly reviewed in this section. Laser Raman gas-diagnostic techniques have been reviewed by Williams and Stenhouse (76B). The determination of the density and temperature of gaseous systems and the concentration of species present is considered and the techniques discussed include (a) rotational scattering, (b) vibrational scattering. (c) resonant scattering, and (d) coherent anti-Stokes spectroscopy. Borarski et al. (64B) discussed flame measurements utilizing Raman scattering and Stephenson and Aiman (7*5B)used laser Raman to probe a premixed laminar flame. They measured CO and 0 concentrations and flame temperature in the reaction zone and postflame gases. Recent reviews on coherent Raman spectroscopy, in general, include those by Eesley (68R) and Harvey (7013). In a very interesting article Sochet et al. (74R) discussed
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orchard leaves are presented. A multichannel (5 P M T s ) direct diagnostic technique in flames. Drake and Rosenblatt (65B) discussed rotational Raman scattering from premixed and diffusion flames, and Setchell and Miller (73B) measured Raman scattering from nitric oxide in ammonia-oxygen flames. Eckbreth (67B)discussed the averaging that can occur for pulse Raman signals from turbulent combustion media. Several workers utilized coherent anti-Stokes Raman spectroscopy (CARS) to study flames. M a k o and Rimai (72%) implemented space and time-resolved coherent anti-Stokes Raman spectroscopy for combustion diagnostics. Laufer et al. (72B) measured angularly resolved CARS in gases, Eckbreth and Hall (66B) implemented CARS thermometry in a sooting flame, Hall et al. (69B) measured the spectra of H,O vapor in flames using CARS, and Beattie et al. (633) measured the rotational temperature of N2 in diffusion flames using CARS. Acoustic Measurements in Flames. A few papers have appeared recently dealing with the measurement of acoustic phenomena in flames. T h e general area appears ripe for innovation and development. Allen et al. (77B) in a very brief note presented some preliminary data on the observation of optoacoustic pulses in a flame. Howard and Greenhalgh (78B) discussed flame atomic acoustic spectrometry as a new technique applied to the determination of Na. With a repetitively pulse dye laser they could actually “hear” sodium when 1000 ppm solutions were aspirated which facilitated turning their dye laser to the correct wavelength. Roberts (79B)reported the amplification of an acoustic signal by a laminar, premixed flame.
DEVELOPMENTS I N INSTRUMENTATION, MEASUREMENT TECHNIQUES, A N D PROCEDURE Spectrometer Developments. One of the major trends in spectrometer design for flame spectrometric methods, perhaps the major trend, is the development of systems capable of either simultaneous or rapid sequential multielement analysis. One of the most interesting designs to appear in the last two years is a multiple entrance slit vidicon spectrometer described by Busch et al. (4C). While requiring complex input optics this system provides a unique output spectral format t h a t can overlap and/or stack diverse spectral regions in a unique manner compatible with the area sensing capability of vidicons. Harnley et al. (18C) have described a continuum source-echelle system for background-corrected simultaneous multielement atomic absorption measurements. Flame or electrothermal atomization measurements are possible for up to 16 elements. This same system has been described by Harnly and O’Haver (17C) for background correction for the analysis of high-solid samples by graphite-furnace atomic absorption. A self-scanned photodiode array spectrometer for flame atomic absorption measurements has been described by Chuang et al. (6C). Multielement, single-element multiline, and single-element modes were evaluated. In a series of papers Salin and Ingle (26C-29C) described the design and performance of a time multiplex multiple-exit-slit multielement spectrometer for flame and electrothermal atomization atomic absorption measurements and for flame atomic fluorescence spectrometry. Rose et al. (24C) described the use of an oscillating mirror rapid scanning spectrometer as a detector for simultaneous multielement determinations. The system was applied to multielement carbon-cup vaporization atomic absorption spectrometry and microwave-induced plasma atomic emission spectrometry. Felkel and Pardue ( I IC) presented the evaluation of an echelle spectrometer-image dissector system for simultaneous multielement determinations by atomic absorption spectrometry. Data are included for the simultaneous determination of Cu, Cr, Mn, Fe, Co, and Ni. This is the same basic instrument described for atomic emission spectrometry with a dc plasma (12C). Horlick and Yuen (19C)described a modular Michelsen interferometer for Fourier transform spectrochemical measurements from the mid-infrared to the ultraviolet. This instrument is capable of simultaneous multielement determinations using flame emission and ICP sources. Korba and Yeung (22C) have extended the application of Fabry-Perot interferometry in multielement flame emission analysis. Several simultaneous determinations such as Na, Ca, and K in urine, serum, and
FLAME EMISSION, ATOMIC ABSORPTION AND FLUORESCENCE
the use of multichannel pulsed Raman spectroscopy as a reading spectrometer attachment has been developed by Cann et al. (5C). Each channel can be individually scanned. Finally in a review, Rose and Caruso (25C) have described the application of rapid scanning spectrometry to atomic spectroanalytical analysis. A very unique system for selective spectral-line modulation atomic absorption spectrometry has been described by Cochran and Hieftje (7C). In part, the basic idea of this system is to be able to implement continuum source atomic absorption spectrometry with relatively simple instrumentation. T h e key to this system is a conventional pneumatic nebulizer/spray chamber and slot-burner-supported flame that form the modulating atom cell, and a mirrored chopper directs the continuum radiation alternately around or through this atom cell, thus performing the selective modulation function. T h e analytical flame is placed between the continuum source and the modulating flame. Another continuum-source based atomic absorption spectrometer has been described by Bower et al. (2C). In this case the key element is a resonance monochromator detector based on a pyrolytic graphite furnace which is fed with desolvated analyte. A microcomputer-controlled dual wavelength spectrometer has been described by Defreese et al. (9C). Atomic spectrochemical applications include such two channel measurements as two elements simultaneously, internal standardization and background correction. Many workers are “computerizing“ their spectrometers. Two such reports are presented by Willmott and Mackenzie (34C) and Barnett and Ediger ( I C ) for atomic absorption spectrometers. Fisher et al. ( I 3 C )also report on an automatic sampler for flame atomic absorption. Three instruments have been described that combine flame atomic emission and fluorescence measurement capability. Ullman et al. ( 3 I C ) described a computer-controlled multielement AE/AF spectrometer system based on a slewed-scan monochromator. The AE/AF system described by Brinkman e t al. ( 3 C ) was built from “stock” components and applied to the analysis of trace metals in petroleum and petroleum products. Gustavsson and Ingman (16C) described an AE/AF instrument based on an image dissector echelle spectrometer. Furuta et al. (14C) designed a slewed-scan monochromator for flame atomic emission spectrometry. A unique aspect of their system was the use of a n S I T detector which relieved the rather strict accuracy requirement of most slewed-scan systems, as the S I T could monitor a 5 nm window. As mentioned in the introduction the ICP is making inroads to AA applications. An instrument has been described by Magyar and Aeschbach (23C) that combines flame, graphite tube and argon plasma capability on one instrument. Such multiple source instruments may become more common in the future. Developments of more general interest in instrumentation include a discussion by Salmon and Holcombe (30C) of an off-axis imaging system for improved spatial resolution and spectral intensities. All mirror image rotators were presented by Klueppel et al. (21C)and discussed in more detail by Dixon (IOC). Glaser et al. ( I X ) described a new interactive method for Abel inversion. Coleman and Walters ( 8 C ) described the design of a large rigid, modular optical bed for versatile optical and spectroscopic experimentation. This is a truly remarkable and effective system for experimentation in analytical spectroscopy in the most general sense. It has evolved over several years in the laboratories of J . P. Walters at the University of Wisconsin. He has discussed this system and his approach to research in a recent book chapter (32C) which is highly recommended reading. Hunter and Hieftje (20C) described a unique directly digital flow controller with rapid response time and high precision. This controller is suitable for computer control and amounts, in a sense, to a gas analogue of a weighted resistor analogue-to-digital convertor, Le., a gas DAC. Gas flows were controllable over a dynamic range of 256 (8 bits). Webster (33C) discussed t h e gas flow modeling of variable area flowmeters, the “rotameters“ most frequently found on flame instruments. P r i m a r y Source Developments. In this section developments in hollow cathode lamps and electrodeless discharge lamps are covered. Brief mention is made of laser sources
which are more fully covered in the section on Atomic Fluorescence Spectrometry. The preparation of cathodes for use in demountable hollow cathode lamps by electrolytic deposition rather than by sputtering is discussed by Niemczyk and Erspamer (54C). Such lamps produce slightly more intense and much cleaner spectra, with better long term stability than the sputtered cathodes. Wolfe and Vickers (66C) studied the optimization of pulsing conditions for hollow cathode lamps for atomic fluorescence spectrometry. Essentially each lamp had to be individually optimized but, in general, a large duty factor was desirable. T h e spectral properties of pulsed hollow cathodes were discussed by Otruba et al. (59C). Lamps were pulsed from 10 to 100 Hz at 50 to 550 mA. The increased intensity afforded by pulsed operation greatly improved atomic fluorescence performance, and line broadening was not severe enough to prevent atomic absorption measurements. Dewalt et al. (42C) have described a stable supply for modulating hollow cathode lamps. A coaxial boosted output hollow cathode lamp for atomic absorption has been described by Myers (53C). T h e output intensity was 10 to 20 times the intensity of commercial hollow cathode lamps for Al, Mo, Ti, V, and Cu. Sullivan and Van Loon (62C) described a demountable boosted-output spectral lamp for atomic absorption and fluorescence measurements. Atomic absorption measurements were presented for Ni, Pb, Sn, Cu, and Mg; and nondispersive atomic fluorescence measurements for Zn, Cd, Ni, Ag, and Cu. In a later article Sullivan (63C) presented an improvement in design enabling As, Se, and T e lamps to be constructed and operated successfully. Some other hollow cathode lamp studies carried out in the last two years include a study of sample volatilization in a hot-type hollow cathode by Broekaert (37C),a low temperature hollow cathode lamp developed by Torok and Zaray ( 6 4 C ) , new lamp designs discussed by Gough and Sullivan (45C) and Dyulgerova and Zechev ( 4 3 C ) , measurement of electron temperatures in a hollow cathode discharge by Meks and Niemczyk (48C),and use of a high current hollow cathode tube as a source of metastable atoms by Lombardi et al. (47C). Japanese workers, Daidoji et al. (4OC, 4 1 0 have described water cooled arsenic and selenium hollow cathode lamps and their application to atomic absorption speci rometry. Thermostated electrodeless discharge lamps in atomic spectroscopy have been reviewed by Bartley et al. (35C). Michel et al. (49C-61 C) have described a reproducible method for preparation and operation of microwave excited electrodeless discharge lamps based on simplex optimization and applied their method to both Cd and Se EDL‘s for use in atomic fluorescence spectrometry. Goode and Otto (44C) have also critically evaluated the fabrication details and operating conditions influencing microwave-excited electrodeless discharge lamps. An improved high-intensity microwave-discharge lamp for atomic absorption and fluorescence spectrometry was described by Lifshitz et al. (46C). An inexpensive microwave source for electrodeless discharge lamps based on a modified commercially available microwave oven has been described by Chilukuri and Lichten (39C) and Norris and West (55C) have described a iystem using a twin-port power divider that allows one to simultaneously operate two electrodeless discharge lamps from one microwave generator. Outred and Howard (60C) have measured microwave radiation leakage from microwave-excitfld light sources. Although leakages >10 mW cm (the recommended maximum level in the U.K or U S A . ) a t 10 cm froin the discharge were recorded from a quarter-wave Evenson cavity, it was concluded that leakage can be kept within acceptable limits by simple precautions such as keeping the discharge within the cavity if possible, and, when using a vacuum system, shielding the discharge section with a mesh screen. Radio-frequency-excited electrodeless discharge lamps for use in atomic absorption and atomic fluorescence spectrometry have been described by Novak and Rrowner (5(iC,57C). These lamps are operated in a pulsed mode and run at 13.5 MHz. T h e performance of Zn, Cd, and Hg lamps was evaluated. High frequency discharge lamps operated at 20 and 85 MHz have also been described by Murayama et al. (5’2C) and Tsujii et al. (65C). Cd, Zn, and P b lamps were eLaluated and found to be 10 to 100 times as intense as hollow cathode lamps. ANALYTICAL CHEMISTRY. VOL 52, NO 5, APRIL 1980
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A couple of comparative studies carried out recently are those by O’Haver and Harnly (58C) who compared the radiant power of an Eimac xenon arc lamp and hollow cathode lamp finding the Eimac to be generally superior and Caroli and Delle Femmine (38C) continued their comparative work on hollow cathode and glow-discharge light sources. For many years now lasers have been heralded as the ideal source for atomic absorption and atomic fluorescence spectrometry. However, reliable, tunable systems in the far-ultraviolet are still not available. Some work in this area has been reported by Blit et al. (36C) who described a system for generating continuous wave ultraviolet radiation tunable from 285 to 400 nm by harmonic and sum frequency generation and Stickel et al. (61C) who reported the generation of coherent continuous-wave radiation tunable from 211 to 216 nm. In addition excimer lasers may provide the tunable source of the future for the far-ultraviolet. The book edited by Omenetto on “Analytical Laser Spectroscopy” (14A)provides information on these systems as well as many other laser advances and applications. Lasers are currently having a major impact on atomic fluorescence measurements and these developments are discussed in that section of this review.
Background Correction in Atomic Absorption Spectrometry. Some general papers on background correction in atomic absorption spectrometry are reported on in this section. Related sections entitled Zeeman Atomic Absorption and Flame Molecular Absorption Spectrometry should also be consulted. Bath e t al. (67C) described a sequential hollow cathode system for background correction in atomic absorption spectrometry. In this system a H2 lamp is mounted in line with a double windowed demountable hollow cathode lamp and the lamps are pulsed out of phase to effect the background correction operation. Herber and DeBoer (69C) reported a modification to the Varian AA6 background system that allows the independent monitoring of the background signal which is not normally available as an output. Siemer (71C) discussed some design considerations in background corrected atomic absorption instrumentation. In particular the Koirtyohann and Pickett approach is critically examined. It was concluded that successful correction can be achieved for nonatomic absorption, but, if a nonmodulated emission signal is present, the estimate of atomic absorption may be erroneous. Guthrie et al. (68C) discussed background correction problems related to the determination of chromium by graphite-furnace atomic absorption spectrometry. They could not correct for background with a D2 lamp on a PE603-HGA2100 because the D2 lamp emission was too weak a t Cr 357.9 nm. Nakamura and Kawase (70C) discussed, in a general sense, errnrs of background correction by a deuterium lamp in atomic absorption spectrometry. Relative lamp alignment problems, differences in spatial and time behavior of the background, and analyte signals were discussed. Finally, serious background absorpton errors were shown to occur with perchloric acid digests of plant material for the determination of Cu accompanied by high Ca concentrations by Simmons (72C). Sample Introduction into Flames. It seems that a flame spectroscopist’s imagination is best at devising new approaches for introducing samples into a flame. One would almost think that over the years just about every approach has been tried but such is not the case and several new and effective systems and approaches have been developed over the last two years. Skogerboe and Olson (87C) in a paper entitled “Aerosols, Aerodynamics, and Atomic Analysis” raised several interesting points with respect to aerosol transport which are often ignored when critically assessing the performance of a particular determination and a potential interference effect. They make the point that some high salt effect interferences may be traced to aerosol dynamics. Fry and Denton (78C)characterized a “high solid“ nebulizer for flame atomic absorption spectrometry. This nebulizer was based on the “Babington” approach and was capable of neutralizing samples with very high solid content (Le., tomato soup). O’Reilly and Hicks (85C) aspirated aqueous coal slurries directly into a conventional atomic absorption spectrometer and obtained relative precisions of *1-5% and accuracies of 5-25%. Savage and Hieftje (86C)showed that the efficiency of pneumatic nebulization could be enhanced by application of a large electric field to the burner-nebulizer 294R
ANALYTICAL CHEMISTRY, VOL. 52, NO. 5, APRIL 1980
system. Cresser (76C)discussed the use of an impact cup to reduce nebulization efficiency as a n aid to directly running high analyte concentrations, presumably in lieu of dilution. An auxiliary benefit was a generally reduced level of vaporization interferences as only very small droplets are finally transported to the flame. The addition of 1% glycerol to the sample solution was found by Van Eck (91C) to improve the nebulization precision. There is considerable interest in being able to carry out flame-based analytical determinations on small sample volumes. Fry et al. (79C) described a microsampling nebulizer technique based on small Teflon funnels. These allowed both 1 dilutions and additions to be carried out on the sample. Uchida et al. (90C) presented a “one drop” method for flame atomic absorption spectrometry that allowed the analysis of 100 1 L portions of solution. Wilson (92C)described a venturi sampler for aspirating small sample volumes in atomic absorption spectroscopy. Up to 6 elements could be determined with a 2-mL sample volume. A Teflon sampling manifold for use with small injections was described by Eaton and Schiemer (77C). Sample volumes of 100 1 L were readily aspirated. The injection method of flame atomic absorption spectrometry has recently been reviewed by Berndt and Slavin (74C). Zagatto et al. (94C)and Wolf and Stewart (93C)have both coupled flow injection analysis techniques and flame atomic absorption and emission systems. A branch capillary for ionization buffer addition in flame atomic absorption determinations of Na and K was discussed by Szydlowski (88C). This method saved time and improved precision and accuracy. A similar system was reported and discussed by Niemojewski (84‘2). Several auxiliary sample introduction/atomization systems have been and continue to be coupled to flame systems. Howlett and Taylor (80C) used the “Delves Cup” approach for the measurement of Ag in blood by atomic absorption spectrometry. A Pt microloop was used by Berndt and Messerschmidt (73C) to introduce small amounts of drinking water into a flame for P b and Cd analyses. Electrothermal atomization of Os is difficult because of volatility problems. Mallett et al. (83C) pulsed the osmium generated in a furnace into a N20-C2H2flame for atomic absorption determination. Khalighie et al. (82C) investigated the use of a water-cooled silica tube as a n “atom trap” in a flame. The analyte (Cu in this case) is preconcentrated on the silica tube which is subsequently allowed to heat up releasing the collected analyte which is measured by atomic absorption spectrometry. An arc has been used by Kantor et al. (81C) to “nebulize” samples into a flame for atomic absorption determinations. Bruhn and Harrison (75C) have used cathodic sputtering in a glow discharge as a means of atomization for atomic absorption spectrometry. Ca, Mg, Zn, Au, Ni, and Sn solution residues were analyzed. Tsujii and Kuga (89C) carried out the direct analysis of metals by cathodic sputtering atomic absorption spectrometry. Brass was analyzed and they indicated that the use of a two channel instrument was desirable in that internal standardization could thus be used.
The Zeeman Effect in Analytical Atomic Spectrometry. Several reviews and general articles have appeared in the last couple of years. A general discussion of Zeeman-effect-based background correction in atomic-absorptjon spectrometry was presented by Brown (95C). A major review has also appeared entitled “Applications of the Zeeman Effect in Analytical Atomic Spectroscopy” written by Stephens (107C) one of the prominent workers in this field. An extensive discussion of the theory of the Zeeman effect for background correction in analytical atomic absorption spectrometry has been present by a de Loos-Vollebregt and de Galan (96C). These same two authors also discussed the shape of analytical curves in Zeeman atomic absorption spectrometry (97C). Stephens (108C) discussed the merits of having a signalcomparison parameter to facilitate comparison of Zeeman background-corrected atomic absorption spectrometers. In effect the proposed parameter is a measure of the ideality of the background-correction process and equations are derived for use with instruments involving the transverse Zeeman effect, the longitudinal effect and for those involving ac magnetic modulation. The feasibility of high-frequency field modulation for Zeeman-modulated atomic absorption spectrometers was discussed by Stephens ( I l O C ) . It was shown
FLAME EMISSION, ATOMIC ABSORPTION AND FLUORESCENCE
how this approach can be implemented with a conventional hollow cathode lamp by fitting a water-cooled field coil around the lamp. In a continuing series of papers on applications of the Zeeman effect to analytical atomic spectroscopy Murphy and Stephens (102C) discussed the use of rf excited hollow cathode lamps as Zeeman modulated sources. Stephens (106C) compared experimental and theoretical detection limits in Zeeman effect background corrected determinations and Stephens and Murphy ( 1 1I C ) discussed the effect of line interferences on a Zeeman modulated source spectrometer. Koizumi (99C)illustrated the correction of potential spectral overlap interferences that might affect conventional atomic absorption spectrometers by use of a Zeeman effect based instrument. In a later paper Sotera et al. (IO5C) indicated that, in fact, t h e spectral overlaps studied ( P b on Sb, S b on P b , and Xi on Sb) were either relatively insignificant or readily overcome by simple means such as alternate line choice. Qtruba et al. (103C) described the analytical capability of a simple Zeeman based atomic absorption spectrometer. Pleban and Pearson (104C)determined P b in whole blood and urine using Zeeman-effect flameless atomic absorption spectrometry. Koizumi et al. ( I O O C , IO1C) and Stockton and Irgolic (112C) describe the use of a furnace Zeeman-effect atomic absorption spectrometer as a detector for high pressure liquid chromatography. Finally, the applicability of other magnetic based effects to atomic spectroscopy are being studied. Stephens (109C) discussed the detection of Hg vapor by magnetically induced optical rotation and Kitagawa e t al. (98C) studied the application of the Faraday effect to the trace determination of Cd by atomic spectroscopy with an electrothermal atomizer. Laser Enhanced Ionization. Work continues at NBS on laser enhanced ionization or as it is sometimes called optogalvanic spectroscopy. Turk et al. (119C)discussed the basics and application of laser enhanced ionization to analytical flame spectrometry emphasizing that optical detection systems were not necessary and hence optical noise source irrelevant. In a series of three more papers this group presented the application of this method to the determination of atomic species in analytical flames (114C, 117C) and also discussed stepwise excitation using two lasers (118C). A general article on optogalvanic spectroscopy has appeared written by King and Schenck ( 1 1 6 0 . Detection limits for Na, Mg, Mn, Cu, and P b in air-C,H, and air-H, flames were on the order of ppb or better. Keller et al. (115C) reported on optogalvanic spectroscopy in a uranium hollow cathode discharge and Bridges ( I I3C) discussed the characteristics of an optogalvanic effect in cesium and other gas discharge plasmas. Gonchakov e t al. (114C) reported on the determination of picogram amounts of Na in a flame by stepwise photoionization of atoms. General Developments in Procedure. Developments in procedure are generally documented in following sections devoted to the specific techniques. However some general articles have been grouped in this section which is divided into four subsections, sample pretreatment, reference materials, standard addition method, and analytical calibration curves. In many situations it is difficult to get a meaningful sample from the site to the laboratory. Boutron (122C)discussed the reduction of contamination problems in sampling Antarctic snows for trace element analysis. Pellenbarg and Church (136C) discussed the storage and processing of water samples for trace metal analysis by atomic absorption spectrometry a n d Subramanian et al. (142C) studied the preservation of trace metals in samples of natural water. Loss of Al, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, P b , and Zn stored in Pyrex and Nalgene containers was studied as a function time by graphite-furnace atomic absorption spectrometry. Loss was minimal at p H
