Instrumentation
Jon C. Van Loon Departments of Geology and Chemistry and The Institute for Environmental Studies University of Toronto Toronto M5S 1A1 Ontario, Canada
Direct Trace Elemental Analysis of Solids by Atomic (Absorption, Fluorescence, and Emission) Spectrometry The direct elemental analysis of solids (methods not requiring chemical pretreatments) is an important priority in analytical methods development. In the case of the trace element analysis techniques, atomic absorption spectrometry (AAS) , atomic emission spectrometry (AES) and atomic fluorescence spectrometry (AFS), many strategies for direct solid sample analysis have been proposed. However, with the exception of arc and spark AES, no approach to direct solid sample analysis for these spectrometric techniques has received widespread acceptance. In addition, most of the proposed methods for solid sample analysis, including arc and spark AES, are plagued by poor precision and difficulties with standardization. Recently, particularly due to the rapid upsurge in interest in the inductively coupled plasma, there has been renewed research activity into methods for direct analysis of solid samples. The following is an overview of this activity set in the perspective of the more traditional approaches. AAS and AES are the most widely utilized techniques of trace elemental analysis. In the case of the latter, the inductively coupled plasma is the atomizer of greatest current interest. Relatively little activity is evident in analytical AFS. One of the major obstacles to wide acceptance of this technique is the serious problem due to the scatter of incident radiation by particles in the atomizer, a problem greatly compounded when solid samples are directly introduced. Thus, in the forseeable future, work on the direct trace elemental analysis of solids will undoubtedly focus on AAS and AES. Table I presents a summary of the methods for the direct trace analysis of solids by atomic spectrometry. These methods are discussed in the following pages. 0003-2700/80/0351-955A$01.00/0 © 1980 Artierican Chemical Society
Flames Historically, the flame has played a dominant role in analytical atomic spectrometry. A variety of flames has been utilized depending on temperature requirements. In AAS and AFS the air-acetylene flame has been favored in most applications, with airhydrogen and nitrous oxide-acetylene flames being used for the hydrideforming and more refractory elements, respectively. In AES where the role of the atomizer is excitation, as well as atomization, nitrous oxide-acetylene, oxygen-acetylene and cyanogen-oxygen flames provide the higher temperatures required for the atomization and excitation of the more refractory samples. The direct analysis of solids in flames has involved a wide range of flame types with the higher temperature flames being favored on a theoretical basis for most complex sample types. It is surprising, therefore, how much use has been made of the airacetylene flame in this application. The simplest and generally most primitive methods for the analysis of solids by techniques of atomic spectrometry consist of the introduction of the solid directly into a flame or aspiration of the solid suspended in a liquid into a flame. Powdered sample rolled in filter paper was introduced into the flame for flame emission analysis by Ramage (1). This method was later improved and automated by Roach (2), and by Stewart and Harrison (3). If the sample is rigid it can be simply held in the flame. Powders have been introduced either in a container or as a coating on a rigid object. For example, sample mixed with NaCl was deposited on an iron screw and placed into an air-acetylene flame (4) . By measuring the atomic absorption thus produced, rubidium in rocks could be determined. Other related methods
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980 · 955 A
Table 1. Summary of Methods of Direct Solid Sample Trace Analysis by Atomic Spectrometry Conventional
Emission
Hybrid techniques
solid-in-flame solid-in-plasma arc spark electrothermal laser glow discharge
furnace-arc chloride generator laser-plasma furnace-plasma spark-plasma
Absorption
solid-in-flame arc spark electrothermal laser cathodic sputtering
capsule-in-flame hollow graphite " T " tube furnace-flame arc/spark-flame chloride generator
Fluorescence
laser laser-spark electrothermal
furnace-flame
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/
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involve samples held in cups, e.g., the Delves Cup (5), wire loops and boats. Techniques of this type are limited by the relatively low temperature envi ronment of the substrate and sample. Hence only the more volatile elements can be determined. In an a t t e m p t to make this type of approach applicable to the less volatile elements, Venghiattis (6) mixed the powdered sample with a solid propellant and introduced the pellitized sample into a flame. Re producibility with this method was generally unsatisfactory. Sample can be suspended in a slurry and aspirated into the flame. In a method typical of this approach Willis (7) suspended particles of rock less t h a n 44 μ in size in water and sprayed the mixture into air-acetylene, nitrous oxide acetylene and oxygen-hydrogen flames. T h e results obtained had an accuracy suitable for geochemical sur vey work. Arcs and Sparks Arc and spark optical emission spectroscopy has historically been an important method of solid sample analysis. Although application to con ducting samples is best the procedure has been successfully applied to a very wide range of samples including or ganic material. Many of the methods . to be discussed for the introduction of solid samples into other atomizers originated with arc/spark technology. For conducting materials the sam ple is machined into an appropriate form (often pointed) and used as the electrodes in the system. An arc or spark is then generated point to point between the electrodes. For noncon ducting materials the sample is com monly mixed with carbon powder and packed into an electrode; the electrode is then made the cathode (hotter t h a n the anode) in a carbon arc discharge.
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One simple method of solid intro duction involves sprinkling or sifting of sample into the arc from above. Furnaces can be used to volatilize sample into an arc or spark. For exam ple, the Preuss furnace (8) is a resistively heated device mounted below the arc, which can hold up to 3 g of sample in a boat. T h e vapors pro duced are carried in an inert gas stream into the arc. There have been many ingenious methods suggested for continuous solid sample introduction into arcs and sparks. These include forcing a sample up through a hollow electrode using a piston (9) and spreading a sample out thinly on a cellulose tape and feeding the tape between the elec trodes of an arc or spark (10). Arc/spark emission spectrometry is fraught with matrix problems—these being more pronounced for noncon ducting t h a n conducting samples. I t is usually necessary to standardize using samples of closely approximating com position. Reproducibility is also a problem. Methods have been pro posed to stabilize the arc. Perhaps the most accepted is the Stallwood jet (11) whereby a stream of gas (air, nitrogen or argon), introduced concentrically, is circulated around the electrode. This alters the condition of evaporation and excitation and reduces the matrix effects considerably. Arcs and sparks have found very limited application in AAS. Atomization may be carried out in an arc or spark, the gap of which is placed in the optical beam of an AAS instru ment (12). As with arc or spark, AES analysis precision leaves much to be desired. Electrothermal Atomizers L'vov (13) first suggested the analy sis of solid samples by electrothermal
956 A · ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
AAS in his paper proposing furnaces as a method of AAS atomization in 1959. A number of resistively or in ductively heated, metal or graphite devices including rods, filaments and furnaces have since been developed, the most widely utilized approach being the resistively heated graphite furnace. Much of the pioneering work is summarized in references (14) and (15). A good review in this field by Langmyhr (16) has recently appeared. Electrothermal AAS finds its forte in the analysis of liquid samples con taining analyte below the detection limit of flame AAS. However, there are still a number of deficiencies with commercial electrothermal and these are outlined by L'vov (15). T h e most serious is the failure to atomize sam ples under isothermal conditions. T h u s , the integral value of the absorbance (the best signal parameter to em ploy in electrothermal AAS) is depen d e n t on the rate of furnace tempera ture rise and on the reproducibility of this parameter with time. Interfer ences are more complex under nonisothermal conditions. In addition most electrothermal atomizers are cooled at the ends. This leads to nonuniform heating along the length of the fur nace, thus yielding memory effects and reduced sensitivity. Electrothermal atomization with AAS has successfully been used for the direct analysis of some types of solid samples, including highly organic samples and other materials of a rela tively simple matrix. T h e main re quirement for success in this applica tion is t h a t the matrix and analyte have widely different volatilities. Drawbacks to direct solid sample analysis by electrothermal atomiza tion AAS include problems from ex cess background absorption signals (Zeeman background correction may give some additional relief in this re gard), difficulties in standardization, relatively poor precision, blockage of the optical beam by skeletal remains of the sample and restriction of sam ple size to only a few milligrams. Carbon furnace emission spectros copy has been used for many years and is potentially applicable to solids. Recently, commercially available atomic absorption furnaces have been used for this purpose (17). T h e wide spread occurrence of such devices makes this approach potentially at tractive. Because of the relatively moderate temperatures attainable by these furnaces, the method is mainly for the more volatile elements. Reduced Pressure Discharges For conducting samples, use of a Grim-type discharge lamp is receiving increased attention. This AES method is particularly attractive where high
precision is essential. H u m a n and Butler (18) have developed a fluorimeter for the isolation of spectral lines emitted by the discharge source. T h e device has been particularly use ful for the analysis of steel samples. Kenawy et al. (79) extended the use of the glow discharge to nonmetallic samples. T h e sample is mixed with copper powder and pressed into tab lets. T h e technique was used to ana lyze chalcogens and halogens using their vacuum ultraviolet lines. T h e glow discharge techniques give best precision for the determination of ele ments at levels of above 0.001%. Cathodic sputtering (20) has been used for the analysis of the conductive samples by both AAS and AFS with precision comparable to Grim dis charge emission. T h e sample is made the cathode in a discharge cell operat ing under electrical conditions similar to a hollow cathode lamp. Time reso lution of the background and analyte signals (21) is possible, making back ground correction even in AFS rela tively simple. Plasmas T h e introduction of the inductively coupled plasma as an atomizer for an alytical spectrometry is credited with revitalizing optical emission spectrom etry. T h e advantages of this atomizer have been widely publicized, particu larly by Fassel (22). While other plas mas are used, e.g., microwave and DC arc, the inductively coupled plasma exhibits good qualities of stability and freedom from chemical interferences. These latter two qualities have led to the widespread acceptance of induc tively coupled plasma emission spec trometry as an exciting new analytical technique. As a result, sales of this equipment have recently skyrocketed. T h u s , a top priority must be given to the development of methods for the direct analysis of solids using this at omizer. T h e very high temperatures achieved in the inductively coupled plasma make this device particularly attractive as an atomizer for solids. A number of proposals have already been made for the direct introduction of powders into the plasma (23). Dagnall et al. (24) used a fluidized bed chamber to entrain powders into an argon carrier gas flowing into a plasma. In this device, powdered sam ple was placed on a sintered glass disc through which argon was flowing. A cyclone chamber was placed above the fluidized bed chamber to separate large particles. Beryllium and boron were analyzed in magnesium oxide. Hoare and Mostyn (23) placed pow dered solid sample into a borosilicate cup at the base of a plasma torch. T h e cup was then mechanically vibrated. A
relatively low argon flow rate through the vibrating powder carried the solid into the plasma. This device was used for the analysis of trace impurities in powdered electrode materials, lithium salts and alumina. Solids can be elutriated in an ener getic spark between two electrodes held above a powder (25). A low argon carrier gas flow is used to carry the fine particles into the plasma. Geochemical rock powders ground to pass a 200-mesh sieve were analyzed. Attempts have been made to nebu lize solids, suspended in a slurry into the plasma (28) in a similar way as was used in AAS. However, the usual problems (which adversely affect pre cision) intrinsic to passing slurries through a nebulizer are of course in herent in this approach. Kleinmann and Svoboda (26) di rectly vaporized solid residues into the plasma from a graphite disc mounted in the central body of a plasma torch. T h e applicability of this technique to other solids is uncertain. Recently, Salin and Horlick (27) used a graphite rod (made from a graphite cup electrode), in a tube re placing the central aerosol tube in a conventional plasma torch, to intro duce powdered sample directly into the plasma. Sample is placed into the top of the rod. T h e plasma is initiated with the rod at the height of the top of a conventional aerosol injector tube. T h e rod is then raised into the plasma. In this preliminary study the samples tested were a powdered standard (a type usually used in DC arc work), NBS Coal and Orchard Leaves. T h e method was compared with conven tional DC arc optical emission spec trometry and was found to give better detection limits for As, Bi, Cu, Ga, In, Na, P b , Tl and Zn. Similar detection limits were observed for Ag, Be, Cd, Cr, Fe, Mg, Mn, Sb and Sn, and detec tion limits were not as good for B, Ca, Ge, Mo, Ti and B. Lasers A laser can be readily used to vapor ize a sample in a spot of 1 to 30 μια.. An emission, absorption or fluores cence measurement can then be made. Laser excitation has been used alone or in combination with an auxil iary spark for AES analysis, the latter giving a better detection limit of be tween two and five times (29). A CO2 laser (30) and a ruby laser (31) have been used for analysis by AAS and AFS, respectively. In these applica tions a wide range of samples has been analyzed including rocks, ores, and bi ological samples. In the future the ability of the laser to focus on a very small spot should be utilized to advan tage, for example, in the analysis of fluid inclusions in rocks, individual
958 A · ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
mineral grains and small areas on tis sue samples. A distinct advantage of the laser ap proach is the possibility of resolving the analyte and background signals in time. Disadvantages include problems arising from inhomogeneity problems (such a small area is vaporized) and the relatively poor shot-to-shot repro ducibility of existing lasers. Hybrid Devices for Direct Solid Sample Analysis In the preceding, methods have been outlined in which, in most cases, the atomizer performs the multiple functions of sample breakdown, va porization and atomization. Recently, there has been renewed interest in methods in which sample breakdown and, to a varying extent, vaporization occur in a device separate from the at omizer. Such an approach has several potential advantages depending on the technique of atomic spectrometry employed. In most cases some resolu tion of analyte and background signals in time occurs. With reference to AAS and AFS a wider range of solid sam ples can be analyzed mainly because of the reduction in chemical and back ground interferences obtained. Also solid samples of widely differing ma trix and analyte volatilities can be an alyzed. Atomic Absorption and Fluorescence Spectrometry L'vov and colleagues introduced two atomizers t h a t were primarily de signed to handle solids for atomic ab sorption analysis. These were a capsule-in-flame atomizer (32) and a cir cular cavity furnace (33). In the capsule-in-flame atomizer the powdered sample mixed with graphite powder (to prevent melting or sintering into a bead) was placed into a cavity in the center of a porous graphite capsule. This capsule was mounted horizontal ly between two water-cooled cylindri cal holders. Current from a step down transformer was passed through the rod for heating. T h e capsule was placed in a flame with the optical beam passing above the rod. An a i r acetylene or nitrous oxide-acetylene flame was used depending on the vola tility of the element being determined. When the capsule was heated, the vapour from the sample passed through the walls of the graphite into the flame where atomization occurs. In the circular cavity furnace the powdered sample, mixed with graph ite powder, was placed in a cavity be tween the casing (pyrolytical graphitelined) and a porous graphite inner tube. T h e assembled furnace was heated electrically. As the furnace is heated, vapors from the sample pass through the walls of the porous inner
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tube into the optical beam. Although the same heater is used to heat the sample and the inner tube atomizer, the inner tube becomes heated before the vapors of the sample pass into it. This gives optimal thermal conditions for the dissociation of the more ther mally stable oxide compounds. Comparing these two atomizers the following was obtained. T h e range of elements which could be analyzed was appreciably greater with the capsulein-flame approach (i.e., 50 elements were analyzed compared to about 35 with the circular cavity furnace). A wide range of substances could be ana lyzed by the capsule-in-flame atomiz er (chemical reagents, rocks, metals, slags, semiconductors, organic samples and graphite materials) compared to only graphite powders and silica with the circular cavity furnace. Although other materials will likely be analyzed with the circular cavity furnace, the sample types will be restricted to those in which the analyte volatility greatly exceeds t h a t of the major ma trix constituents. Standardization was done for these two atomizers using graphite powder containing the analyte elements of in terest. T h e integral method of absorbance measurement should always be used because this minimizes the effect of matrix interferences. Robinson et al. (34) introduced a hollow graphite " T " tube atomizer. In this device a solid sample was vapor ized by induction heating in the verti cal arm of the " T . " T h e aerosol thus produced flowed in a stream of inert gas into the horizontal crosspiece of the " T " tube where atomization oc curs. These workers recorded a sub stantial reduction in nonspecific ab sorption for seawater and organic tis sue samples compared to conventional furnaces. Other types of solid samples were not run. Koop et al. (35) and Ip et al. (36) used a commercially available graph ite furnace to volatilize solid samples into a flame (combined furnaceflame) for AAS and AFS analysis, re spectively. A Perkin-Elmer HGA 2000 furnace was fitted at one end with an aluminum end-cap with an inlet tube for inert gas and at the other end with a short quartz transfer tube into the flame. Solid samples were placed into the furnace and were heated to high temperature. T h e aerosol thus pro duced was swept into the flame where atomization occurred. In the atomic absorption mode, nonspecific back ground interference was reduced by at least an order of magnitude in most instances compared to conventional furnaces. Atomic absorption analyses done included As, Se and Hg determi nation in sulfide ores, P b in rocks, and Fe, Mn, Cd, and Zn in botanical sam
960 A · ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
ples and air and water filters. In atom ic fluorescence Fe, Mn, Cd and Zn were analyzed in air and water filters and in botanical and zoological sam ples. In the case of atomic fluores cence, as predicted, light scatter in the air-acetylene flame prevented the ap plication of this approach to more re fractory sample types. Standardization for the combined furnace-flame techniques need not be done by using exactly similar materi als. In the case of air filters, aqueous standards on a blank filter were satis factory. Aqueous standards were also used for the determination of P b in rocks. In all cases the integral method of signal reading should be employed. Another method in which the vola tilization and atomization zones of an atomizer are separate is arc or spark vaporization followed by flame atom ization (37). Solid samples were vapo rized in the arc or spark and were car ried into a flame in a stream of inert gas. This approach suffers from exces sive sparking times necessary to achieve a steady state signal.
Plasma Skogerboe et al. (38) used a chloride generator to volatilize metals from residues as volatile chlorides into an atomic absorption flame or microwave plasma according to the following re action: Δ
M n Ym(s) + excess HCl( g ) ->- M Cl n r g ) + HmY T h e system can be used at tempera tures u p to 1000 °C and hence is ap plicable to about 30 elements which form volatile chlorides with boiling points below this value. Abercrombie et al. (39) used a pulsed transversed excited CO2 laser (maximum power 17 kW) to vaporize powdered solid samples. T h e resultant aerosol was swept in an argon stream into an inductively coupled plasma. Twenty elements in air particulate could be analyzed in up to 3400 sam ples per day. This approach, used for geochemical prospecting, suffers from inhomogeneity problems because of the very small sample which is vapor ized. Carr and Horlick (40) used a similar approach with a ruby laser for analyz ing aluminum brass, steel, rock, ce ramics and biological samples. The greatest problem with this method was power fluctuation, which adverse ly affects shot-to-shot reproducibility. Several researchers have vaporized samples using electrothermal devices. T h e resulting aerosol is entrained in an argon flow and injected into the in ductively coupled plasma.
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Nixon et al. (41 ) used a resistively heated tantalum filament confined under a quartz dome. Vaporized sample flows directly from the top of the dome into the sample introduction orifice of the plasma. Although t h i s device was used only for liquids, preliminary studies suggest its applicability to solid samples. Grime and Vickers (42) used this approach for sample introduction into flames for flame emission. Recently Gunn et al. (43) vaporized samples from a resistively heated graphite rod enclosed under a glass dome. T h e device was applicable to Ag, As, Au, Be, Cd, Ga, Hg, In, Li, Mn, P, P b , Re, Sb, Ti and Zn. Poor detection limits were obtained for carbideforming elements, and with this problem in mind Kirkbright and Snook (44) and Kirkbright (45) employed a 0.1% trifluoromethane (freon 23) in argon mixture. Formation of volatile fluorides extended the method to B, Zr, Mo, W and Cr. Detection limit improvements compared to argon alone were one to two orders of magnitude. With equipment similar to Gunn et al. (43), Hull and Horlick (46) determined Ca, Ag, Mg, P b , Cu, Zn, Ga and Mn. Signal appeared approximately 1 s after the atomize cycle of the furnace was initiated. High levels of Na cause a spacial shift in emission b u t the integrated intensity is not affected. In a commercial offering (47) solids are vaporized in a capillary arc. T h e aerosol thus produced is swept in an argon stream into the inductively coupled plasma. This approach has been utilized for conducting samples only. A high voltage spark was used by H u m a n et al. (48) to produce particles from solid conducting materials. T h e particles were transported to the plasma in a gas fed through the spark chamber. Copper was analyzed in aluminum alloys, iron and brass. T h e use of this technique for injection of solids into flames for AAS and AFS analysis was also discussed.
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962 A · ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
In most cases, whether it be by AAS, AFS or AES, when solids are analyzed directly it is necessary to employ a standard t h a t is similar in composition to the samples. This reflects the strong matrix dependency of the signal in most of the approaches. Generally, the devices discussed under the heading of "hybrid devices" show reduced matrix dependency. T h e techniques which might be expected to be least matrix dependent are those involving the inductively coupled plasma. It is often difficult to obtain suitable reference samples of a wide range of solid samples. T h u s , as a substitute, it may be possible to make up synthetic standards from carefully weighed pure constituents t h a t are
subsequently combined into the de sired form. In all cases, to minimize matrix dependency it is important to use signal integration. Atomic Absorption, Emission or Fluorescence? Atomic fluorescence is generally the least attractive technique for solid sample analysis. This is because of the severe problems due to scatter of inci d e n t radiation by particles in the at omizer. P e r h a p s recent progress in using inductively coupled plasmas as atomizers for fluorescence (49) will di minish this problem. Inductively coupled plasma emis sion spectrometry is a particularly at tractive method for solid sample anal ysis. Although equipment and re search to date have concentrated on analysis of solutions, the extremely high temperature of t h e inductively coupled plasma is highly conducive to efficient vaporization, atomization and excitation. T h u s , the high matrix dependency noted in other atomizers, requiring t h e use of solid standards similar in composition to the samples, should be to some extent reduced with this plasma. Of course, t h e inherent multielement characteristics of AES are another plus for this approach. Atomic absorption spectrometry should continue to develop as a tech nique for solid sample trace element analysis. Reducing the matrix depen dency of the signal requires much fur ther research effort. In this regard the introduction of vaporized sample into flames is a useful approach t h a t should be further investigated.
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Analytical Chemists Exxon Research & Development Laboratories known worldwide for its achievements in the discovery and development of catalysts and pro cesses for refining fuels, has immediate openings for Analytical Chemists. You will be responsible for developing and directing the application of new analytical techniques to separate, identify and analyze compo nents in complex systems. You should have a PhD in Analytical Chemistry and at least 2-10 years' experience in Mass Spec, IR, GC or HPLC. If your experience, interests and credentials match our requirements and you are interested in joining a growing organization, please submit your resume in confidence to: Exxon Research & Development Labora tories, Coordinator of Professional Employment, Dept. ED-14, P.O. Box 2226, Baton Rouge, Louisiana 70821.
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