Application of a Wavelength Scanning Technique to Multi-Element Determinations by Atomic Fluorescence Spectrometry J. D. Norris and T. S. West Chemistry Department, Imperial College of Science and Technology, London S W7 2A Y , U.K.
Two dual-element electrodeless discharge lamps, operated from a single microwave generator via a twoport power divider, enable a separated air-acetylene flame to be illuminated with the intense resonance radiation of four elements. The resultant atomic fluorescence may be rapidly measured by scanning over the appropriate wavelength range. Results obtained for the sequential multi-element determination of zinc, cadmium, nickel, and cobalt show that the sensitivity and selectivity are the same by the scanning technique as by conventional atomic fluorescence spectrometry. Wavelength scans are also given for nickel, cobalt, iron, and manganese, and for selenium, tellurium, nickel, and cobalt combinations.
THECLAIMS OF HIGH SENSITIVITY, high selectivity, and simplicity, which were made when atomic fluorescence spectrometry (AF) was first proposed as a means of chemical analysis, have been illustrated by the numerous papers and reviews which have since been published o n the subject. However, another claim, that of the probable application of A F to multi-element determinations, has not yet been so fully supported by subsequent work. A spectrometer designed specifically for multi-element A F was initially reported by Mitchell and Johansson ( I , 2), who used it for the simultaneous determination of copper, iron, magnesium, and silver. West et a/. ( 3 , 4 ) have described the application of a similar A F spectrometer, capable of the simultaneous determination of up to six elements, to the analysis of calcium, copper, magnesium, manganese, potassium, and zinc in soil extracts (3),and copper, iron, magnesium, manganese, nickel, and zinc in aluminum alloys ( 4 ) . These instruments are complex, requiring individual pulsed single-element hollow cathode lamps, a rotating wheel with an optical filter for each element, which is synchronized with the lamp power supply, and a sophisticated signal processing system. Wdlsh (5) has reported the use of a simpler non-dispersive sevenchannel A F spectrometer, incorporating a solar-blind photomultiplier. More recently, Malmstadt and Cordos (6) have proposed a twelve-channel A F instrument. The potential use of a scanning technique for multi-element AF, employing commercially available atomic emission/ atomic absorption instrumentation, has been discussed previously ( 7 , 8 ) . A recent communication from this laboratory ( 9 ) described a rapid scanning technique, using a n atomic (1) D. G. Mitchell and A. Johansson, Spectrochim. Actu, Purt B , 25, 175 (1970). (2) lbid., 26, 677 (1971). (3) R. M. Dagnall, G. F. Kirkbright, T. S . West, and R. Wood,
ANAL.CHEM., 43, 1765 (1971). (4) R. M. Dagnall. G. F. Kirkbright, T. S . West, and R. Wood, Aiicilysr (Lo/idoii),97, 245 (1972). (5) A. Walsh. Pirre Appl. Chem.. 23, 1 (1970). (6) H. V. Malmstadt and E. Cordos. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1972, Paper 132. (7) G. B. Marshall and T. S . West, Aiiul. Cliim. Acfu, 51, 179 (1970). (8) T. S. West, Pure Appl. Cliem., 23, 99 (1970). (9) D. 0. Cooke, R. M. Dagnall, H. N. Johnson, G. F. Kirkbright, and T. S. West, Lab. Prrrct.. 21, 171 (1972). 226
absorption spectrometer, for the determination of copper, magnesium, manganese, and strontium by atomic emission spectrometry. In a multi-element A F technique, it is necessary to illuminate the flame with sufficiently intense radiation of the resonance lines of the elements to be determined, to stimulate their atomic fluorescence. Xenon arc lamp continua have been employed to obtain the atomic fluorescence of a number of elements using a single source. However, the peak intensities of these lamps normally occur in the region of 400-500 nm, and they are much less intense in the 200-285 nm wavelength range, where A F is frequently the most sensitive flame method of analysis. The detection limits obtainable with these sources are usually at least a n order of magnitude inferior to those with line sources (10). This makes them unsuitable as spectral sources for a scanning technique. The limitations on the construction and operation of multielement hollow cathode lamps have been discussed elsewhere (7, 8). The preparation and operation of a number of multielement electrodeless discharge lamps (EDL) have been reported (7, 11-14). Although some of these multi-element EDL have been used for the determination of up to four elements ( 7 , I I ) , the detection limits are not good for all of the elements, and because of present constructional difficulties it has been found best to restrict these EDL to contain only two elements. Dual-element EDL are used with the scanning technique, and the use of a two-port power divider is described for the operation of two EDL from one microwave generator, thus providing a n economical means of simultaneously illuminating the flame with the intense radiation of the resonance lines of four elements. EXPERIMENTAL
Apparatus. A modified ' Southern Analytical A1740 grating flame spectrometer and ancillary equipment for A F determinations, as previously described (I.?), was used for a n evaluation of the spectral and operating characteristics of the dual-element EDL and for some preliminary studies of the scanning technique. The spectrometer was equipped for scanning a t 75 nm min-1. This instrument has a dc amplification system, and with the scanning technique provides no means of backing off the flame background emission. Therefore, although this instrumental system was not particularly suitable for a scanning technique, satisfactory results were obtained for zinc and cadmium. The following experimental arrangement was, therefore, finally selected for the scanning technique:SPECTRAL SOURCESThe spectral sources were dual-element EDL, operated at 2450 =t 25 MHz with a Microtron 200 (IO) J. D. Winefordner, V. Svoboda, and L. Cline, C.R.C.Crit. Rec. Aiiul. Cliem., 1, 233 (1970). (11) H. A. Fulton, K. C. Thompson, and T. S . West, Anal. Chim. Acto, 51, 373 (1970). (12) M. S. Cresser and T. S. West, ibid., p 530. (13) J. D. Norris and T. S . West, ibid., 55, 359 (1971). (14) Ibid.. 59, 474 (1972).
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Table I. Construction and Operating Parameters and AF Detection Limits for Some Dual-Element EDL A€ Operating Wavelength, detection power, limit, ppma Element watts Contents nm EDL 0.001 213.8 Zinc 60 1 mg zinc, 1 mg cadmium, 3 Torr argon Zinc-cadmium (7) Cadmium 228.8 0.001 Gallium 1 .Ob 417.2 60 5 mg gallium, 5 mg indium, 3 Torr argon Gallium-indium (7) Indium 451.1 0.2* Selenium 1O b 204.1 40-50 5 mg selenium, 5 mg tellurium, 5 mg Selenium-tellurium (7) Tellurium 1, 5 b 283.3 iodine, 1 Torr argon Arsenic 235.1 25c 40-45 4 mg arsenic, 4 mg antimony, 27 mg Arsenic-antimony ( 1I ) 217.6 0.9c Antimony iodine, 3 Torr argon Cobalt 240.7 0.01 60 1 mg cobalt chloride, 1 mg nickel chloCobalt-nickel (13) Nickel 232.0 0.02 ride, 4-5 Torr argon 279.8 0.005 Manganese 60 1 mg manganese chloride, 1 mg chroManganese-chromium (14) 359.3d 0.005 Chromium mium chloride, 4 Torr aigon Nickel 232.0 0.01 60 1 mg nickel chloride, 1 mg chromium Nickel-chromium 359.3d 0,008 Chromi um chloride, 4 Torr argon 324.7 0.01 50 1 mg copper chloride, 1 mg nickel chloCopper-nickel Copper 0.07 232.0 Nickel ride, 4 Torr argon 0.01 240.7 Cobalt 60 1 mg cobalt chloride, 1 mg ferrous chloCobalt-ir on 248.3 Iron 0.1 ride, 4 Torr argon 0.01 405.8 Lead 50 1 mg lead chloride, 1 mg manganese Lead-manganese 0.01 279.8 Manganese chloride, 3 Torr argon 228.8 Cadmium 0.008 45 1 mg cadmium chloride, 1 mg lead chloCadmium-lead 0.3 405.8 Lead ride, 3 Torr argon 248.3 Iron 0.01 60 Iron-manganese 1 mg ferrous chloride, 1 mg manganese 0.005 279.8 Manganese chloride, 4 Torr argon a In argon-separated air-acetylene flame using modified Southern A1740 with 10- or 20-second integration. * In air-propane flame, using unmodified Southern A1740 with 20-second integration (7). In unseparated air-acetylene flame using Techtron AA4 (11). d Combined fluorescence at 357.9 nm, 359.3 nm, and 360.5 nm (14).
Mark I1 microwave generator (Electro-medical Supplies Ltd., Wantage, U.K. Model 2000L) and two three-quarter wave (Broida type) resonant cavities (EMS Model 210L). In order to operate the two resonant cavities from one microwave generator, a two-port power divider (EMS Model 4000L) was used. This was connected t o the microwave generator by a n 18-111. length of coaxial cable (EMS Model 2009L), and to the two resonant cavities by two 54-in. lengths of coaxial cable (EMS Model 2010L). The use of the two-port power divider enabled the output from the microwave generator t o be divided equally between the two resonant cavities. This meant that the EDL in both resonant cavities had t o be operated at the same incident power. However, the majority of the dual-element EDL have optimal operating powers of ca. 60 watts, so that the choice of E D L which could be operated together from one microwave generator was not greatly restricted. The output from the microwave generator was modulated a t 285 Hz, using a Microtron Mark I1 modulator unit (EMS Model 3005L). Discharge was initiated using a Tesla high frequency vacuum tester. BURNER. An air-acetylene flame was supported o n a Meker type circular burner head (Beckman-RIIC Ltd., London, U.K.). This flame was separated with argon using a n emission burner shield (Beckman-RIIC). SPECTROMETER. The spectrometer used was a Techtron AA4 atomic absorption spectrometer (Varian-Techtron Pty. Ltd., Melbourne, Australia), fitted with a Hamamatsu Type R213 UV-sensitive photomultiplier. This spectrometer has a range of monochromator slit-widths u p to 0.3 mm, corresponding t o a spectral band-pass of 1 nm. The only scanning facilities available with this instrument are a t 450 nm min-’, which is too fast for use with conventional chart recorders. Therefore, a variable speed stirrer motor (Citenco Ltd., Borehamwood, U.K. Model KQPS 24) was connected t o the monochromator “wavelength set” control by means of a brass sleeve. This made available a choice of scanning speeds between 10 and 50 n m min-l. This motor could be engaged o r disengaged using the arrangement for the opera-
tion of the “wavelength set” control, so that when disengaged the scanning motor on the spectrometer could be used for resetting the wavelength after a scan. The amplifier, which is modulated a t 285 Hz, was synchronized with the Microtron modulator unit via the connector normally used to synchronize a chopper. The output signal from the amplifier was connected to a Servoscribe potentiometric chart recorder (Smiths Industries Ltd., London, U.K. Model R E 51 1). The optical arrangement used is OPTICAL ARRANGEMENT. shown in Figure 1. The dual-element E D L were placed as close as possible t o the flame (ca. 60 mm from the center of the flame). The resonant cavities were ca. 10 mm apart, and it was not necessary t o place any grounded screening between them to prevent interaction. The only difficulty caused by placing the resonant cavities so close together was experienced in initiating the discharge of both EDL. However, once alight, the close proximity of the two resonant cavities did not appear to affect the operation of the EDL. It was necessary t o cover one of the side holes of the cavity nearer the monochromator t o prevent any possibility of stray radiation from the EDL falling o n the entrance slits of the monochromator. either directly or through the condensing lens. The atomic fluorescence from the flame was focussed onto the entrance slits of the monochromator with a silica condensing lens of focal length 62.5 mm and diameter 25 mm. CONSTRUCTION OF DUAL ELEMENTEDL. The equipment required and general procedure for the construction of EDL have been described elsewhere (15). The fill materials and argon pressures used for a number of dual-element EDL, together with their optimal operating powers and A F detection limits are given in Table I. The preparation of several of these E D L has been described in more detail previously (7, II,13,14). Reagents. All stock solutions (1000 ppm) were prepared from analytical-grade reagents and distilled water. The (15) R. hl. Dagnall and T. S. West, Appl. Opt., 7, 1287 (1968).
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microwaw grneralor
modulator
I
0
Table II. AF Detection Limits and Working Ranges for Zinc, Cadmium, Nickel, and Cobalt Using the Scanning Technique Working Detection Wavelength, range, limit, Element nm PPm PPm 0.02-1.5 0.003 Zinc 213.8 0.02-1.0 Cadmium 228.8 0.002 0.2-10 0.02 Nickel 232.0 Cobalt 240.1 0.2-10 0.02
motor
Figure 1. Instrumental arrangement
Figure 2. AF wavelength scan for zinc, cadmium, nickel, and cobalt stock solutions were diluted as required immediately before use. Procedure. The discharge of both EDL was initiated and they were allowed to stabilize at their normal operating power for ca. 10 min. The wavelength control was set so that a wavelength ca. 5 nm below that of the lowest resonance line required was indicated. The scanning motor was engaged, the solution was nebulized into the flame, and the chart recorder and then the scanning motor were switched on. After scanning to ca. 5 nm above the wavelength of the highest resonance line required, the scanning motor was switched off and disengaged, and the chart recorder switched off. The monochromator was returned to the original wavelength by means of the spectrometer scanning motor. The procedure was then repeated with the next solution. As usual in A F determinations, it is desirable to nebulize a standard solution between the unknown solutions. The peak heights are measured, and those of the unknown solutions compared with those of the standard solutions.
RESULTS AND DISCUSSION Selection of Operating Parameters. The elements to be determined must be analyzed in the same flame and under the same instrumental operating parameters. Since it is unlikely that the operating conditions will be optimal for all of the elements to be determined at one time, a compromise must be made, and either the optimal operating parameters for the least sensitively determined element, or the most favorable conditions for the majority of the elements will have to be selected. 228
The operating parameters used in this work were those found to be optimal for the majority of the elements to be determined. The optimal operating conditions for the multielement determination of zinc, cadmium, nickel, and cobalt are discussed below. Flame Conditions. In order to reduce both the flame background emission and the flame noise, and so to obtain a flat base line for the wavelength scan, a low-background flame, such as argon-separated air-acetylene, is desirable. The flame was slightly fuel-lean and flow rates of 0.6 1. min-l acetylene and 8 1. min-I air were used. A flow rate of 20 1. min-l argon was necessary to separate this flame. The part of the flame viewed by the monochromator was between 5 and 15 mm above the burner head. Monochromator Slit-Width The monochromator slitwidth must be selected so that the resonance lines of the elements whose atomic fluorescence is stimulated by the light source, do not overlap within the spectral band-pass. Preferably this should be arranged so that the peaks produced on the wavelength scan are completely separate, thus enabling more accurate measurements of the peak heights to be made. Since wide spectral band-passes are advantageous in AF, this places a restriction on the elements which may be determined together in a scanning technique. The maximum monochromator slit-width of 0.3 mm, corresponding to a spectral band-pass of 1 nm, was used throughout this work. Scanning Speed. The speed used for scanning must be compatible with the response times of the amplification system and the chart recorder. Under certain circumstances. it is possible that the scanning speed could also be dependent upon the monochromator slit-width. The response time of the Servoscribe chart recorder, which was slower than that of the amplifier, was ca. 1 second. In order to obtain a faithful response (i,e.,peak height = steady state signal), a scanning speed of 25 nm min-l was required. However, at a scanning speed of 75 nm min-I, using the Southern A1740 spectrometer, peak heights were ca. 80% of the steady state signals and percentage standard deviations in peak heights were ca. 6 %. Therefore, considerably faster scanning speeds may be used without too great a loss in sensitivity or reproducibility. The chart recorder was operated on the 10-mV range a t a speed of 30 mm min-'--i.e., 1 mm on the chart paper represented a wavelength scan of 0.83 nm. Wavelength Scans for Zinc-Cadmium-Nickel-Cobalt. A typical A F wavelength scan, between 210 and 245 nm, obtained while nebulizing a solution containing 0.1 ppm zinc, 0.1 ppm cadmium, 1 pprn nickel, and 1 ppm cobalt, is shown in Figure 2 . The wavelength range scanned includes the major resonance line of each of these elements. The actual scanning time for this wavelength scan was ca. 85 seconds, and the total time required for such a determination for the four elements was ca. 100 seconds. Detection Limits. The detection limits obtained for the determination of zinc, cadmium, nickel, and cobalt by
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY 1973
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Figure 3. AF wavelength scan for selenium, nickel, tellurium, and cobalt
Figure 4. AF wavelength scan for nickel, cobalt, iron, and manganese
scanning A F are given in Table 11. Because of the use of peak height measurement, the detection limits were defined as the concentration of the element in aqueous solution which produced heights equivalent to twice the standard deviation in peak height near the lower limit of the working range. These detection limits are of the same order as those previously obtained for these elements by conventional AF, using dualelement EDL and a n argon-separated air-acetylene flame (see Table I). Obviously, a direct comparison, even using identical instrumentation, is of little value, because of differences in the definitions of detection limits for scanning and conventional AF. However, it would appear that there is no significant reduction in sensitivity in the scanning technique. Operating Range. The operating range for a n element in A F is normally taken as the range of concentration over which the atomic fluorescence signal is linearly related to the concentration of that element nebulized into the flame--i.e., from the detection limit to the upper concentration limit. A F operating ranges frequently cover several orders of magnitude, and to obtain a reasonable sensitivity over the whole of this concentration range, adjustments of the instrumental gain or of the power supply, and hence the intensity, of the spectral source must be made. Obviously, it is impractical to alter the instrumental operating parameters during a wavelength scan. This places a restriction o n the composition of solutions that can be determined by the scanning technique, if a reasonably sensitive determination of each element is to be obtained. Thus more than one dilution of a sample may be necessary to obtain a complete analysis. This is the main outstanding problem with any multi-element technique involving atomic spectrometry, and was referred to by West et al. (3, 4) in relation to determinations with their multi-channel A F spectrometer. For the purposes of this investigation, operating ranges with their lowest point ca. 10 times greater than the detection limit were selected, and analytical curves for each element obtained within these ranges. These analytical curves were linear, and the operating ranges used for zinc, cadmium, nickel, and cobalt are given in Table 11. These operating ranges are merely an example of those which are compatible
for use together to enable a sensitive determination of each element to be made with one scan. Interferences. The effects of 500-ppm concentrations of thirty-seven elements (Ag, Al, As, Ba, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Hg, In, K, Li, Mg, Mn, Na, Nb, Ni, Pb, Se, Si, Sm, Sn, Sr, Ta, Te, Th, Ti, V, W, Zn, and Zr) of the A F wavelength scan produced by a solution containing 0.5 ppm zinc, 0.5 ppm cadmium, 5 ppm nickel, and 5 ppm cobalt were investigated. These concentrations represented 1000fold (weight) excesses o n zinc and cadmium and 100-fold (weight) excesses o n nickel and cobalt. N o interference (i.e., less than i5 deviation in signal) was observed with any of these elements. These observations agree with the results of previous A F interference studies for zinc (16), cadmium (17), nickel (13), andcobalt (13,18). Scatter. Normally in AF, it is possible to back off any signal produced by the reflection or scatter of the incident radiation. This is not possible in a scanning technique. However, with the optical arrangement used, no reflection or scatter was observed, except near the detection limit, when high gains were used, and even this was negligible. Reproducibility. The percentage standard deviation in peak height for ten A F wavelength scans obtained while nebulizing a solution containing 0.5 ppm zinc, 0.5 ppm cadmium, 5 ppm nickel, and 5 ppm cobalt, over a period of one hour, with other solutions nebulized in between, was less than 3 for each of the four elements. Wavelength Scans for Other Combinations of Elements. SELENIUM-NICKEL-TELLURIUM-COBALT. A typical A F wavelength scan obtained while nebulizing a solution containing 1000 ppm selenium, 1 ppm nickel, 10 ppm tellurium, and 1 ppm cobalt, is shown in Figure 3. This scan was taken between 200 and 245 nm, and thus incorporates the major resonance lines of each of these elements. (16) M. S. Cresser and T.S. West, Aizal. Chim. Acta, 50, 517 (1970.) (17) R. M. Dagnall, T. S. West, and P. Young, Talanta, 13, 806 (1966). (18) B. Fleet, K. V. Liberty, and T. S. West, Anal. Chim. Acta, 45, 205 (1969).
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NICKEL-COBALT-IRON-MANGANESE. A typical A F wavelength scan obtained while nebulizing a solution containing 1 ppm nickel, 1 ppm cobalt, 1 ppm iron, and 0.5 ppm manganese, is shown in Figure 4. This scan was taken between 227 and 285 nm, and thus incorporates the major resonance line of each of these elements. CONCLUSIONS
A scanning technique for multi-element determinations by AF, in conjunction with dual-element EDL appears to be a feasible method of analysis for a number of combinations of elements. The procedure is simple and all of the instrumentation is commercially available. The sensitivity and selectivity appear to be as good as for conventional A F with the same type of source and flame.
The time taken for a determination of zinc, cadmium, nickel, and cobalt is ca. 100 seconds; that for selenium, nickel, tellurium, and cobalt is ca. 140 seconds; and that for nickel, cobalt, iron, and manganese is ca. 150 seconds. These times may be improved upon by using a faster scanning speed, without too great a loss in sensitivity or reproducibility. The main disadvantage of the technique, as with any multielement atomic spectrometric method, is that the solutions must be of a limited concentration with respect to each element, to avoid more than one dilution, and, hence, more than one analysis of each sample RECEIVEDfor review July 31, 1972. Accepted October 2, 1972. We thank the Science Research Council for the award of a research studentship to J.D.N.
Determination of Sub-NanogramQuantitiesof Silver in Snow by Furnace Atomic Absorption Spectrometry Ray Woodriff, Bruce R. Culver,’ Douglas Shrader, and Arlin B. Super Montana State Unicersity, Bozeman, Mont. 59715 An analytical method for determining microtrace concentrations of Ag in snow is discussed. The method involves preconcentration of the Ag by solvent extraction and its subsequent determination by furnace atomic absorption (FAA). The extractant is a dithizoneCC14 solution. Over two hundred twenty-five snow samples were analyzed by this method. A comparison among microsampling boat flame AA, neutron activation analysis, and furnace AA is presented. The results obtained by the boat technique are generally higher than those obtained by the other two methods. Where neutron activation analysis is &15-40% reproducible, FAA’s reproducibility is 15%. The concentration ranges involved are on the order of 5 X 10-1‘ g/ml. The sensitivity for the FAA method is 5 X 10-13 g/ml.
SILVERIODIDE is the most commonly used agent in weather modification programs. The detection of silver in snow from clouds seeded with silver iodide is of interest in evaluating the success of such programs. Detection of silver in above background levels does not prove that the silver iodide crystals caused nucleation of supercooled cloud droplets and subsequent ice crystal growth and fall out. The silver iodide particles might have been scavenged by natural snowflakes and/or simply have been deposited on the snow surface. However, the absence of above-background levels of silver in an assumed target area indicates that proper targeting was not accomplished. Hence, measurement of the spatial variation of silver concentration in snow from a seeded storm or storms can be a valuable tool in the evaluation of a cloud-seeding experiment. During the past five years there have been two accepted techniques for measuring Ag in snow. The first and most widely accepted is neutron activation analysis (1). This expensive technique requires large samples (1 to 2 liters of Present address, Varian, Los Altos, Calif. 94022. ~~
(1) J. A. Warburton and L. G. Young, J . Appl. Meteorol., 7, 433
(1968). 230
melted snow), a source of neutron flux, and has a fractional standard deviation of + 15-40 in the range of Ag concentrations in snow. Recently another method has come into existence. This less expensive method is a flame atomic absorption technique involving a microsampling boat ( 2 , 3). The claimed detection limit for this technique ranges from 10-lO to 5 x lo-” giml of silver. These reported limits incorporate a preconcentration step and a 1OX recorder scale expansion. AAS has been used for the silver determination in seeded snow, but the procedure lacks reliability since the values reported are dependent upon considerable extrapolation ( 4 ) . Use of the microsampling boat improves this somewhat. Due to the extremely low Ag concentrations in snow, a method must be very sensitive to be useful. Using neutron activation analysis, E. Bollay Associates (5) found that the background concentration of silver in the spring snow pack of the western United States ranged from 0-20 X g/ml. Samples collected from the target areas of cloud seeding programs yielded silver concentrations ranging from 20-200 x lo-’* g/ml (melted snow). The sensitivity of furnace atomic absorption (FAA) for g/ml with a lox preconsilver is on the order of 5 x centration step and no recorder scale expansion. The reproducibility of the method in the 5 X 10-l1 g/ml range is = 5 for repeated determinations of the extractant solution from a single sample. FAA should be an excellent method for microtrace silver determinations. There are, however, many prob(2) Denver, Colorado Meeting, May 18, 1971, on “The determination of silver in terrestrial and aquatic ecosystems.” (Details available from H. L. Teller, Colorado State University, Ft. Collins, Colo.) (3) H. L Kahn, G. E. Peterson, and J. E. Schallis, A?. Absorption Newslett., 7, 35 (1968). (4) F. P. Parungo and C. E. Robertson, J . Appl. Meteorol., 8, 315 (1969). ( 5 ) E. Bollay Associates, Final Report, Bureau of Reclamation Contract No. 14-06-D-5573, 20 pp (1965).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY 1973
