generally be expected to at least reduce, if not eliminate, interference from "fluorescence background" by the proper choice of modulation conditions. Selective modulation should be particularly useful for the identification and quantitation of the minor component in a mixture of two fluorophores. This capability might be useful, for example, in studying unresolved peaks in the chromatography of fluorescent compounds. For very complex mixtures, the combination of a chromatographic separation followed by selective modulation fluorescence spectrophotometry might be especially useful. In fact, since the selective modulation system works in real time, a fluorescence detector for LC and GC could utilize the selective modulation principle. Another area of application might be in energy-transfer studies. It is possible that the energy transferred from a radiationally-excited doner may be able to be modulated by wavelength modulation of the excitation monochromator. On this basis, it may be possible to distinguish an energy transfer process from direct radiational excitation of the acceptor. Of course, there are limitations. First, it must be realized that selective modulation does not in any way reduce the total intensity of the nulled luminescence at the photode-
tector. This nulled luminescence will generate a certain amount of photomultiplier shot noise which will be evident in those regions of the recorded spectrum in which the nulled luminescence is intense. Second, selective modulation does nothing to reduce fluorescence quenching, inner filter effects, and other phenomena which affect the slope of the analytical curve. These effects must be considered as separate problems, to be hahdled as far as possible by the conventional methods such as dilution or standard addition. Finally, selective modulation does not increase signalto-noise ratios, so it cannot be considered useful in situations in which the limiting factor is the low intensity of luminescence. Of course, selective modulation could profitably be combined with a noise-reduction instrument such as an ensemble averager.
RECEIVEDfor review May 22, 1974. Accepted July 19, 1974. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research. From a dissertation to be submitted to the Graduate School, University of Maryland, by W. M. Parks, in partial fulfillment of the requirements for the Ph.D. degree in Chemistry.
Solvent Extraction for Use with Flame Atomic Absorption Spectrometry John D. Kinrade and Jon C. Van Loon Department of Geology, University of Toronto, Toronto, Ontario
A method of solvent extraction is described for concentrating metals from natural waters prior to analysis by flame atomic absorption. Using two chelating agents, ammonium pyrrolidindithiocarbamate (APDC) and diethylammonium diethyldithiocarbamate (DDDC) a total of eight metals can be simultaneously extracted-cadmium, cobalt, copper, iron, lead, nickel, silver, and zinc. The method is simple, fast, free from the effects of many interfering ions, and has high sensitivity and good precision (generally less than 5 % coefficient of variation on a routine basis). Accuracy tested using EPA standard waters was found to be good.
There .~have been few critical studies of solvent extraction/atomic absorption procedures. Those which have been done, e.g., Koirtyohann and Wen ( 4 ) , commonly deal with only a few of the problems which can be encountered. A comprehensive critical study of most of the popular solvent extraction/atomic absorption systems has been done in our laboratory. Some of the important findings at this evaluation are given in the following report together with a mixed ligand, solvent extraction procedure for 8 environmentally important heavy metals. The procedure outlined is simple, fast, and reliable for analysis of natural water samples.
EXPERIMENTAL Solvent extraction methods for concentrating trace metal ions in waters and other environmental samples, prior to atomic absorption analysis, abound. Unfortunately the majority of these have been developed without regard for important theoretical data available from such sources as Stary ( I ) ,Morrison and Freiser (2),and Zolotov ( 3 ) .As a result, available procedures are seldom optimized with respect to pH range, buffer, ionic strength, stability, equilibration time etc. This means it is often impossible for the analyst in a laboratory to obtain good results on a routine basis. (1) J. Stary, "The Solvent Extraction of Metal Chelates," Macmillan, New York, N.Y. 1964. (2) G. H. Morrison and H. Freiser, "Solvent Extractions in Analytical Chemistry," J. Wiley and Sons, New York. N.Y. 1957. (3) Y. A. Zolotov, "Extraction of Chelate Compounds," Ann Arbor-Humphrey Science Publishers, Ann Arbor, Mich.. 1970.
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ANALYTICAL CHEMISTRY, VOL. 46,
NO. 13,
Apparatus. An I.L. (Instrumentation Laboratories Inc.) 153 was used for all atomic absorption measurements. p H measurements were made using a Beckman expandomatic pH meter. Reagents. All chemicals used were of reagent grade or the highest quality available. Standard metal solutions (1000 ppm) were prepared for Ag(I), COW), Fe(III), Mn(II), Ni(II), Pb(II), and Zn(I1) from the pure metal and for Cd(II), Cr(III), Cr(VI), Hg(III, and Mo(I1) from the corresponding salt or oxide. Choice of Solvent. The solvent used to extract the metal complexes formed must show a number of desirable characteristics: extract the desired metal-chelates; be immiscible with the aqueous solution; not tend to form emulsions; have good burning characteristics; and enhance rather than suppress the atomic absorption sensitivity as compared to the metal in water. Work was carried out on a number of likely solvents. Benzene and xylene were eliminated because of the turbulant and unstable flames they produced. Decanol proved to have too pungent an odor. Chloroform, a solvent widely used in colorimetric work, had (4) S.R. Koirtyohann and J. W. Wen, Anal. Chem., 45, 1968 (1973)
NOVEMBER 1974
already been tested by earlier workers (5). This solvent evaporates too quickly leaving the solid complex behind, which then clogs the vaporization chamber. Ethyl acetate, methyl isobutyl ketone, isoamyl acetate and n-butyl acetate were also considered. Of these, ethyl acetate and methyl isobutyl ketone (MIBK) gave the greatest enhancements to metal absorbances as compared to the absorbance of the same quantity of metal in water. However, ethyl acetate proved too volatile to work with so the use of MIBK was settled on. A review of the literature showed t h a t MIBK was one of the more favored solvents in present use (6-8). Choice of Buffer. The use of a buffer is mandatory in routine extraction work. This fact is not recognized by many workers. I t is well known that the quantity of metal extracted is strongly dependent on the p H of the solution (1-3) and the chelating agents will often alter the p H of the solution t o which they are added. The choice of the buffer is very important. It must be stable, have a high buffering capacity, and not participate in any reaction. A number of buffers were studied: borate, phosphate, citrate, acetate, and formate. Solutions containing the formate buffer were unstable and slowly decomposed (organic droplets appeared on container walls) after several days. An acetate buffer was also unfavorable. I t would combine with any lead or silver in solution to form stable P b - and Ag-acetates which were not readily extracted. In the final extraction procedure, a citrate buffer which was stable and did not interfere with the extraction process was used. This buffer, however, does contain considerable cadmium and iron. The buffer should be purified as well as possible of trace metal contaminants by an extraction wash using the chelating agent. In spite of this precaution, a blank must be run with each set of samples. If Ag and P b are not to be analyzed for, an acetate buffer is recommended because of lower Cd and Fe contamination. Choice of Chelating Agent. Two of the main considerations affecting the choice of a chelating agent were that it would extract all or most of the desired metals and that it would extract all these metals equally well over some fairly wide range of p H so that there would be some allowable error in adjusting the p H of the solution. Many procedures available a t present require p H adjustment to within one p H unit or less which can result in serious errors in routine applications. T h e p H dependence of nine different chelating agents was studied. These chelating agents were 8-quinolinol, acetylacetone (HAA), thenoyltrifluoroacetone (HTTA), l-(Z-pyridylazo)2-naphthol (PAN), ammonium pyrrolodindithiocarbamate (APDC) potassium ethyl xanthate (KEX), a-benzoinoxime (CUPRON), diethylammonium diethyldithiocarbamate (DDDC), and sodium diethyldithiocarbamate (NaDDC). 8-Quinolinol. Figure 1 for 8-quinolinol illustrates an undesirable case of pH/extraction dependence characteristic of many popular extracting reagents. In the procedure employed, 200 ml solution of 0.25 ppm metal were extracted with 5 ml of 1%oxine in acetone and 35 ml of MIBK. Separatory funnels were shaken for 2 minutes and allowed to sit for 15 minutes. Surprisingly most papers do not provide detailed data on p H dependence. The variation in absorbance is plotted for a variation in p H of the aqueous solution. Note that the shapes and positions of the curves vary strongly with pH. In general, the shapes of extraction curves are not readily explainable. For example, Morrison and Freiser (2) state the following in a discussion of the distribution of 8-quinolinol between chloroform and water. In this system "the situation is complicated by the ionization of 8-quinolinol in the aqueous phase, so t h a t although the distribution coefficient of the 8-quinolinol remains essentially constant, the stoichiometry of the extraction varies very dramatically," as the p H changes. 8-Quinolinol then, does not satisfy the earlier stated requirements. It does not extract all the metals equally well a t any one p H under the given conditions. Most of the other chelating agents studied also showed this strong p H dependence. APDC + DDDC Combination. Of all the nine chelating agents studied APDC showed the best characteristics. Figure 2 shows the extraction curves. Note that in comparison to 8-quinolinol there is now a range of pH's over which most of these metals can be extracted equally well. T h a t is, there is a plateauing of the absorban( 5 ) E. Lakanen, At. Absorption Newslett., 5 , 17 (1966). (6) J. H. Culp, R. L. Windham, and R. D. Whealy, Anal. Chem., 43, 1321 (1971). (7) K . K. Kuwata, K. Hisatomi, and T. Hasegawa, At. Absorption Newslett., I O , 111 (1971). (8) Y . K. Chau and K. Lum-Shue-Chan, Anal. Chim. Acta, 48, 2 0 5 (1971).
2.4
Trace Ag pH 8-1 I 1.6 1.4 C
x h t 9 t 1.2
0.6 k-
I
.
\
0-\
n
PH Figure 1. pH dependence of extractions with 8-quinolinol (scale 2.5 = absorbance; scale 1.0 = 2.5 X absorbance: all scales are 1 . 0 . unless otherwise stated, for all figures)
F/). 0.2
j
t 0.0
0
\
/*-• ' '
2
4
\.,
'.,
--..?
6
*
8
'
' '
.A*-
IO
\
\. 12
'
14
PH Figure 2. pH dependence of extractions with APDC ces so that, as the p H changes, the absorbance remains the same. APDC alone, however, was not good enough since two of its complexes, manganese and iron, were quite unstable and, as seen in Figure 2, silver was not extracted very well. Various combinations of pairs of chelating agents were then studied in detail and the combination APDC DDDC gave the best results. Figure 3 shows the extraction curves resulting from using this mixture of chelating agents. As before, little can be said to quantitatively explain the shapes of these curves. At pH's higher than 8 (in CC14 and CHCl,?),DDDC is transferred almost completely into the aqueous phase (2). Both APDC and DDDC are quickly destroyed in acid solutions.
+
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
1895
minute and is then allowed to sit for 5 to 10 minutes. (vi) After the waiting period is up, the aqueous phase is drained and the organic phase is collected and analyzed for its metal content by flame atomic absorption. By this proceudre, eight metals can be extracted: Ag, Cd, Co, Cu, Fe, Ni, P b , and Zn. Determination of Molybdenum and Chromium. If it is necessary to determine Mo and Cr6+, the chelating agents APDC and DDDC can be used separately. Chromium(V1) 1s extracted at a pH of 2 to 4 using 5 ml of 1%w/v solution of DDDC in water and an acetate buffer. (or no buffer a t all if the low acid p H is used). A sensitivity of about 20 ppb is achieved. Molybdenum is extracted between a pH of 0 to 2 using 5 ml of a lob w/v solution of APDC in water. At these low pH's, a buffer is normally not required. A sensitivity of about 45 ppb is achieved (using a nitrous oxide-acetylene flame).
Cd Scale 2.5
Zn Scale 2.5
$
1.4
C
;
1.2
0
2 a
1.0 0.8
0.6 0.4 0.2
' / ' , , k.4
0.0 0
2
I 4
I
,
,
6
I 8
I
I IO
I
I 12
14
PH Figure 3. pH dependence of extractions with APDC -k DDDC
It is apparent that all of the metals with the exception of silver could be extracted well between pH of 3 and 6. If silver is to be analyzed along with the other metals, the extraction must be carried out between a pH of 4.5 and 6.0. In general, the addition of DDDC seems to have a stabilizing effect on all the metal complexes in the system. The distribution ratios for metal complexes of APDC + DDDC at pH 5.0 are as follows: Ag 1800, Cd 2500, Co 8000, Cu 5000, Fe 2800. Ni 4200, P b 2300, and Zn 160. Procedure. The final procedure developed involves six steps for each extraction: ii) 200 ml of an aqueous sample solution is poured into a 250 ml separatory funnel. (ii) 4 ml of a citrate buffer is added and the pH of the solution is checked to ensure that it is around 5.0 (the buffer is 1.2M sodium citrate and 0.7M citric acid). (iii) 5 ml of the chelating solution is added (this is 1%w/v in each of APDC and DDDC in water). (iv) 35 ml (or less) of MIBK is added. (v) The separatory funnel is shaken for 30 seconds to a 1896
ANALYTICAL CHEMISTRY, VOL. 46, NO.
RESULTS AND DISCUSSION Variation in Ratio of APDC to DDDC. Three different mixtures of chelating solution were prepared to determine if the ratio of APDC to DDDC made a difference during extraction. The different ratios of APDC to DDDC in grams per 100 ml of solution were 1.0:1.0, 1 3 0 . 5 , and 0.5: 1.5, respectively. The results are shown in Table I. It is recognized that for a set of extractions it does not matter what the exact ratio of chelating agents is since all the sample solutions would be extracted with this same chelating solution. However, it does appear that there is little difference resulting from the three mixtures tested. In all cases, the chelating solution should be prepared fresh daily and filtered to remove the insoluble material introduced by the APDC. (In all work to follow, a ratio of 1.O:l.O was used). Shaking and Waiting Times. I t is necessary that the solutions be shaken long enough that equilibrium may be achieved for the distribution of the metal complexes between the aqueous and organic phases. T o determine the time required to reach equilibrium, a number of identical solutions (250 ppb of each metal in a 200-ml solution at p H 5.0) were prepared and, after the chelating agent and MIBK were added, each was shaken and allowed to sit for a different length of time. With the exception of silver, as long as the solutions are shaken for a t least 30 seconds and left for 5 minutes or more, equilibrium may be assumed to have been reached. The silver complex, however, appears to he destroyed as the shaking time is increased. Therefore, if silver is to be analyzed along with the other metals, the shaking time is critical, while the time that the mixture is left to sit before the aqueous phase is drained, is of lesser importance. Precision and Accuracy. The precision of the method was tested by individually preparing ten solutions all a t a p H of 5.0 and each containing the same quantity of each of the eight metals. The precision was expressed in terms of the coefficient of variation, and the results are shown in Table I1 for solutions containing 250 ppb and 25 ppb. Note that the solutions containing 250 ppb of metal were shaken for one minute and allowed to sit for 10 minutes while the separatory funnels containing the 25 ppb solutions were inverted at a constant rate, instead of being shaken, for one minute. The precision for the 250-ppb solutions is very good except for silver, whose complex is very sensitive to the duration and vigor of shaking. Simply inverting the separatory funnel, either a given number of times or a t a constant rate for a fixed time interval, is felt to be far more reproducible and therefore leads to the higher precision for the silver determinations. T o give some idea of the performance of the procedure during routine use, the EPA water standards 1 and 2 were analyzed at random during analysis of other samples. Typical results are given in Table 111.
13, NOVEMBER 1974
Table I. Variation in Ratio of APDC to DDDC APDC; D DDC
Metal absorbances
(grams per 100 m l
Pb
Fe
iii
cu
co
Cd
Zn
.%
0.468 0.491 0.487
0.723 0.731 0.735
0.603 0.610 0.611
1.185 1.194 1.203
0.786 0.791 0.795
1.288 1.290 1.282
1.870 1.870 1.870
1.170 1.052 1.140
solution)
1.5:0.5 1.O:l.O 0.5:1.5
Table 11. Precision Measurements Expressed as 9’0 Coefficient of Variation Ag
Solution Concentration
7.16 1.86
250 ppb (solutions shaken) 25 ppb (solutions inverted)
Cd
co
cu
Fe
Ni
Pb
Zn
0.26 0.92
0.67 5.23
0.52 4.27
0.99 6.46
0.37 5.51
0.41 4.26
0.29 2.25
Table 111. Typical Results Routinely Obtained on EPA Water Standards (ppb) Sample
EPA 1 EPA 2
Fe
n . 7
Pb
11 (10) 18 (18) 27 (28) 78 (79) 368 (402) 93 (92)
cu
Cd
12 (9) 76 (67)
1.7 (1.8) 15 (16)
Table IV.Stability of Metal-APDC-DDDC Complexes Metal a Time (hours)
Ag
834
100
223/4
100
80’4
984 a
28.0 0.1
Cd
Co
Cu
Fe
100 100 100 100 100 95.0 100 100 94.0 100 100 100 58.0 100 100 100
Xi
Pb
Zn
100 100 100 64.4 100 56.0 100 100 27.1 100 100 100
Given in per cent. metal left in the MIBK after a given time.
long as the extractions and analysis are carried out in the same day, no problems will arise. This can be contrasted with the stability using APDC alone where after 16 hours the levels of Fe, Pb, and Zn were lowered to 60. ’75, and 25%, respectively. Sensitivity and Range of Linearity. There are several ways of describing sensitivity. The present paper uses the definition of sensitivity as that concentration which produces a 1% absorption. T o determine the sensitivity for each metal, a number of aqueous standards ( 0 , 1, 5 , 10, 20, 25, 50, 100, 250, 500, 750, 1000, 5000, 10,000, and 15,000 ppb in each metal, pH 5.0) were prepared and their absorbances measured after extraction. From these standards, the range over which there is a linear relationship between concentration and absorbance was also determined. The results are shown in Table v. Interferences. In the analysis of water samples, a number of ionic species may be encountered a t concentration levels far greater than those of the metals being determined.
Table V. Sensitivity and Range of Linearity
Sensitivity, PPb Range of linearity, PPb
4
Cd
co
cu
Fe
Xi
Pb
Zn
0.6
0.8
1.5
0.8
1.3
1.3
2.5
0.6
0-200
0-400
0-350
0-400
0-350
0- 3 00
0-5000
0-200
Stability. An important requirement in this work is that the extracted metal complexes remain stable in the MIBK as long as it takes to carry out a number of extractions and to analyze the solutions. Three solutions (pH 5.0, 250 ppb in each metal) were extracted and the organic phase was drained into a polyethylene bottle and set aside for later analysis. Just before these “stability” solutions were to be analyzed, three new identical solutions were extracted and their absorbances compared to the absorbances of the stability solutions. Each time the stability solutions were analyzed a t a later date, again three new solutions were extracted and the absorbances of the two sets of solutions compared. The results are shown in Table IV. As is evident, all the metal complexes are stable for a t least S3/4 hours. After this time, three of the metal complexes (Fe, Zn, Ag) started to break down. Therefore, as
In this study F, Ca, K, Mg, Na, phosphate, silicate, and biodegradable detergent a t 50- and 100-ppm levels were individually added to solutions containing 0.25 ppm of‘ each of the metals. The absorbances obtained from the analysis of the solutions were then compared to the absorbances of solutions containing no added “interference ion.” It might be noted that most of these ions are known masking agents (3);however, only the biodegradable detergent appeared to affect the metals to any degree. Application to Other Metals. Several other metals were also considered: mercury, chromium(III), and manganese. Mercury was not apparently extracted by any of the chelating systems considered. The major problem with this element was its very poor sensitivity (approximately 2.5 ppm). Chromium(II1) has always had the notorious reputation of forming the very stable hexahydrate complex, Cr ( H ~ O ) Gand ~ + ,can be extracted only under rather rigorous
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
1897
conditions such as boiling under reflux or adding some chemical to oxidize the Cr(II1) to Cr(V1) (I, 2). Manganese is best extracted at a pH of 7.0 or higher by such complexes as HTTA, DDDC, HAA, Oxine, and NaDDC. Further study on the extraction of other metals with this procedure
might be fruitful. At present it appears that antimony is also extracted under the described conditions. RECEIVEDfor review January 16, 1974. Accepted July 10, 1974. Work supported by the National Research Council of Canada.
Comparison of Continuous Wave and Pulsed Continuum Sources for Atomic Fluorescence Flame Spectrometry D. J. Johnson, F. W. Plankey, and J. D. Winefordner‘ Department of Chemistry, University of Florida, Gainesville, Fla. 326 1 1
In atomic fluorescence spectrometry, a continuum source can be used to determine many elements whlle avoiding the one source per element restriction. Previous use of continuum sources has resulted in relatively high detection limits when compared to line sources and their low output in the 200- to 250-nm spectral region has limited their use for many elements. The use of a high powered pulsed continuum source and cw source wlth large solid angle collection efficiency is described, and llmlts of detection are compared for the two source systems. Also, analog and dlgltal (photon counting) detectlon systems are used for each source, and these results are compared. Although the pulsed source was expected to give a larger signal-to-noise ratio than the cw source, the opposite results were found. This was posslbly due to the useable source flux (at the atomizer) ratlo determined largely by solid angle conslderatlons. The overall convenience of the cw source wlth elther detection system and the detection limits found for 13 elements indicate that thls source has practical value in AFS.
Although low intensity (not laser) spectral continuum sources have several obvious advantages as primary sources in atomic fluorescence spectrometry, e.g., the possibility of using only one source for exciting many elements, the possibility of wavelength scanning to compensate for any scattering interference, and the increased stability-long and short term-as compared to many line sources, such sources have been primarily a tool for physical studies and a curiosity for analytical studies. The major analytical limitations of such sources have been primarily associated with their meager output in the UV (200-350 nm) and their generally low fluxes over atomic absorption lines. Nevertheless, if the average photon flux reaching the absorption cell could be increased by increased input power to the source or by increased gathering (solid angle) efficiency of the excitation source optics and/or if the peak photon flux could be increased by pulsing the source and the detector system could be made to respond to the fluorescence signal primarily during the source “on-time”, then it would seem that a spectral continuum source could be of considerable use in atomic fluorescence spectrometry. Veillon, Mansfield, Parsons, and Winefordner ( I ) first showed that a cw continuum source (150-W Osram Lamp) Author to whom reprint requests should be sent. (1) C. Veillon, J. M. Mansfield, M. L. Parsons, and J. D. Winefordner. Anal Chem., 38, 204 (1966).
1898
could be used in atomic fluorescence spectrometry with a turbulent flame atomizer (H2-02 or Hs-Ar-entrained air); these authors were able to excite the resonance lines of 14 elements, including Zn and Cd but were distressed with the appreciable scatter signal. Ellis and Demers (2), shortly thereafter, described the use of a 450-W xenon arc lamp to excite eight elements in atomic fluorescence spectrometry (H2-entrained air flame). Dagnall, Thompson, and West ( 3 ) and Manning and Heneage ( 4 ) also used a 150-W xenon arc (Osram type) lamp for atomic fluorescence studies, and Bratzel, Dagnall, and Winefordner ( 5 ) described the use of a 150-W xenon arc lamp, made by Eimac, for atomic fluorescence flame spectrometry. Cresser and West (6) determined detection limits and interferences for 13 elements in atomic fluorescence spectrometry with an air-acetylene flame and a 500-W xenon arc lamp. Omenetto and Rossi (7) used a mercury vapor discharge arc lamp as a continuum source to excite several elements in atomic fluorescence flame spectrometry. In all of the above cases, the cw continuum sources resulted in detection limits from one to three orders of magnitude higher (worse) than with intense line sources (e.g., electrodeless discharge lamps), and even worse results for elements with resonance lines below about 250 nm ( e . g . ,Ni, Cd, Zn, Sb, Se, Te, Fe, Co, etc.). Certainly, the detection limits obtained in the above studies could have been improved considerably by use of better entrance optics between the source and atomizer and/or between the atomizer and spectrometer, and/or by use of a spectrometer with a larger LR product ( L = luminosity and R = resolving power), and/or by use of flames with greater atomization efficiencies assuming the changes in fluorescence quantum yields and flame background are not too great (8). The present atomic fluorescence study involves the application of a pulsed xenon lamp as well as a point source cw xenon arc lamp with a special mirror system enabling the transfer of nearly all the radiation produced by the source into the flame atomizer. In the former case, either an analog or digital boxcar detector is utilized whereas, in the latter case, either a lock-in amplifier or a synchronous photon counter is used. In both cases, flames with good atomization charac(2) D. W. Ellis and D. R. Demers, Anal. Chern., 38, 1943 (1966). (3) R. M. Dagnall, K. C. Thompson, and T. S. West, Anal. Chim. Acta, 36, 269 (1966). (4) D. C. Manning and P. Heneage, At. Absorption Newslett., 7, 60 (1968). (5) M. P. Bratzel, R. M. Dagnall, and J. D. Winefordner, Anal. Chirn. Acta, 52, 157 (1970). (6) M. S. Cresser and T. S. West, Spectrochlm. Acta, Pari& 25, 61 (1970). (7) N. Omenetto and G. Rossi, Anal. Chlm. Acta, 40, 195 (1968). (8) V. Svoboda. R. F. Browner, and J. D. Winefordner, Appl. Spectrosc., 26, 505 (1972).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
