phate, fluoride, citrate, cyanide, EDTA, etc., can often be effective. Occasionally the cobalt is isolated by precipitation as cobalt(II1) hydroxide or as potassium hexanitritocobaltate(III), which is then processed for reaction with the color reagent. The method to be used in any given analysis will depend more upon the kind and amount of interfering elements in the sample than upon the sensitivity of the color reaction.
RECEIVED for review June 19, 1972. Accepted August 15, 1972. Thanks are expressed to The African-American Institute for a Graduate Fellowship to M.J.J., and to the National Science Foundation for financial support by grant G P 9447. Condensed from a dissertation submitted by Mirjehan J. Janmohamed to the Graduate School of The University of Texas at Austin in partial fulfillment of the requirements for the Ph.D. degree, December 1970.
Fluorometric Method for Determining Nanogram Quantitiesof Nitrite Ion Lawrence J. Dombrowski and Edward J. Pratt Sterling- Winthrop Research Institute, Rensselaer, N. Y.12144 A sensitive fluorometric method has been developed for measuring nitrite ion. The procedure involves diazotization of p-chloroaniline (PCA) and coupling with 2,6-diaminopyridine (DAP). The resulting azo product then is further derivatized with ammoniacal cupric sulfate to produce a highly fluorescent triazole compound (excit. max. 360 nm; fl. max. 430 nm). The intensity of the fluorescence is linearly dependent upon the nitrite concentration. The procedure permits detection of 2 nanograms nitrite ion per ml utilizing a 10-ml sample. The precision at this level is 7%. The triazole exhibits strong fluorescence in acid media and shows fluorescence quenching by hydroxide ion suggesting radiationless deactivation of the excited triazole imine anion. The coupling reaction between DAP and PCA was examined and found to be strongly pH dependent. Greatest reaction velocity was observed near neutral pH. The determined second-order rate constant at pH 5 and temperature of 22 OC was 85 i 5 liter mole-' sec-I. Optimum conditions are presented for obtaining maximum nitrite detection sensitivity.
RECENTLY, THERE HAS BEEN great concern about the potential health hazard caused by nitrite ion. Nitrites frequently are used as a preservative or for color-fixation in food products. Nitrosamine can result from an interaction between nitrite ion and existing secondary amines. For example, N-nitrososarcosine readily is produced from creatine and nitrite ion ( I ) . Creatine is present in the muscular tissue of many vertebrates and is a normal constituent of meat. Two in vitro studies involving nitrite and secondary amines have shown that gastric juices of mammals provide an excellent medium for the production of nitroso derivatives (2,3). Indeed, a recent investigation revealed the presence of simple alkyl nitrosamines in food products which had been treated with nitrite (4); their presence in food products has been demonstrated detrimental to human health. In view of the continuing widespread use of nitrites, a sensitive method for measuring trace levels of nitrite ion seemed desirable. Previous determinations of this species (1) M. C. Archer, S. D. Clark, J. E. Thilly, and S.R. Tannenbaum, Science, 174,1341 (1971). (2) N. P. Sen, D. C. Smith, and L. Schwinghamer, Food Cosmet. Toxicol., 7, 301 (1969). (3) J. Sander, Arch. H y g . Bakteriol., 151,22 (1967). (4) T. Fazio, J. N. Damico, J. W. Howard, R. H. White, and J. 0. Watts, J. Agr. Food Chem., 19,250 (1971). 2268
were made by the classical Griess colorimetric method (5). This absorptiometric procedure permits measurement of 0.05 part per million nitrite ion after diazotization of sulfanilic acid and coupling with 1-naphthylamine. A variety of colorimetric and ultraviolet procedures are available (6-11) but generally these are not so sensitive as the Griess method. Recently Dombrowski and Pratt (12) have shown that an azo dye derived from nitrite ion and 2,6-diaminopyridine (DAP) could be further derivatized to produce a highly fluorescent compound. Nanogram quantities of primary aromatic amines were measured. The most sensitive amine detected was p-chloroaniline. The sensitivity of arylamine measurement afforded by this procedure prompted us to consider adapting it, after some modification, to the fluorometric determination of nitrite ion. This paper describes the use of DAP and p-chloroaniline (PCA) as reagents for nitrite ion which is detected and measured sensitively via fluorescence resulting on oxidation of the coupled diazo derivative with ammoniacal cupric sulfate. The extent of the fluorescence (excit. max. 350 nm; fl. max. 430 nm) was linearly dependent upon the amount of nitrite initially present. As little as 20 nanograms in 10 ml of water can be measured with a precision of7z. The proposed procedure is more lengthy than the Griess method; however, it provides a fivefold increase in nitrite detection sensitivity. This study was made to determine optimum conditions and reagent concentrations necessary to achieve this sensitivity. EXPERIMENTAL
Instruments. The fluorescence instrumentation used is described elsewhere (12). Absorption spectra were obtained with a Beckman D K 2 spectrophotometer. (5) P. Griess, Ber., 12,427 (1879). (6) K. Hutchinson and D. F. Botty, ANAL.CHEM.,30,54 (1958). (7) M. K. Bhatty and A. Townshend, Atial. Cllim. Acta, 56, 55 (1971). (8) J. M. Pappenhagen and M. G. Mellon, ANAL.CHEM.,25, 341 (1953). (9) A. M. Hartley and R. I. Asai, ibid., 35,1214 (1963). (10) D. F. Kuemmel and M. G. Mellon, ibid.,28,1674 (1956). (11) J. H. Wetters and K. L. Uglum, ibid., 42,335 (1970). (12) L. J. Dombrowski and E. L. Pratt, ibid.,43,1042(1971).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
Reagents. 2,6-Diaminopyridine (Aldrich Chemical CO.) was further purified by heating it with hot benzene containing decolorizing charcoal. The heated solution was filtered rapidly and pure DAP recrystallized from the chilled filtrate. For this investigation an 0.025% solution of D A P in p H 5 buffer was prepared fresh daily. Ammoniacal cupric sulfate solution was prepared by dissolving 1 gram of anhydrous CUpric sulfate in 10 ml of water containing 3 ml of 28 % ammonium hydroxide. A 4 x ammonium sulfamate solution in water and 0.06 PCA in 0.1M hydrochloric acid also were made fresh daily. A standard nitrite solution was prepared which contained 100 nanograms of nitrite ion per milliliter of water. Triple distilled water was used in the preparation of all reagents and the standard solution. Buffer. A pH 5 acetate buffer was prepared by mixing 400 ml of 1 M sodium acetate and 116 ml of 1 M hydrochloric acid, then diluting to 1 liter with distilled water. Benzene Solvent. Mallinckrodt analytical reagent benzene was used without further purification. Its spectral acceptability required that it exhibit no significant fluorescence at 430 nm when excited with light having a maximum wavelength of 360 nm. Preparation of Calibration Graph. Transfer 0.2 to 2.0 ml of the standard nitrite solution to a series of test tubes, each containing 0.1 rnl of 6 M hydrochloric acid. Add 0.3 ml of 0.06% p-chloroaniline solution, mix, and immerse test tube in an ice water bath for 15 minutes. Add 0.3 ml of 4 % ammonium sulfamate, mix, and after 5 minutes add 2 ml of the 0.025x DAP reagent. Mix thoroughly and place in an ice water bath for 30 minutes. Quantitatively transfer the solutions to a 60-ml separator with the aid of a few milliliters of distilled water. Add 6 ml of benzene and extract for 0.5 minute. Discard the aqueous layer and wash benzene extract with 2 X 6 ml pH 5 acetate buffer followed by 2 X 6 ml distilled water. Discard all washings and quantitatively transfer the benzene layer to a 10-ml volumetric flask with the aid of a few milliliters of benzene. Place flask on steam bath and, using a displacing stream of nitrogen, evaporate benzene to dryness. Add 2 ml of distilled water to flask followed by 0.4 ml of the ammoniacal cupric sulfate solution. Stopper flask, mix, and place in a boiling water bath for 30 minutes. Cool flask to room temperature under tap water and acidify contents with 0.45 nil of 6 M hydrochloric acid. Dilute flask to 10-ml volume with distilled water, mix, and filter with suction through a fine glass frit funnel. Measure the relative fluorescence of the clear filtrate at 430 nm using an excitation wavelength of 360 nm. Two milliliters of distilled water served as the blank and was carried through the complete analytical procedure. RESULTS AND DISCUSSION
Spectral Characteristics. Figure 1 shows the uncorrected excitation spectrum ( A ) and emission spectra (B, C, D, E ) for the fluorogen resulting from processing various amounts of nitrite. Fluorescence spectrum F results from processing a 2-ml water blank. The excitation and fluorescence maxima are 360 nm and 430 nm, respectively, for the nitrite dependent species. The fluorogen previously has been characterized as a triazole and is formed by oxidation and subsequent cyclization of the azo dye (13). Ammoniacal cupric sulfate was an excellent oxidant for this reaction (12). The absorption spectra of 2,6-diamino-3-(4 '-chloropheny1azo)pyridine (compound I) and of the triazole (compound 11) in 0.1M hydrochloric acid are shown in Figure 2 . The conversion of compound I into compound II results in a pronounced bathochromic shift in the absorption maximum and a subsequent -~
(13) G. Charrier and M. Jorio, Atti Accad. Naz. Liiwi, 26, 170
(1937).
'0°1
WAVELENGTH IN nm
Figure 1. Excitation (curve A ) and Buorescence (curves B,C, D,and E ) spectra Final nitrite concentrations for B.C,D, and E , are 4, 8, 12, and 16 nanograms per ml. Curve F is spectrum resulting from a processed 2-ml water blank. (All spectra recorded with meter multiplier switch position 0.030; sensitivity control reading 30) 0.6-
0.5-
-
0.4 U W
0.3(r
v?
0.I
350
300
450
400
Figure 2. Spectrum I [2,6-diamino-3-(4 '-chloropheny1azo)pyridine] and spectrum I1 (triazole) in 0.1M hydrochloric acid reduction in absorption intensity. With respect to nitrite ion concentration, the molar absorptivities calculated at the maxima for compound I and I1 are 26,000 and 10,000, respectively. Effect of pH on Triazole Fluorescence. To determine fluorescence dependence on pH, a quantity of the triazole was synthesized as described above. After its preparation with ammoniacal cupric sulfate, the pH of the solution was adjusted to 5 and the triazole extracted with benzene. The triazole was obtained by evaporation of the organic extract. Its purity was estimated by thin layer chromatography. A fixed quantity of the triazole then was dissolved in aqueous solutions of known and varied acidity and the fluorescence measured at 430 nm with 360 nm excitation. The actual fluorometric readings at the specified values of pH are given in Table I. These data show that in relatively strong acid the fluorescence intensity is constant and a maximum. As the pH is raised, the fluorescence decreases and is approximately 50% of its maximum value at pH 9.5. The most significant changes occur in strong base where there is about a tenfold loss in sensitivity. No shift in either the excitation or fluores-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
~
500
WAVELENGTH IN nm
~~
~~
~
2269
~-
~~
Table 1. Dependence of Triaz.de Sensitivity Slit arrangement Meter multiplier switch Concentration PH 1M HCl 0.1M HC1 3.0 4.0 7.5 9.5
0.1M NaOH 1.OM NaOH
~
~~
Fluorescence on pH 30
No, 3 0.010 100 ng/ml
j z 801
Meter scale deflection 32.0
33.0 25.0 21.5 16.5 16.5 5.5 3.5
2
0.040 0.050
65
0.030
cence wavelength maxima was observed in the pH-fluorescence intensity study. In this case, the triazole cation is more fluorescent than the neutral species and the hydroxide ion considerably quenches the fluorescence. A similar p H effect was reported for some related compounds by Weisstuch and Testa (14). They observed fluorescence quenching of 2- and 3-aminopyridine by base and explained it as a proton transfer between the excited hydrogen-bonded aminopyridine and hydroxide ion followed by radiationless decay of the imine anion. Because of the structural similarity between the triazole and the monosubstituted aminopyridine, it is likely that a n excited triazole molecule deactivates by the same mechanism as proposed by Weisstuch. The results from this p H study suggested to us that the disubstituted aminopyridine (DAP) should also yield a fluorescence p H profile similar to the one reported here in Table I. Subsequent investigation of DAP fluorescence did indeed produce the expected results and hydroxide ion considerably quenched the fluorescence (IS). The triazole in dilute hydrochloric acid was stable for at least 48 hours. At the end of this time, the aged solutions gave approximately the same fluorescence response as those that were freshly prepared. Effect of DAP Concentration on Nitrite Detection Sensitivity. The effect of varying the DAP concentration between 0.005-0.050~ was studied on a 2-ml volume of standard containing 120 nanograms of nitrite ion. The standard solution was carried through the complete analytical procedure using 2 ml of the DAP reagent. Results of the fluorometric measurements are shown in Table 11. The fluorescence sensitivity is relatively insensitive toward changes in DAP concentration over the interval investigated. (14) A. Weisstuch (1968).
and A. C. Testa, J. Pliys. Cliern.,
12, 1982
(15) Unpublished data, Sterling-Winthrop Research Institute, 1972. 2270
20-
002
0.04
0.06
008
010
012
PCA CONCENTRATION, (%1
59
60 65 65 66
40t/ 0
Table 11. Effect of DAP Concentration on Nitrite Detection Sensitivity Sensitivity setting 30 Slit arrangement No. 3 Meter multiplier switch 0.030 Nitrite concn final dilution 12 ng/ml DAPconcn, Meter scale deflection 0.005 0,010 0.020
//
Figure 3. Effect of PCA concentration on nitrite detection sensitivity Final nitrite concentration is 15 nanograms per ml. (Meter multiplier switch 0.030; sensitivity control reading 30)
Effect of PCA Concentration on Nitrite Detection Sensitivity. The effect of varying the PCA concentration on nitrite sensitivity was examined between the concentration interval 0.010.12%. In this case a 2-ml standard was employed containing 150 nanograms of nitrite ion. The standard was treated with 0.3 ml of the PCA reagent and the fluorogen was prepared as described above. The results of this study are presented in Figure 3. The fluorescence was relatively constant and a t its maximum between 0.08 to 0.12 % of added PCA. It decreased sharply at lower PCA levels. At PCA concentrations between 0.01 to 0.06%, a n enhancement in sensitivity can be achieved by simply extending the time for the diazotization reaction beyond the allotted 15 minutes. From a practical standpoint, the shortest effective reaction time is most desirable. It is evident, then, that if maximum nitrite sensitivity is to be attained, sample volumes larger than 2 ml should be treated with proportionately larger amounts of the PCA reagent. Rate of Coupling between DAP and PCA. The rate of coupling between DAP and diazotized PCA to produce the azo derivative was conveniently followed by monitoring the decrease in DAP fluorescence as the latter compound was consumed in the reaction. DAP possesses strong native fluorescence (excit. max. 345 nm; fl. max. 408 nm) whereas diazotized PCA and the azo product are nonfluorescent. The experimental procedure used depended on the measured fluorescence when the excitation and emission monochromators of the instrument were set a t 345 nm and 408 nm, respectively. A cuvette containing 1.5 ml of 3.0 x 10-jM DAP in a specified buffer solution was placed in the fluorescence spectrometer and irradiated by the excitation source. Exactly 0.5 ml of 1.2 x 10-3M diazotized PCA solution was rapidly introduced into the cuvette and the fluorescence intensity then measured a t specified times. Figure 4 shows the time dependence of DAP fluorescence. The reaction rate was investigated at p H 1, 3,4, and 5. Several conclusions are evident from Figure 4. In strong acid, DAP fluorescence is essentially constant with time indicating that no significant coupling has occurred. However, as the p H is raised there is a corresponding increase in the rate of coupling. For instance at p H 3 the reaction is 65 % complete at 15 minutes. At pH 5 the reaction has reached 100% completion within 3 minutes. Further examination at a higher p H of 7 revealed even a greater reaction velocity. In this latter case, however, high speed recording equipment would be necessary to obtain the complete fluores-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
Table 111. Rate Data for DAP and PCA Coupling DAP 5.0 x 10-eM PH 5.0 Temperature 22" 3~ 1°C kl, (sec-1) k2 = kl/PCA concn, PCA concn, x 10-2 liter mole-' sec-l 10-4~ 0.45 0.39 87 1.13 0.92 82 2.25 2.07 91 3.38 2.99 90 4.50 3.75 83 5.63 4.37 80 Table IV. Limit of Nitrite Detection Blank 10 ml water 20 ng nitrite ion in Standard 10 ml water Sensitivity setting 30 Slit arrangement No. 3 Standard Blank Ratio DeterMeter Meter (standard1 Meter Meter mination blank) multiplier reading multiplier reading No. 1 0.003 44 0.010 48 3.6 0.010 53 0.003 54 3.2 2 0.010 50 3 0.003 52 3.2 4 0.010 46 0.003 40 3.8 5 0.003 44 0,010 43 3.2 6 0.010 53 0.003 50 3.5 0.003 50 3.7 7 0.010 56 8 0.010 53 0.003 48 3.7
0
2
I
I
I
I
4
6
8
IO TIME I N MINUTES
12
I
14
16
Figure 4. pH dependence for coupling reaction between DAP and PCA Exactly 0.5 ml of 1.2 X low3M diazotized PCA added to 1.5 ml of 3.0 X 10-j M DAP buffered at pH 1,3,4, and 5. Reaction temperature is 22' + 1 "C
"8A 1.6
1.40
z 2
1.2-
U Y U
: 1.0I 0
cence decay profile. This pH study suggests that the active species in the coupling reaction with diazotized PCA is the free nonionized DAP. The concentration of free DAP obviously reaches a maximum near a neutral p H and thus would account for the observed increased coupling rate with decreasing acidity. Although the coupling rate was greatest at pH 7, it was decided to employ pH 5 for the assay. The latter value was chosen as a precaution since diazo compounds generally are relatively stable in acid media but react at a higher pH to produce inactive diazotates. The decay of D A P fluorescence at pH 5 was exponential and followed the first-order rate law. (See Figure 5 for a representative plot.) Although first-order kinetics are observed, the reaction rate is pseudo-first-order and is actually dependent upon the diazotized PCA concentration. This is exemplified by the data in Table 111 which show variation of the experimentally derived first-order rate constant kl with PCA concentration. The fact that the amount of PCA >> DAP accounts for the seemingly first-order dependence for the reaction. The true second-order rate constant k pis obtained by dividing ki by the PCA concentration. The constancy of this ratio is illustrated in Table I11 and supports the second-order dependence for the reaction. The average value for kz was 86 f 5 liters mole-' sec-1. Limit of Nitrite Detection. In this study, the limit of detection is defined as that quantity of nitrite which produces a signal three times greater than the signal resulting from a processed blank. Preliminary studies established this value at approximately 20 nanograms of nitrite ion in 10 ml of water (2 ppb). To determine the precision at this level, eight blanks and eight nitrite standards were analyzed by the complete analytical procedure. Because this experiment involved larger volumes for the blank and standard, two changes in
-I
0.8-
o'6 0.4
0
20
40
60
80
T I M E IN SECONDS
Figure 5. First-order plot for the coupling reaction between DAP (5.0 X 10- 6M)and PCA (5.63 X M) at pH 5 and at 22 "C reagent concentration were made in the procedure. Exactly 0.3 ml of 0 . 6 x PCA and 2 ml of 0.10% DAP were employed. The actual instrument readings resulting from processing the blanks and standards in pairs are shown in Table IV. The product of the (meter multiplier switch) X (meter reading) for a single standard divided by its corresponding blank product yielded the ratio values listed in Table IV. An average value of 3.5 was obtained for the standard/blank response. The standard deviation based on the eight determinations was 7%. When larger amounts of nitrite ion were processed, the precision was lowered to about 4 %. Fluorescence Blank. All the glassware used in this study was scrupulously cleaned and rinsed with triple distilled water. Because of the sensitivity of the developed fluorogen, it was not surprising to observe blank fluorescence responses such as those listed in Table IV when processing supposedly nitritefree water. The excitation and fluorescence maximum obtained from the processed blank were the same as those obtained from processing authentic nitrite ion. Trace contamination of the distilled water and/or the reagents by nitrite ion was suspected of producing the blank fluorescence.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
2271
In order to minimize its contribution, a n attempt was made to determine the origin of the blank fluorescence. An examination of distilled water from several different sources was made by processing fixed volumes through the complete assay procedure. No significant differences could be found, and all waters produced comparable blank responses. Similar results also were obtained on ultra pure triple distilled water. An examination of the reagents was conducted by carrying out the analytical procedure on 2 ml of distilled water. In this case, one reagent concentration was varied while the remaining reagent concentrations were held constant. The resulting blank response at 430 nm utilizing 360 nm excitation then was recorded. This entire process was repeated until all the reagents were individually screened. The results of this study did not establish any correlation between a specific reagent concentration and the magnitude of the blank signal. In summary, the fluorescence blank response could not be identified positively as originating from either the solvent or
from the reagents. Regardless, the fluorescence resulting from processing a fixed volume of high purity water (blank) through the complete analytical procedure is minimal and reproducible provided that fixed reagent concentrations are employed. It is recommended then that when analyzing a n unknown sample, both the sample and its corresponding blank be identically treated with respect to volume of solution and reagent concentrations. The effect of diverse ions on nitrite detection sensitivity was not investigated. Interferences can be expected from those ions which either are not compatible with nitrite ion or can effect the diazotization and coupling reactions. These have been extensively examined by Rider and Mellon (16).
RECEIVED for review May 17, 1972. Accepted August 28, 1972. (16) B. F. Rider and M. G. Mellon, IND.ENG.CHEM.,ANAL.ED., 18,96 (1946).
Design and Operation of Temperature-Controlled Multiple Element Electrodeless Discharge Lamps for Atomic Fluorescence Spectrometry B. M. Patel,’ R. F. Browner, and J. D. Winefordner2 Depurtment of Chemistry, Unicersity of Florida, Gainesaille, Florida 32601
The preparation of multiple-element electrodeless discharge lamps for Hg, Cd, In, Ga, TI, Zn, Cu, Fe, Mg, Ag, Ge, Sn, Pb, Th, U, and Zr is described. The lamps are excited using a temperature-controlled antenna system. The spectral radiant output from each individual element in a multiple-element lamp i s very temperature sensitive, but largely uninfluenced by the presence of the other elements (or compounds). Plots of the variation of spectral radiant output with temperature allow the rational choice of a compromise operating temperature for several elements in each lamp. Alternatively, the optimum temperature for each element present may be selected in turn. A comparison i s made between the output stability of the multiple-element lamps and the corresponding single-element lamps. Atomic fluorescence detection limits are given, using both multiple-element and single-element lamp sources.
ATOMICLINE SOURCES which emit the spectra of more than one element, and which can therefore be used for multiple-element analysis without the need to change lamps, can be a great convenience in atomic absorption and atomic fluorescence spectrometry. The hollow-cathode lamp is not well suited to this function, primarily because of the problems which occur due to selective sputtering of the more volatile components of the cathode material. However, sophisticated alloying techniques allow lamps emitting the spectra of up to five elements to be prepared, although their cost is high and their performance less than ideal. On IAEA Fellowship from the Atomic Energy Commission, India Radiochemistry Division, Bhabha Atomic Research Center, Trornbay, Bombay 85, India. ” Author to whom reprint requests should be sent. 2272
Multiple-element electrodeless discharge lamps (EDL) containing more than one element or compound wirhin the sume encelope, have been described for Bi-Hg-Se-Te, Cd-Zn, and Ga-In by Marshall and West (I), for As-Sb by Fulton, Thompson, and West ( 2 ) , and for Co-Ni by Norris and West (3). Also, dual-element lamps, with each element contained in a separate concentric compartment of the lamp, have been described by Aldous, Alger, Dagnall, and West ( 4 ) and by Cresser and West (5). However, constructional problems would not allow lamps containing a large number of elements to be prepared with the concentric design. Both of these source types, when operated with resonant cavity excitation, require a very careful choice of fill-material in order that there may be a n adequate vapor pressure of each component under operating conditions. This has restricted multiple-element lamps ( 2 , 3) t o materials of comparable volatility. In previous communications ( 6 , 7), Browner, Patel, Glenn, Rietta, and Winefordner demonstrated that the spectral radiant output of an EDL, operated with an “A” antenna, is very sensitive to changes in the lamp operating temperature. A (1) G. B. Marshall and T. S. West, And. Chim.Acra, 51,179 (1970). (2) A. Fulton, K. C. Thompson, and T. S. West, ibid., p 373. (3) J. D. Norris and T. S. West, ibid., 55, 359 (1971). (4) K. M. Aldous. D. Alger, R. M. Dagnall, and T. S. West, Lab. Pract., 19,587 (1970). (5) M. S. Cresser and T. S . West, Anal. Chim.Acta, 51, 530 (1970) (6) R. F. Browner, B. M. Patel, T. H. Glenn, M. E. Rietta, and J. D. Winefordner, Specrrosc. Lett., in press. (7) R. F. Browner, M. E. Rietta and J. D. Winefordner, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1971, Abstract No. 136. ,
I
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
