Fluorescence Determination of Sub-Parts per Billion Hydrogen Sulfide in the Atmosphere Herman D. Axelrod, J o e H. Cary, Joseph E. Bonelli, and J a m e s P. Lodge, Jr. National Center f o r Atmospheric Research, Boulder, Colo. 80302
ESTIMATES OF HYDROGEN sulfide concentrations in the "unpolluted" atmosphere are few in number and low in reliability, but suggest values below 6 parts per billion (ppb). Few existing analytical methods are adequately sensitive for actual measurements at such low concentrations. Sanderson, Thomas, and Katz ( I ) demonstrated that a method which uses lead acetate paper is sensitive but that problems develop with stain fading and with changes in humidity. Jacobs, Braverman, and Hochheiser ( 2 ) determined H2S in the ppb range using the methylene blue method, but required larger volumes of air, since either long sampling periods or high flow rates were necessary. High flow rates resulted in low trapping efficiencies. Andrew and Nichols (3) developed a n instrument based upon fluorescence. Although these authors claimed high sensitivity, they were not able to check the method with air samples containing low concentrations of H2S, nor to cite data on the efficiency of collection, especially at high flow rates. Karush, Klinman, and Marks ( 4 ) used a fluorescence method for the determination of disulfide groups. Grunert, Ballschmiter, and Tolg (5) further refined the method for the determination of sulfur as sulfide in organic compounds. In the following work, the method outlined in the latter study has been applied to the measurement of atmospheric H2S. The method shows high sensitivity and reliability, and is suitable for the measurement of HsS background levels. EXPERIMENTAL Apparatus. Fluorescence measurements were made with a Turner Model 110 Filter Fluorometer or a Perkin-Elmer Model 203 Spectrofluorometer. Turner general purpose filters were used for the filter fluorometer. The excitation filter peaks at 365 nm, while the emission filter passes all wavelengths longer than 415 nm. (The fluorescence material can be excited a t lower wavelengths than the optimum 499 nm.) The cells used were 12 X 75 mm tubes. The spectrofluorometer was adjusted to a 499-nm excitation wavelength and a 519-nm emission wavelength. The measurements were made at 23 "C, air-conditioned room temperature. Hydrogen sulfide was removed from air samples by passing the air through fritted bubblers designed by Wartburg, Pate, and Lodge (6). Reagents. All chemicals were reagent grade quality. The water used was first passed through an ion exchange column and then distilled. Fluorescein mercuric acetate (FMA) was prepared in a manner similar to that of Grunert, Ballschmiter, and Tolg
(5): 14.0 grams of Hg(OAc)z were dissolved in 200 ml o glacial acetic acid, and 7.53 grams of disodium fluorescein were dissolved in 100 ml of water. The Hg(OAc)2 solution was heated to 50 "C and the fluorescein solution was added dropwise with constant stirring. The resulting red-orange precipitate was filtered through Whatman 41H filter paper, washed several times with water to remove the excess fluorescein, and vacuum dried for several days. The yield was about 50%. The powder was placed in a dark bottle and kept in a silica gel desiccator. The working solutions were also kept in dark bottles. Samples of dry synthetic air containing trace gases at various concentrations were obtained from a dynamic precision gas dilution system (7). Fluorescence calibration curves were prepared from dilute sodium sulfide solutions. Crystals of reagent grade sodium sulfide (NarS.9H20) were washed with distilled water, dried quickly on filter paper, and weighed. The sample (240 mg/l. or 1 X 10-3M Na2S) was dissolved in 0.1N NaOH and the appropriate dilutions made with 0.1N NaOH to obtain a working concentration of 1 X lO-7M NazS. The addition of Na2S to FMA caused a linear decrease in the fluorescence intensity of the FMA. The calibration curves were made by the same procedure used to analyze the samples (see below). The sodium sulfide solutions remained stable for several days in clear glass containers exposed to room light. Procedure. Air was drawn through a bubbler containing 10 ml of 0.1N or 1.ON NaOH at a rate of 2 l./min for up to 60 minutes, using syringe needles operating as a critical orifice to control the rate of flow, as suggested by Lodge et a/. (8). After the sampling period, the bubbler was disconnected and the solution analyzed for S2- as follows: Water was added to increase the sample volume to 10 ml. For 0.1N NaOH sampling solutions, 1 ml of 1.1 X 10-6M FMA reagent (90.5 pg/l.) in 0.1N NaOH was pipetted into the bubbler. The solution was mixed well and the fluorescence measured. With a 1.ON NaOH sampling solution, 2 ml of 1.0 X 10-6M FMA was added to the sample. The solution was mixed well and immediately thereafter 8 ml of 1.ON H & 0 4 was added to lower the NaOH concentration to 0.1N. The resulting solution was mixed well and the fluorescence measured. In either case, if the fluorescence had been completely quenched, a 1 :10 dilution of the sample FMA solution with 0.1N NaOH was made and additional aliquots of 1.0 x 10-6M FMA were added. All fluorescence measurements should be made in 0.1N NaOH. The reagent blank was 0.1N NaOH and the maximum FMA intensity was measured from a 1 X 10-7M FMA-O.1N NaOH solution. RESULTS AND DISCUSSION
(1) H. P. Sanderson, R. Thomas, and M. Katz, J . Air PoNution Control Assoc., 16, 328 (1966). (2) M. B. Jacobs, M. M. Braverman, and S. Hochheiser, ANAL. CHEM., 29, 1349 (1957). (3) T. R. Andrew and P. N. R. Nichols, Analyst, 90, 367 (1965). (4) F. Karush, N. R. Klinman, and R. Marks, Anal. Biochem., 9, 100 (1964). (5) A. Grunert, K. Ballschmiter, and G. Tolg, Tuluntu, 15, 451 (1968). (6) A. F. Wartburg, J. B. Pate, and J. P. Lodge, Jr., Enciron. Sci. Teclznol., 3, 767 (1969). 1856
Na2S as Working Standard. The salt Na2S.9H20 appears to be a suitable working standard for sulfide analysis. The crystals are deliquescent but the rate of water adsorption is so slow that the crystals can be washed, dried, and weighed (7) H. D. Axelrod, J. B. Pate, W. R. Barchet, and J. P. Lodge, Jr., accepted for publication in Arrnos. Enuiron. (1969). (8) J. P. Lodge, Jr., J. B. Pate, B. E. Ammons, and G. S. Swanson, J . Air Pollution Control Assoc., 16, 197 (1966).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
without further water interference due t o adsorption. Standardization of the F M A with Na2S.9H20 was compared to the use of diphenylthiourea as recommended by Griinert, Ballschmiter, and Tolg (5). Identically weighed molar concentrations of the two reagents gave calibration curves within 2 of each other. An assay of reagent grade NazS.9 H 2 0 was made in order to determine its purity. Using the method of Bethge (9) and preparing samples (as described above) of about 2 mg/ml S2- in 0.1N NaOH, the purity of Na2S.9H20 was found to be only 3 % below the theoretical amounts as determined by the initial weighing. The stability of dilute Na2S was also examined. A 1 X lO-'M Na2S in 0.1N NaOH was prepared from a higher concentration stock solution. Aliquots of the diluted solution were analyzed with F M A at various times after preparation. After a period of six hours, the S2concentration varied by only 3 %. Stability of FMA. The FMA-NaOH solutions were stored in dark bottles. The diluted solutions (10-6M) were stable for several weeks and the stock solution (lO-4M) was stable for 2 months. However, during these time periods, the intensity of the F M A fluorescence continually diminished; hence the F M A must be restandardized with each series of determinations. The fluorescence intensity was not affected by variations in ionic strength, but was affected by changes in pH. Contrary to the findings of Griinert, Ballschmiter, and Tolg (3,the FMA intensity fell off rapidly at NaOH concentrations greater than 0.15N. Karush, Klinman, and Marks ( 4 ) noted a degradation of F M A in 1.0 N N a O H , and observed a consequent decrease in intensity. NaOH concentrations between 0.05 and 0.1N did not affect the F M A intensity; 1.ON H 2 S 0 4added to neutralize any excess NaOH, did not alter the fluorescence. A solution containing 50% ethyl alcohol-50 0.1NNaOH increased the fluorescence by 50%. An attempt was made to place F M A in the bubblers to capture the H2S directly from the air. However, the F M A was strongly affected by aeration and by exposure to sunlight. Thus, the F M A must be added to the sample following the bubbling procedure. These data are not in agreement with the findings of Andrew and Nichols (3). Sulfide Analysis and Stability. The useful range of analysis of S2- is 0.5 to 10 X 10PM in 0.1N NaOH, 1 X lO-'M FMA. F o r our instrument, this is the optimal range; thus the sample concentration should be adjusted to fall into this bracket. For atmospheric sampling, one can determine 0.2 ng of S?- in 1 ml of trapping solution. The reproducibility of determinations on single samples was 1 %, while parallel sampling of the same air showed average differences between bubblers of about 10% at concentrations below 10 ppb H2S. Attention was also given to the solution stability of the captured H,S. Because this method depends o n fluorescence emission, methods of precipitation capture could not be employed to stabilize the S2- unless the precipitate can be redissolved prior to the addition of the FMA. Studies were made to determine the stability of S2- In 0.1N NaOH and 1.ON NaOH solutions under field sampling conditions. Using the previously mentioned gas dilution system, 0.9 ppb H,S was passed through glass tubing from the apparatus to a sun-exposed portion of the laboratory roof. The air stream was sampled at 2 l./min in each of three fritted bubblers containing 0.1N and 1.ON NaOH. The surfaces of the bubblers were, respectively, clear, wrapped in black tape, and wrapped with an inner layer of black tape and a n outer layer of white tape. The outside air temperature was usually 80 O F or (9) P. 0. Bethge, Anal. Chim. Acta, 10,310 (1954).
Table I. Loss of Sulfide during Sampling. Loss in NaOH Bubbler 0.1N 1.ON Clear 14 8 Wrapped in black tape 31 9 Wrapped with an inner layer of black tape and outer layer of white tape 25 4 4 0.9 ppb H2S, 2 I./rnin, 30 rnin, 3 samples for each category.
Table 11. Efficiency of Capture of H2S5 Efficiency in NaOH ppb H2S 0.1N 1 .ON 10 99 88 3 98 89 ... 0.8 93 0.2 80 82 a Series bubblers, 3 samples at each concentration, flow rate 2 l./rnin.
higher in the shade, and the sunlight was very bright. The concentration of HsS found in the outside bubblers was compared with the concentration measured inside the building under cooler, darker conditions. Table I shows the stability of the sulfide with respect to the trapping solution. The 1.ON NaOH solution shows less loss of sulfide than the 0.1N solution. However, as a criterion for choosing a n acceptable procedure, the loss of the sulfide must be weighed against the ease of analyzing 0.1NNaOH us. 1.ON NaOH. The black bubblers successfully protected the sample from sunlight, but became very warm while the white ones remained cool. Thus, the data indicate that the sulfide is destroyed by both heat and sunlight, However, sampling can be done with clear bubblers if they are protected with a heavy cloth. Measurements were made to test the stability of samples with time. Sixty liters of a I-ppb HsS-air stream were passed through the bubbler. After 24 hours the sample showed no loss of its initial strength, and after 72 hours, only a 2 loss, Efficiency of Capture. The efficiency of capture of H2S by 0.1N and 1.ON NaOH was measured by sampling known concentrations with two fritted bubblers in series. Table I1 shows the results of these tests for various concentrations of H2S. In all cases, the efficiency was at least 80% and increased with increasing H2Sconcentration. Interferences. Numerous reagents were checked as possible interferents. IO5-fold mole excesses of C1-, S042-, P043-, NOa-, C2H302-,CH02-, and c03'- were added to separate 2 X 10-8M S2- solutions. N o significant changes in fluorescence were noted. A slight quenching effect was observed with similar excesses of Br-, SO3*-, and NO2-, while about a 10 enhancement was observed with excess I-. As representatives of classes of compounds, methyl acetate, ethyl alcohol, acetone, acetamide, mercapto ethanol, and ",OH were added in 1O5-foldmole excesses, and were found not to interfere. Cysteine and cystine in concentrations of 1 0 - 7 M c a u ~ e ad 20 interference for a comparable concentra: tion of S2-. However, according to Karush, Klinman, and Marks (4,various disulfides react at different rates; thus a general statement about the interference of these compounds cannot be made.
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Table 111. Panama Canal Zone-Atmospheric Site Time Albrook Tower Top 0930-1030
Albrook Tower Bottom Fort Sherman
1145-1245 1400-1500 0930-1030 1145-1 245 1400-1500 0930-1030 1145-1 245
H a s Samples ppb H2S 0.07 0.21 0.26
0 0.11 0.32 0.06
0.29
Some of these materials were trapped in the 1.ONNaOH, giving a fluorescence nearly equal to that of the FMA in the Turner filter fluorometer. The analyzed samples therefore showed a higher fluorescence than the FMA without Sa-. This problem was overcome by measuring the fluorescence of the sample prior to the addition of FMA, and subtracting the preanalyzed fluorescence from the final value. Possible alternatives which were not investigated include the use of different wavelengths and/or narrower pass bands for both excitation and fluorescence. Field Sampling. On 31 October 1968, the method described above was applied to the measurement of atmospheric H2S at previosly established sites (11) in the Panama Canal Zone. Air was passed at 2 l./min through 10 ml of 1.ON NaOH for one hour using 5 % KHCO, prefilters. The samples were analyzed the following day. Results are shown in Table 111. The NO, level was about 1 ppb, so no attempt was made to correct for any possible error from this source. The measured values are 10 to 100 times lower than the values predicted by Junge (12). However, the SOz concentrations obtained during the same sampling period were also much lower than Junge’s values.
No interference was noted for Mn2+, Ni2+, Co2+, Cuzf, Cd2+, Pbz+, Fez+, Fe3+, K f , and Na+ at a 1000-fold mole excess. This is not in agreement with Griinert, Ballschmiter, and Tolg (5). Air containing NO2 and SO2 was drawn through a bubbler containing 2 X lO-7M S in 1 .ONNa0J-I. At concentrations greater than 10 ppb these gases tended to react with the Sachanging it to a form that resists analysis. Sixty liters of gas with 100 ppb of these contaminants would eliminate all of the Sa- present. Prefilters impregnated with 5 % K H C 0 3 as described by Pate, Lodge, and Neary (10) eliminated the SOz interference while allowing H2S to pass at concentrations as low as 1 ppb. Lower H2S concentrations were not checked. However, NO2 could not be removed from the air prior to reaction with the Sa-. Anti-oxidants such as glucose, ascorbic acid, and fructose, were added to the Sa- solution, but they did not inhibit the NOa interference. Sulfamic acid also did not help. Thus, NO2 remains the major interferent. Hydrogen sulfide determinations made recently in the Panama Canal region showed an enhancement of fluorescence levels caused by large amounts of organic materials in the sampled air (as evidenced by gas chromatographic data).
RECEIVED for review March 3, 1969. Accepted August 15, 1969. Presented in part at the 10th Conference on Methods in Air Pollution and Industrial Hygiene Studies, San Francisco, Calif., February 19-21, 1969. The Panama experiments were supported by the U. S. Army Research Office Department of Defense. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
(10) J . B. Pate, J. P. Lodge, Jr., and M. P. Neary, Anal. Chirn. Acra, 28, 341 (1963).
(11) J. P. Lodge, Jr., and J. B. Pate, Sci., 153, No. 3734,408 (1966). (12) C. E. Junge, “Air Chemistry and Radioactivity,” Academic Press, New York, N. Y., 1963.
ACKNOWLEDGMENT
The authors acknowledge the helpful assistance of John B. Pate, Arthur F. Wartburg, and Miles D. LaHue.
X-Ray Emission Analysis of Paints by Thin Film Method J. D. McGinness, R. W. Scott, and J. S. Mortensen‘ Analytical Research Department, Sherwin- Williams Research Center, Chicago, 111. 60628
QUANTITATIVE X-ray emission analysis of paints is seriously complicated by the extreme variation in the absorption coefficient of the matrix due to the wide variety of compounds which may be used for the pigmentation. Methods for determination of specific elements in complex systems by means of matrix matching, dilution, and thin film-controlled mass have been reported (1-6). However, none of these techniques alone have been found adequate when applied to the analysis of paint. 1 Deceased. (1) E. L. Gunn, ANAL.CHEM., 29, 184 (1957). (2) Ibid.,33,921 (1961). (3) B. J. Mitchell, ibid.,p 917. (4) H. A. Liebhafsky, H. G. Pfeiffer, E. H. Winslow, and P. D. Zemany, “X-Ray Absorption and Emission in Analytical Chemistry,” John Wiley and Sons, New York, 1960. (5) S. A. Bartkiewicz, and E. A. Hammatt, ANAL.CHEM., 36, 833,
(1964). (6) M. L. Salmon, “Advances in X-Ray Analysis,” Vol. 5, W. M. Mueller, Ed., Plenum Press, New York, 1962, p 389. 1858
In the present study the several techniques for solving the matrix problem have been combined. The liquid paint sample is diluted in an internally standardized varnish matrix and then analyzed in the form of a dried thin film for several pigment elements in this single sample preparation. Preliminary comments on this approach were reported in 1965 (7); the final procedure and some initial results were reported by the authors in 1966 (8). This paper constitutes a report of the results obtained by this procedure and the conclusion that the procedure essentially eliminates matrix or interelement effects in the analysis of paints by X-ray emission.
(7) J. D. McGinness, P. J. Secrest, and D. J. Tessari, “Standard Methods of Chemical Analysis,” 6th ed., Vol. 111, Part B, F. J. Welcher, Ed., D. Van Nostrand Co., New York, Chap. 54, 1966. (8) J. D. McGinness, R. W. Scott, and J. S. Mortenson, Paper No. 157 (unpublished), Fifth National Meeting of the Society
for Applied Spectroscopy, June 1966, Chicago, Ill.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
