100-ppb concentration range in a continuous fashion can be achieved using the FPD detector and the calibration/monitoring procedure developed and tested in this work. With the exception of Son, the sulfur gases studied can be successfully separated and quantitated in the low ppb concentration range by the use of a cold trap, followed by GC analysis with a specially treated Porapak QS column and an FPD detector.
ACKNOWLEDGMENT Thanks are due to S. Guerin and R. Grozelle for their technical assistance and to J. MacMillan, S. Prahacs, R. Wostradowski, and B. Fleming for their very helpful suggestions during the course of this project. LITERATURE CITED (1) D. F. Adams. "Limitations in analysis of malodorous, sulphur-containing gases in the ambient air", Air pollution research section, College of Enaineerina Research Division. Washinaton State Universitv, Pullman, Wash., Paper .N ; 67-AP-22. (2) H. L. Helwig and C. L. Gordon, Anal. Chem., 30, 1810-1814 (1958). (3) M. D. Thomas and R. J. Cross, lnd. Eng. Chem., 20,645-649 (1928). (4) D. F. Adams, W. L. Bamerberger, and T. J. Robertson, J. Air Pollut. Control Assoc., 18 (3), 145-148 (1968). (5) R. R. Austin, G. K. Turner, and L. E. Percy, Instruments, 22, 588-589 (1949). (6) Model "Titrilog" by Consolidated Electrodynamics Corp., Pasadena, Calif. (7) Barton Titrator Models 286 and 400 by Barton Instrument Co., Monterey, Calif. (8) P. J. Klass, Anal. Chem., 33, 1851-1854 (1961). (9) H. C. McKee and W. L. Rolliwitz, J. Air Pollut. Control Assoc., 8, 338-340 (1959). (10) J. S. Nader and J. L. Dolphin, J. Air Pollut. Control Assoc., 8, 336-337 (1959). (1 1) Dohrmann C-200 Microcoulometer by Dohrmann Instrument Co., San Carlos, Calif. (12) D. F. Adams, G. A. Jensen, J. P. Steadman, R. K. Koppe, andT. J. Robertson, Anal. Chem., 38, 1094-1096 (1966).
(13) T. R. Andrew and P. N. R. Nichols, Analyst (London), 90, 367-370 (1965). (14) A. J. McCormack, S. C. Tong, and W. D. Cooke, Anal. Chem., 37, 1470-1476 (1965). (15) S. S. Brody and J. E. Chaney, J. Gas Chromatogr., 4, 42-46 (1966). (16) MicroTek Instruments Inc., Baton Rouge, La. (17) R. J. Robertus and M. J. Schaer, Environ. Scl. Technol., 7, 849-852 (1973). (18) T. L. C. de Souza, D. C. Lane, and S. P. Bhatia, Anal. Chem., 47, 543-545 (1975). (19) R. K. Stevens, J. D. Mulik, A. E. O'Keeffe, and K. J. Knost, Anal. Chem., 43, 827-831 (1971). (20) R. E. Pecsar and C. H. Hartmann, J. Chromatow. Sci., 11, 492-502 (1973). (21) F. Bruner, A. Liberti, M. Possanzini, and I. Allegrini, Anal. Chem., 44, 2070-2074 (1972). (22) F. Bruner, C. Cameili, and M. Possanzini, Anal. Chem., 45, 1790-1791 (1973). (23) F, Bruner, P. Ciccioli, and F. Di Nardo, Anal. Chem., 47, 141-144 (1975). (24) B. H. Devonald, R. S. Serenius, and A. D. Mclntyre. Pulp Pap. Mag. Can., 73 (3), (1972). (25) Robert A. Austin Co., P.O. Box 3655, Industry, California, Bulletin No. 100-470. (26) R. K. Stevens, A. E. O'Keeffe, and G. C. Ortman, Environ. Sci. Technol., 3, 652-655 (1969). (27) R. K. Stevens and A. E. O'Keeffe, Anal. Chem., 42 (2), 143A-148A (1970) - -, (28) A. E. O'Keeffe and G. C. Ortman, Anal. Chem., 38, 760-763 (1966). 129) Ramesh Chand. "lmnroved Permeation Devices for Calibration". Paoer presented at the 66th Annual Meeting of the Air Pollution Control Association, Chicago, Ill., June 1973. (30) J. E. MacMillan, "Development of the Meloy flue gas monitor for sampling and measurement of sulphur compounds", Final report on CPAR Project No. 157, submitted to the Canadian Forestry Service, Dept. of Environment, Aug. 1974. (31) H. P. Williams and J. D. Winefordner, J. Gas Chromatogr., 4 (7), 271-272 (1966). I
~I
RECEIVEDfor review June 28, 1976. Accepted August 26, 1976. We are indebted to the Cooperative Pollution Research (CPAR) Organization for the financial support given to us to carry out this work. Presented a t the 27th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1-5,1976, Cleveland, Ohio.
Concentration and Determination of Selenium from Environmental Samples Milton McDaniel, Arun D. Shendrikar, Kenneth D. Reiszner, and Philip W. West" Department of Chemistry, Louisiana State University, Baton Rouge, La. 70803
Radiotracers were used to evaluate a variety of procedures for concentrating selenium for its determinationwith the heated graphite atomizer (HGA)atomic absorption technique. The concentration procedures studied included extraction, internal electrolysis, coprecipitation, and hydride generation. Although hydride generation using sodium borohydride was chosen as the best method for concentrating and introducing samples into the HGA,the data in this report indicate that published procedures may liberate as little as 10% of the total inorganic selenium from the solution. Optimization of the hydride generation step as well as the other steps necessary for the determination of selenium resulted in a procedure which is sensitive, straightforward, and free from interference.
The importance of selenium as a toxin and carcinogen has been well documented ( I , 2). The natural abundance as well as the wide use of selenium or its compounds in manufacturing and industrial applications necessitate effective methods for measuring this element, especially a t trace levels.
Although numerous methods have been developed for the determination of selenium, no thorough examination has been made of the concentration steps necessary for its determination at the low levels occurring in environmental samples. Adequate studies of possible interferences are also lacking, particularly in the case of recent methods based on the selective volatilization of selenium as its hydride. This study establishes the optimum conditions for the generation of the hydride and its subsequent injection into a heated graphite atomizer. Additionally, several other approaches, both traditional and new procedures for concentrating selenium, have been critically examined. All work was done with the aid of a radioactive selenium tracer. The selenium hydride results are particularly enlightening due to the fact that they invalidate the popular belief that published procedures for the generation of the hydride are nearly 100% efficient. The ability of selenium to form a hydride has been used for some time to increase the selectivity and sensitivity of its determination. The hydride generation has often been coupled with atomic absorption techniques using flame atomization (3-6). Recent innovations have in-
2240 * ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
chded generation of the hydride with sodium borohydride or aluminum (7) in strongly acidic media coupled with the flameless atomizers for increased sensitivity ( 5 ) .Unfortunately little work has been done on the measurement of the efficiency of the generation of the selenium hydride and its transfer to the atomizer. The data obtained from the radiotracer investigations indicate that the efficient generation of the volatile hydride with sodium borohydride and subsequent injection of the sample into the HGA can be accomplished only by the careful selection of several parameters, such as acidity and mechanical design of the apparatus. The data indicate that as little as 10% of the selenium is injected into the HGA by published procedures while the present procedure is about 90% efficient. No interference was noted with the 39 possible interfering species tested.
Np-
EXPERIMENTAL Instrumentation. The Perkin-Elmer HGA 2000 graphite furnace was used with a Perkin-Elmer model 403 atomic absorption spectrophotometer. Nitrogen was used as the inert gas. A demountable hollow cathode lamp from a design by Wolcott (8),was used as a light source. Its energy output was 10 times that of conventional hollow cathodes and it proved to have a substantially longer life. It was operated at 50 mA with helium as the filler gas. An electrodeless discharge lamp or a commercial demountable hollow cathode would probably have worked equally well. A Heathkit power supply model PS-4 was used to power the lamp through a 2000-Q variable resistor. Measurement of radioactivity was done using a scintillation counter with a well-type sodium iodide crystal, a Harrison high voltage power supply and a Canberra model 895 timer and scaler. Apparatus. The reactor shown in Figure 1 was used for efficient reduction of selenium and volatilization of the resultant hydride when solutions of sodium borohydride were used as the reducing agent. The bubbler frit was of fine porosity (25-50 mm pore size) and the drying tube was 1.9 X 15 cm long. The trap consisted of 6-mm 0.d. glass tubing bent into a U and each leg was filled to 13 cm with 3-mm diameter glass beads. It was immersed in liquid nitrogen to a depth slightly higher than the level of the glass beads. Reagents. All reagents were reagent grade and high quality deionized water was used throughout. The sodium borohydride was obtained as a powder (No. 87658 Aldrich Chemical Co., Inc., Milwaukee, Wis.) except where comparisons were made to published procedures which used alternate reagents. Solutions of sodium borohydride were made immediately prior to use. The radioactive W e tracer was obtained from New England Nuclear, Boston, Mass. A selenium stock solution was made by dissolving elemental selenium in a minimum amount of nitric acid. Further dilutions were made when necessary. The calcium chloride and Drierite were 8-16 mesh. Tracer Procedure. Radioactive selenium added to samples containing 1pg of selenium was used throughout to determine the efficiency of the various concentration techniques tested. Mass balances were made when possible. Gaseous selenium hydride was absorbed in sodium hydroxide to obtain mass balances when studying the hydride generation procedures. Solids such as copper wire, zinc residue after reaction, and the desiccants were counted for 75Seand included in the mass balance. Recommended Procedure. A 50-ml sample, after being brought to 4.0 N with respect to hydrochloric acid, containing 0.005-2.0 pg of selenium was placed in the reactor (Figure 1).Twenty-five ml of sodium borohydride solution containing 1.00 g of sodium borohydride were placed in the separatory funnel. Ten g of calcium chloride were placed in the drying tube. The cold trap was placed in liquid nitrogen and allowed to cool for 1min. After positioning the four-way valve so that the gas would pass through the trap and be vented to a hood, the sodium borohydride solution was slowly added to the sample over a period of 3 min. The system was purged with nitrogen 1min prior to the sodium borohydride addition and continued to 1 min after the addition of the reagent. The fwr-way valve was then turned so that the cold trap was by-passed and the connections were changed to by-pass the reactor and drying tube. After connecting the gas flow to the atomizer, the cold trap was warmed a t room temperature for 15 s and then a t 80 O C in a water bath for 30 s. The instrument was then zeroed, the atomizer heated to the correct temperature, and a baseline was recorded. The four-way valve was then opened so that the hydride was swept into the atomizer and the absorbance was recorded. Ami-
Figure 1. Apparatus to generate, dry,
and collect selenium hydride
crogram of selenium was found to give a response of approximately 0.4 absorbance unit.
RESULTS AND DISCUSSION Although a variety of methods have been reported in the literature for concentrating selenium only those reported in Table I were considered promising enough for further investigation. Recoveries were evaluated using radioactive 75Se.As can be seen from these data, the most promising concentration procedures were the generation of selenium hydride with sodium borohydride and the reduction of selenium on copper wire, Le., internal electrolysis on copper. Although internal electrolysis was somewhat less efficient than hydride generation, it was rejected primarily because of the mechanical difficulty involved in placing the sample in commercial furnace atomizers. The technique could probably be applied effectively if hollow-T atomizers such as the one developed by Robinson and Wolcott (9) were used. The data demonstrated the need for improvement in hydride generation techniques. I t was particularly interesting to note that acid plus zinc produced little selenium hydride. Instead, approximately 88%remained as unreacted material or sludge. Procedures using sodium borohydride (3-5) yielded higher recoveries. However, as these studies show, the recoveries were far from optimum. Because radiochemical tracer techniques were not used in the previous studies, experimental evaluations were limited to intuitive judgments. The use of borohydride was sufficiently promising that a detailed investigation using tracer techniques was undertaken to optimize the generation of the hydride. The drying and trapping operations necessary for efficient concentration and injection of the selenium into atomizers such as the HGA were an integral part of the studies. The use of a liquid nitrogen trap for collection of the selenium hydride was deemed necessary for two basic reasons. First, the trap allowed the hydride to be revolatilized and injected into the HGA in a pulse, producing an increase in apparent sensitivity. Second, the trap removed the selenium hydride from the hydrogen that was generated by the decomposition of the sodium borohydride. The elimination of hydrogen from the input gas stream to the HGA minimized noise levels and prevented damage to the HGA that otherwise resulted from ignition of hydrogen a t exhaust ports. Conditions necessary for maximizing selenium hydride generation required a study of the amount of sodium borohydride to be used and the concentration of acid required. Naturally the mechanical design of the hydride generation
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
2241
~~~~
~
Table I. Methods for Concentrating Selenium Method Conditionsa
Recovery
74%. 1 cm wire/pg Se required Boil 50-ml sample 30 min in 1 N HCl with 30 cm Cu wire Reduction on copper wire Coprecipitation and pH 5.0). Allow 60%. Various parameters were On Fe(0H)Z 100-ml sample 20 mg Fe (Fe(NO& in 0.5 N "*NO3 precipitate to stand 60 min before filtering. studied. See also Ref. 10 On Al(0H)s Same as above, substituting Al(NOd3 for Fe(N03)3 40%. Various parameters were studied 30% 50-ml sample extracted with mixed ligands (see Ref. 1 1 ) Extraction Reduction to the hydride With Zn Sample in 4 N HC1. Add 4 g Zn and flush 1min with N2 after reaction 8%. 88%retained on Zn sludge. See also Ref. 6 acid subsides 10%.See also Ref. 3 With NaBH4 Sample in 0.6 N HC1. Pellets of NaBH4 addedb With NaBHd Sample in 6.0 N HCl. Pellets added, agitated and N2 purgedb 40-60%. See also Ref. 5 With NaBH4 See recommended procedure 90%
+
+
Except where noted these conditions are optimum.
Literature conditions used.
26ml ~oiutiono f mw, Addilbn tlm * 3mln. Sarnph size- S o d . 10 . micrograms Selmlum
-
Collbrollon Curve pH O f Solution 4.0 Solutlon VOlUm8* !?OdS.
100
60.
NaBH,
Micrograms of Selenium
Added (Grams)
Figure 2. Effect of the amount of sodium borohydride used on hydride generation
100
-
90 u 80 0 n e 70 6050 403020-
#-+++-.
.u
-
7'
-
#'
-
IO-
,#'
r
,.-*d
+
I
I
I
,
1
,
I
Figure 3. Effect of acid concentration on the efficiency of hydride generation
apparatus was also of prime importance. Several authors have studied the effect of acid concentration on the generation procedure (3, 4 ) , however, none have used radiochemical tracer techniques with their obvious advantages to evaluate the procedure. The amount of sodium borohydride used has not been studied in detail. As the data in this paper indicate, the amount or form of the sodium borohydride used was not optimum. Although pellets have been widely used for introducing sodium borohydride into the reaction vessel ( 3 , 4 ) ,a 5% solution was chosen because it allowed a more constant rate of generation of hydrogen, thus eliminating the need for a balloon reservoir for stripping the solution of selenium. Pellets were found to be only 40 to 60% as efficient as borohydride 2242
Figure 4. Calibration curve for selenium using atomic absorption spectrometry
solutions. It was deemed probable that pellets reacted so fast that intimate contact with the bulk of solution could not be made during the short reaction time (20 s), since borohydride solution added over a 20-s period gave results comparable to those obtained through use of the pellets. These initial experiments were carried out in a three-neck round-bottom flask for convenience. The solution was bubbled with nitrogen or stirred with a magnetic stirrer to reproduce conditions reported by other workers. When the quantity of sodium borohydride added was varied from 0.1 g to 1.25 g per determination, the generation of selenium hydride reached a constant level a t about 1.0 g of reagent as indicated in Figure 2. These studies carried out using radioactive selenium were verified later using atomic absorption spectrometry. The effect of acid concentration on the evolution of the hydride was equally dramatic as shown in Figure 3. Maximum stability and reproducibility would be expected a t the two plateaus occurring a t acid concentrations between 1.0 and 2.0 N and a t concentrations higher than 4.0 N. The optimum concentration was found to be 4.0 N HC1 because concentrations below this caused a yield loss and concentrations higher than 4.5 N caused an optical interference due to HC1 or its decomposition products. A drying step was included in the procedure because moisture entering the liquid nitrogen trap would freeze out and plug the trap. Also, any moisture condensed would reduce the efficiency of revolatilizing the hydride since it is soluble in water. Drierite was the drying agent initially used because several other investigators ( 5 ) had used it in drying various hydrides. Studies made using radioactive selenium indicated
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Table 11. Interference Study
Group I Group I1 Group I11 GroupIV Group V
Li+, Na+, K+ AA+, Cu2+ Be2+,M 2+ ka Sr2+ Ba2+,Hg2+,Cd2+,ZnZi B033-, l$40;2-, Ck3+, kl3+ C032- Sn4+ SiO32- Pb2+ NH4+,'N03', HPOdl-, Sb3+,HAs0d2-, Bi3+, Vn-3-
Group VI Cri+lbr2072-, Te032GroupVII F- C1- Br- I- Mn2+ Group VI11 FeS+,Cb2+, NiZi that between 10 and 40% of the hydride was being retained on the Drierite. Magnesium perchlorate was ruled out because of a possible explosive reaction with the liberated hydrogen, some of which would be in the atomic form. Sulfuric acid was investigated but was found to be unsatisfactory. Calcium chloride, however, proved to be quite satisfactory retaining an average of 2% of the selenium hydride: and a t no time retaining more than 4%. The fact that calcium chloride has a very high heat of solution caused the drying tube to become very hot when water was absorbed. At such temperatures, selenium hydride exhibits relatively little solubility. After approximately 20 determinations, the calcium chloride became saturated with water, and was replaced to eliminate the possibility of water getting into the cold trap. The trapping of selenium hydride at reduced temperatures is a critical step in the proposed procedure and was also examined with radiotracers. Dry ice temperatures were found to offer only limited recovery, therefore liquid nitrogen is recommended. Initially, the cold trap was a hollow 6-mm tube. Use of radioactive selenium showed that 30 to 40% of the selenium passed through the tube without being trapped. This problem was eliminated by filling the trap with 3-mm glass beads, thereby reducing the loss to less than 1%. With the entire apparatus in operation, it was found that the selenium hydride was being transferred from the solution to the HGA with an efficiency approaching 90%. Since this is the best that we were able to obtain, it seems obvious that procedures operating at less than optimum conditions would give considerably lower recoveries. The losses would naturally be compounded at each successive step in the analytical procedure. The method developed here for the determination of selenium with the HGA was designed using the optimum conditions established above. Approximately 90% of the selenium in the sample is evolved and injected into the atomizer. The acid concentration of 4.0 N was chosen as the minimum that would allow removal of the maximum amount of selenium from solution, yet would not necessitate deuterium background correction to eliminate interference from HCl. The atomizer was operated continuously at 1800 "C during the injection of the sample and the lamp current was maintained at 50 mA. The use of a more intense light source, such as the demountable hollow cathode, for atomic absorption determinations is highly recommended. As a result of the noise level
being reduced to a minimum by the use of a demountable hollow cathode lamp, the volume of the cold trap being minimized to increase peak sharpness, and the almost quantitative amount of selenium reaching the light path, the method gave a detection limit of 0.1 ppb in a 50-ml sample and a sensitivity of 10 ng. A typical calibration curve is given in Figure 4. Determinations can be done in less than 10 min per sample for the simple apparatus used here while more complex equipment could.be devised which would enable 15 to 20 determinations per hour. To determine the effect of diverse ions on the determination of selenium by this procedure, enough of each ion was added to give a 100-ppm concentration except for Na+, Mg2+,Ca2+, Zn2+,A13+,and Fe3+ where the concentration was brought to 200 ppm for each of these ions (see Table 11).The interferences were studied by groups. The desired amount of each ion in a particular group was added to a sample containing 1.0 kg of selenium in 50 ml of solution and a determination was made by the recommended procedure. One determination was made in which all the ions were present. In no instance was interference of more than 5% observed, all results falling within the experimental error of the procedure. These results would be expected because of the extreme selectivity of hydride production, cold trap concentration, and atomic absorption determination. It is interesting that other authors (12, 13) have not met with equal success. In most cases, there was a suppression of signal. Of the species studied here, Siemer and Hagemann (12) encountered serious interference from both copper and arsenic while Smith (13)observed interferences from these as well as silver, nickel, cadmium, cobalt, iron, lead, antimony, and zinc. The increased efficiency of the procedure developed here may account for the reduction of interference. In the procedure of Siemer and Hagemann (12)only 0.01 g of NaBH4 was used compared to the 1.0 g used here. Smith (13) acknowledged that his procedure may exaggerate the effect of possible interferences.
LITERATURE CITED (1) A. R. Stahi, "Preliminary Air Pollution Survey of Selenium and Its Compounds", US. Department of Health, Education, and Welfare, NAPCA, 1969. (2) K. Schwarz, Nutr. Rev., 18, 193 (1960). (3) F.J. Schmidt and J. L. Royer, Anal. Lett., 6, 17 (1973). (4) F. J. Fernandez, At. Absorp. Newsl., 12, 93 (1973). (5) E. J. Knudson and G. D. Christian, Anal. Lett., 6, 1039 (1973). (6) Y. Yamamoto. T. Kumamaru, Y. Hayashi, and M. Kande, Anal. Lett., 5,717 (1972). (7) P. D.Goulden and P. Brooksbank, Anal. Chem., 46, 1431 (1974). ( 8 ) D. K. Wolcott, Louisiana State University, Baton Rouge, La., personal communication, 1974. (9) J. W. Robinson and D. K. Wolcott, Anal. Chim. Acta, 74, 43 (1975). (IO) I. M. Koremman, Zh. Obshch Khim., 25, 2399 (1955). (11) S. L. Sachdev and P. W. West, Environ. Sci. Techno/., 4, 749 (1970). (12) D. D. Siemer and L. Hagemann, Anal. Lett., 8, 323 (1975). (13) A. E. Smith, Analyst (London), 100, 300 (1975).
RECEIVEDfor review December 29,1975. Accepted August 16, 1976. We wish to acknowledge support for this research from the National Science Foundation, RANN Grant GI35114x1 and ESR74-18932.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
2243
