MW
217 30 1 266 273
233 295 146 146 146 146 148 1&6
118 116 118 I18
90 80
M 34 46 94
90 78 78
MF
NAME
C 9 H 1 2 N 05 P S SUMITHION ClOH12N304PS O X Y G E N ANALOG O F GUTHION C14 H 1 0 N 2 0 3 S 46-DIPHENYL-1 2.3 5 - O X A T H 1 4 D l A 2 I N E - -2.2-OIOXIDE C11 H12 N 0 3 C L S CHLORMEZANONEITRANCOPAL~ C13 H16 N 3 0 4 N A S METHAMYYRONEIDIPYRONE POWOER U L M E R I C ~ ~ H I B C L N ~ S CHLOROTHENITAGATHENI C6M18S 2 2.4.4-TETRAMETHY L-3-THtAPENTANE C6H18S TERT-BUTYL SULFIDE C7H140S ISO-0UTY L THIOL NOR-PROPANOATE C7H14OS NOR-BUTYL THlOL NOR-PROPIONATE C7HllOS E T H Y L THIOL NOR-PENTANOATE C7 H 1 4 O S E T H Y L THIOL I S 0 PENTANOATE C5HlOOS E T H Y L THIOL ACETATE C5HlOOS M E T H Y L THIOL NOR-BUTYRATE c5 HI0 0 s NOR-PROPYL THIOL ACETATE C5H100 S ISO-PROPYL THIOL ACETATE C3H60S M E T H Y L THIOL ACETATE cos CARBON OXYSULFIDE 02 SULFUR D I O X I D E H2 S H Y D R O G E N SULFIDE c H4 METHANETHIOL I M E T H Y L MERCAPTAN# C2H602S DIMETHYLSULPHONE C2H6N2S B I S I M E T H Y L I M I N OsuLPnuA I C2H60S 2-MERCAPTO ETHANOL C2HSOS SULFOXIDE D I M E T H Y L
s
s
Part of reordered list of sulfur compounds after the SSP procedure had been applied
45 spectra, all thiol esters, out of 8782 were found to meet these 29 criteria. No additional compounds were found to meet this criteria. Thus, a rule based on these 29 features was able to separate alkyl thiol esters from any and every other class of compounds in the file. In further experimentation with these criteria, it was found that 13 of 29 features could be eliminated without finding additional spectra that met this criteria. The features eliminated were: Peaks present: Peaksabsent: Losses absent: Losses present:
45 51,52,65,66,80,93,106,107,108 30, 31 89
Figure 1.
Last, a spectrum of nor-heptyl thiol nonhexanoate thought to be a bad spectrum because it did not meet the criteria was re-run and found to meet the identification criteria derived from the cluster analysis. Thus, the 16 features given below appear to be able to characterize straight-chain alkyl thiol esters. The criteria are:
All these spectra were run on a Bendix TOF mass spectrometer (6). There were no thiol esters with aromatic or saturated rings of other functional groups. Thus, the classification rules are to be considered applicable only to this limited class of compounds. The matrix array of the spectra features arranged in the linear order found by the SSP program was manually inspected, and 29 features were picked out and found to characterize alkyl thiol esters. The features consist of peaks and losses that were found to be always present or absent in the class. These 29 features were then processed against the entire 8782 spectra from the master file. Only
We are indebted to D. Black for the TOF mass spectra. We wish to thank H. M. Fales, J. R. Slagle, M. Shapiro, and R. C. T. Lee, for thoughtful discussions and also wish to thank R. J . Feldmann for the SAIL program used in the SSP procedure.
( 6 ) W . H. McFadden, R. M. Seifert, and J. Wesserman, Anai. Chem., 37, 560 (19 6 5 ) .
Received for review July 23, 1973. Accepted December 26, 1973.
Present 0 27, 29, 41, 42 43, 47, 57, 61
Losses: Peaks:
Absent
M-1 31, 32, 36 37, 38, 50
ACKNOWLEDGMENT
Modified Approach for Submicrogram Determination of Selenium in United States Geological Survey Standard Rocks V. Lavrakas, T. J. Golembeski,' G. Pappas,2 and J. E. Gregory3 Department of Chemistry. Loweil Technological Institute. Lowell. Mass. 07854
H. L. Wedlick Department of Radiological Sciences, Lowell Technological Institute, Lo well, Mass. 07854
The determination of trace elements in the environment is of increasing importance; many trace elements are related to the health of plants and animals. Selenium causes livestock poisoning ( I , 2), poisoning of humans (3), and anemia and hypertension ( 4 ) occur where certain limits are exceeded. Some evidence exists that selenium del Present address, Yew E n g l a n d N u c l e a r Corp., B i l l e r i c a , Mass. * P r e s e n t address, New E n g l a n d N u c l e a r Corp., Boston, M a s s . 3 Present address, U n i r o y a l Corp., N a u g a t u c k , Conn.
( 1 ) I. Rosenfeid and 0. A. Boeth, "Selenium Geobotany, Biochemistry and Nutrition," Academic Press, New York, N.Y. 1964. ( 2 ) W H Allaway and E.E Cary, Ana/. Chem.. 36, 1359 (1964) (3) R. H . Tomlinson and R C. Dickson, Proc. inter Conf.. April 19, 66 (1965). ( 4 ) K . P McConnell, Proc inter. C o n f . . Dec. 1. 139 (1961)
952
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, N O
7 , JUNE 1974
creases the toxicity of methylmercury added to diets containing tuna ( 5 ) .Atmospheric selenium has also been extensively investigated as an indicator of atmospheric sulfur pollutants (6). Various chemical methods have been used to determine the presence of trace quantities of selenium in materials; however, most suffer from the deficiency that interfering ions must first be removed and that the presence of selenium at trace levels in reagents, unless corrected, will contribute to erroneous results. Many of the analytical techniques lack the required sensitivity and accuracy to determine submicrogram levels of selenium. For example, other workers, using a coprecipitation and photometric ( 5 ) H E Gantheretai Science 175. 1122 ( 1 9 7 2 ) ( 6 ) K K Sivasankara Pillayefa/ Envfion S o Techno/ 5, 76 ( 1 9 7 1 )
method in the determination of selenium in sea water, silicates, and marine organisms, reported a value of 2.5 0.1 ppm for the USGS standard rock G-1 ( 7 ) ; using neutron activation analysis (NAA), the selenium content has been found to be approximately one thousand times smaller than this value (8, 9 ) . NAA provides a simple method for the determination of submicrogram quantities of selenium in a variety of materials (8, 10). However, chemical separation is often necessary to ensure radiochemical purity. Selenium in the USGS standard rocks has been determined by Brunfelt and Steinnes using NAA followed by dissolution of the sample and then repeated distillations to separate selenium from the other activated elements present (9). As this procedure is time-consuming and as other elements (Ge, As, Sn, and Sb) distil under the conditions described ( 1 1 ) it was believed that the use of solvent extraction, based to a large extent on the work of McGee, Lynch, and Boswell (12), could be used to isolate the selenium. In addition, Brunfelt and Steinnes had 'used the spectrum obtained from a 3-x 3-in. well type NaI(T1) scintillation detector to establish radiochemical purity, and based their calculations on the 401-KeV peak, the peak area evaluated according to the method of Covell (13). The method of Covel1 yields accurate results only if the peak area is free from extraneous rays and if its base may be accurately represented by a straight line ( 1 4 ) . Previous work failed to provide sufficient evidence for either of these criteria, especially that of extraneous rays. In this investigation, the determination of selenium in standard rocks via solvent extraction is rapid, highly selective, and convenient. The radiochemical purity and the chemical yield of the extracted selenium is favorable.
*
EXPERIMENTAL Apparatus. A lithium drifted germaniam, Ge(Li), gamma-ray detector a n d a 3-inch well type sodium iodide thallium activated, NaI(Tl), gamma-ray detector were used and housed in a 4-inch lead shield. T h e gamma-ray intensity and pulse-height distribution was obtained on a Canberra-Geos multichannel analyzer. as selenous acid was obtained Reagents and Samples. from New England Nuclear Corp., Boston, Mass. T h e purity of this isotope was checked by observing the gamma-ray spectra obtained with a Ge(Li) detector and a multichannel analyzer. Yo extraneous gamma-rays were found in the energy region of interest. Analytical or certified ACS grade chemicals were used. T h e water used to prepare comparator selenous acid solutions was demineralized, followed by quartz distillation. T h e following samples obtained from the United States Geological Survey, Washington, D.C., were investigated for their selenium content: AGV-1 (andesite), BCR-1 (basalt), W-1 (diabase), DTS-1 (dunite), G-1 and G-2 (granites), GSP-1 (granodiorite), and PCC-1 (periodotite). T h e original samples of G-1 and W-1 specified to pass 80 mesh showed segregation a n d the presence of large grain sizes (f5).New samples obtained showed greater homogeneity both within a bottle a n d between bottles. Procedure. Activation Analysis. T h e 74Se (n, y ) 75Se reaction was used in the activation analysis procedure. Activations were performed in nuclear reactors located either a t the Atomic Energy Research Establishment, Harwell, England, or a t the Rhode Island Nuclear Science Center, Naragansett, R.I., a t fluxes of a p Y K Chau and J P. Riley, A m i . Chim. Acta. 33, 36 (1965). A J Blotcky, L J Arsenault, and E P. Rack, Ana/. Chem . 45,
1056 (1973). A . 0. Brunfelt and E Steinnes, Geochim. Cosmochfm. Acta. 3 1 , 283 ( 1 967). H . J M . Bowen and P. A . Cawse. Anaiyst. 8 8 , 721 (1963) V J. Molinski and G . W. 1.eddicotte. "Radiochemisrry of Selenium," NAS-NS-0030 (Rev.).p 16, 1965 T McGee. J. Lynch, and G . G . J.
Boswell, Talanta.
1 5 , 1435
(19681 D. F Covell, Ana/. Chem.. 31, 1785 (1959) K Heydorn and W. Lada. Anal Chem . 44,2313 (1972) M . Fleischer and R. E. Stevens, Geochim. Cosmochim. Acta, 26, 525 ( 1 962)
T a b l e I. K
1)
Values for the Extraction of Selenium,A K I I value,
Trial No.
1 2
3 4 5 6 7
8 9
Counts, Organic Phase
Temperature, OC
Net counts of organic phase
Net counts of aqueous phase
Counts, Aqueous Phase
25 25 25 25 25 25 60 75 85
63,300 66,000 65,971 69,173 61,275 66,345 80,090 97,125 110,850
10,872 11,241 11,973 12,632 11,567 12,153 6,407 860 172
6.03 5.79 5.51 5.48 5.30 5.46 12.5 113 644
Shaking time = 30.0 sec. Average a t 25 " C , 5.60 zt 0.26
proximately 1013 thermal neutrons cm- secQuartz tubing, 4-mm i.d. by 14 cm, washed in aqua regia and rinsed with quartz-distilled water were used to contain the rock samples and comparator solutions. T h e samples were arranged alternately on the inside periphery of t h e rabbits to provide a random distribution to the neutron flux. Errors from self-shielding were considered negligible as reproducible results of the comparators were achieved. Nuclear reactions other than radioactive capture which might result in the formation of 75Se or other radioisotopes are listed by Koch (16). None of these reactions would produce interfering quantities of 75Se. Selenium is also one of the fission products of 235Ubut, a s the amount of uranium is of t h e order of parts-per-million in the rocks, t h e small amount of selenium produced by the fission reaction is negligible. Chemical Separation. After neutron activation in a neutron fluence of approximately no/cm2, the contents were allowed to stand unopened for 10 days to permit short-lived isotopes to decay. T h e contents of each quartz vial was transferred to weighed nickel crucibles and the weight of the rock determined. Five ml of standard solution (10.22 mg Se per ml) was added and the rock dried a t 110 "C for about 1.5 hours. Sodium peroxide fusion was then carried out; the melt dissolved in HC1 and the solution made 4 M HBr. The selenium was immediately extracted with freshly prepared 1%phenol in benzene. The selenium in the organic layer was reduced to elemental selenium by the addition of a solution of hydroxylamine hydrochloride. T h e selenium was transferred to a preweighed test tube, purified, and the chemical yield determined. T h e radiochemical purity of the sample was checked after each separation by observing the gamma-ray spectra obtained with the Ge(Li) detector. Solutions of comparator were prepared from a known concentration of selenous acid. After irradiation, the outside of the quartz vial was checked for radioactive contamination; none was found which eliminated t h e need to wash them in aqua regia. The contents of each vial was emptied into a preweighed test tube and a measured aliquot was transferred to a counting vial. T h e samples were left open and allowed to evaporate so t h a t the size and geometry of the standard comparators was the same as the unknowns. T h e comparators were counted and corrected for background. T h e weight of the selenium and its activity would then give the specific activity (cpm/fig) used to calculate the selenium content of the rock samples from their respective counting d a t a .
RESULTS AND DISCUSSION Losses of selenium due to drying, fusion, and dissolution were studied by the use of the 75Se tracer. The tracer was added to non-irradiated samples of the various rocks and the sample counted a t each stage; recovery a t each step was essentially 100%. The 75Se tracer was used also to determine the yield in the solvent extraction step. There was no change in recovery with shaking times greater than 60 seconds; however, the distribution coefficient varied considerably with temperature as shown in Table I. Overall recovery, for the rock samples analyzed, ranged from 4% to 83%. It is suspected that incomplete (16) R . C . Koch, "Activation Analysis Handbook," Academic Press, New York and London, 1960. ANALYTICAL CHEMISTRY, VOL. 4 6 , NO. 7, JUNE 1974
953
Table 11. Content of Selenium i n Various USGS Standard Rocks, ppm
-
w-1 0.116 0.116 0.114 0.115
G-2
G 1"
0.0078 0.0072 =k 0,001
0.0076
AGV-1
0.0054 0.0065 0.0083
DTS-1
0.0092 0.0086 0.0072 0.0084 0.0067 j ~ 0 . 0 0 1 4 0.0084 f 0 . 0 0 0 8
0.0069 0.0075 0.0074 0.0077 0.0074 f0.0003
PCC-1
GSP-1
0.0267 0.087 0.0333 0.084 0.0441 0.066 0.0541 0.0396 XtO.012 0.079 r0.011
Determined by distillation, ion exchange, and precipitation (V.L.).
I'
Table 111. Trace Selenium Concentrations Determined by Several Analytical Techniques
Table IV. Ratio of Selenium to Sulfur for USGS Standard Rocks Se ppm
Total S, 7/O"
G-1 G-2
0.0076 0.0067
AGV-1
0.0084 0.0074 0.0396 0.079
0.0175 0.02 0.0135 0.01 0.01
4.2 3.3 8.2 8.4 7.4
0.01
4.0
Se, ppm
USGS rocks
w-1
G-1 G-2 AGV-1 DTS-1 PCC-1 GSP-1,
This work
0.115 0.0076 0.0067 =t0.0002 0.0084 i 0.0008 0.0074 f 0.0003 0.0396 i 0.012 0.079 Xt 0.011
Other investigators
0,110," 0 . 17b 2.5 ztO.l,c < O . l b
w-1
0.005(1)"
DTS-1
0.008 (2)' 0.004 (2).
0.022 (2)a 0.059 (2)'
PCC-1 GSP-1
0.115
0.05
Ratio of Se to S
x 10-5 x 10-5 x 10-4 X 10-6
x 10-5 x 10-4 1 . 6 x 10-4
Fleischer and Stevens 116).
Brunfelt and Steinnes (9). Landstrom et al. ( 1 7 ) . Chau and Riley (7).
transfer of the selenium from the benzene-water interface accounted for the low yield of some samples. The radiochemical purity of the 75Se tracer, 75Se comparator solutions, and the solvent extracted 75Se from the rock samples as determined by gamma-ray spectrometry using a Ge(Li) detector indicated that no interfering elements were present. The 401-KeV peak area count obtained by counting the sample with the NaI(T1) detector was due not only to counting the 401-KeV gamma of 75Se but is also due to counting the coincident summing of gamma photon interactions of the other cascade gammarays of 75Se. This effect resulted in a high count rate in the 401-KeV area of the spectrum which varies strongly with sample size, counting geometry, and interfering gamma-rays. Performance tests were run to determine the reproducibility of counting a standard sample and it was found that taking the sample to dryness in a standard counting vial and counting in a well type NaI(T1) detector resulted in acceptable reproducible results. Spectra obtained with the use of the high resolution Ge(Li) detector were observed in a qualitative manner to verify the purity of 75Se by the absence of the other gamma-emitting radionuclides. The spectra obtained via the well type NaI(T1) detector was used for quantitative information; the high efficiency NaI(T1) detector gave a statistically significant count within a reasonable counting time. The Ge(Li) detector was also used to observe the gamma-ray spectra of the raw irradiated rock sample. The presence of gross quantities of interfering gamma-rays, even a t 200 days post irradiation, indicated that chemical '
954
ANALYTICAL CHEMISTRY, VOL. 46, NO. 7, JUNE 1974
separation is required prior to counting the 75Se. In addition, it was found that once the rock sample has been fused and dissolved, the solvent extraction of selenium requires only a few minutes as compared to other laborious and time-consuming procedures (coprecipitation, ion exchange, distillation). The results of this investigation are shown in Table 11, and compared to the averaged results of other investigators (7, 9, 27) in Table ID.The results compare favorably with but one exception (G-1). Finally, the ratios of Se:S have been shown to be for magmatic rocks, about 1:6000, representing an average selenium content of 0.09 ppm (18). Rankama and Sahama have indicated that in most cases the Se:S ratio is about 1 x 10-4 and that, with the exception of sea water, this approximate ratio is typical (19). Table IV indicates that our results approximate this value.
ACKNOWLEDGMENT The authors express their thanks to Francis DiMeglio of the Rhode Island Nuclear Science Center, Narragansett, R.I., and to Donald F. C. Morris of Brunel University, Uxbridge, England, for the activation of the samples and for advice and encouragement during this work, respectively. Received for review August 28, 1973. Accepted January 14, 1974. (17) 0. Landstrom, K . Samsahl, and C. G. Wenner, "Modern Trends in Activation Analysis," NBS 312, I (1969). (18) V . M. Goldschmidt, "Geochemistry," Oxford University Press, Oxford, 1962. (19) K . Rankama and G. Sahama, "Geochemistry," University of Chicago Press, Chicago. Ill., 1950.
