3194 (1975) Equation 22 should read logla not in. (21) J . D. Garcia, R J. Fortner, and T. M Kavanagh. Rev. Mod. Phys., 45. 111 (1973). (22) N. F. Mott and H. S. W . Massey, "Theory of Atomic Collisions", Oxford University Press, London, 1949. (23) J. Fhilibert, in "XAay Optics and Microanavse," H. H. Patlee, V. E. Cossktt, and A. Engstrom. Ed., Academic Press, New Yo&. N.Y.. 1963, pp 379-392. (24) K. F. J. Heinrich. Adv. X-ray Anal., 11, 197 (1968).
(25) (26) (27) (28)
W . Pabst, Nucl. Instrum. Methods, 120, 543 (1974). J. M. Khan, D. L. Potter, and R. D. Worley, Phys. Rev., 145, 23 (1966). 8. L. Henke and E. S.Ebisu, Univ. of Hawaii, AFOSR 72-2174. F. Folkrnann, J. Borggreen, and A. Kjeldgaard, Nucl. Insbum. Methods, 119, 117 (1974).
RECEIVED for review April 15, 1977. Accepted June 27, 1977.
Determination of Sulfur in Environmental Materials by Thermal Neutron Capture Prompt Gamma-Ray Spectrometry Edward T. Jurney," David B. Curtis, and Ernest S. Gladney University of California, Los Alamos Scientific Laboratory, P. 0. Box 1663, Los Alamos, New Mexico 87545
Thermal neutron induced prompt y-ray spectrometry has been investigated as a technique to measure elemental S concentrations in complex materials. Comparison of analytical results on standardized materials shows the methald to be accurate. Repeated analyses of 100 mg of eiementali S gave a 1YO standard deviation from the mean. The standard deviation increased with decreasing quantities of s. Standardized Orchard Leaves containing 500 pg of S had a 13% standard deviation from the mean of repetitive analysis. Calcium and K are the only common elements that directly interfere with the analysis.
One of the major problems associated with coal combustion as a source of energy is the effect of the oxides of 5; on the environment. Studies of the environmental chemistry of S often demand a rapid means of measuring the element a t relatively high concentrations (0.01-1 70) in complex matrices. Traditional analyses for S involve gravimetric methods ( I ) volatilization of both oxidized and reduced sulfur gases ( 2 ) , and colorimetric methods (3). More recently isotope dilution mass spectrometry ( 4 ,neutron activation analysis ( 5 ) ,atomic emission ( 6 ) ,atomic absorption ( 7 ) , ring oven ( 8 ) , PbCIOj titration (9) vacuum ultraviolet spectrometry ( I O ) , and x-ray fluorescence ( 1 1 ) have been used for the determination of S. Chromatographic methods that have been used for the analysis of S include inorganic ion exchange (121, paper chromatography (13),and gas chromatography (14). These analytical techniques are often time consuming, subject t o matrix interferences, or are relatively insensitive, making them inappropriate for use in environmental studies which require analyses of large numbers of samples comprised of varied and complex natural matrices. T h e utility of thermal neutron induced prompt -(-ray spectrometry for careful measurements of B and Cd in environmental matrices has recently been demonstrated ( 1 5 ) . T h e technique provides a means t o do analyses for selected elements in a variety of complicated matrices with a minimal expenditure of manpower. Analysis for S using prompt y-ray spectrometry with ' W f as a neutron source has been utilized in several special purpose applications involving coal (16) and oil (17). However, a study of the general appiicability of the method for S determinations has not appeared in the literature. T h e purpose of the present work is t o quantify the characteristics of the procedure and investigate its applicability
to a variety of matrices using National Bureau of Standard (NBS) Standard Reference Materials (SRM) and other well characterized materials.
EXPERIMENTAL Gamma rays arising from neutron capture by 32Sdominate the prompt gamma spectrum when S in its natural isotopic abundance is irradiated with thermal neutrons. The three most intense prompt transitions produce gammas at 841, 2380, and 5420 keV (18). The 841-keV line appears to be the most useful for analytical purposes; it is the most intense line in the S spectrum and the detector efficiency greatly favors the lower energy gamma. The higher energy gammas may be useful if there are interferences to the 841-keV photon, although the detection limits are significantly degraded by the use of these lines. A prompt gamma-ray spectrum from elemental S contained in a polyethylene vial, taken with a 26 cm3 Ge(Li) detector surrounded by a 25 cm long by 20 cm diameter NaI anticoincidence annulus, is shown in Figure 1. National Bureau of Standards SRMs and standardized coals from the Illinois State Geological Survey (ISGS) were employed in this study. Samples of each material (300-600 mg) were encapsulated in snap-cap polyethylene vials and placed in the thermal column of the Los Alamos Omega West Reactor. At the position of sample irradiation the thermal neutron flux is 2 X 10" n/cm2 s with a Cd ratio of -1000. Prompt gamma rays from the sample are collimated into a well-defined beam that strikes the center of the Ge(Li) detector situated approximately 6 m from the sample. The Ge(Li) detector is surrounded by a 20-cm diameter by 25-cm long NaI annulus for suppression of background caused by the escape of Compton scattered photons from the detector. A detailed description of the facility may be found in Jurney e t al. (19). A small fission counter measured the relative neutron fluence delivered to the sample during each irradiation. All results were normalized to this neutron fluence to account for variations in reactor power during data accumulation. In our experimental configuration, the counting rate for the 841-keV line is 0.11 counts/s/mg of S and for th.e 2380 keV-line 0.033 counts/s/mg of S. In the development work for the analysis of boron (15), we demonstrated that standardization could be accomplished by different methods. A known weight of the element of interest and a known weight of a reference element with a well known neutron capture cross section yielded comparable results. As a matter of convenience, about 100 mg of elemental S was used as a standard in all determinations reported here.
-
RESULTS AND DISCUSSION Results of the analyses of 9 NBS SRMs and ti ISGS standard coals are summarized in Tables I and 11. Sulfur abundances in six of the SRMs have been certified or provisionally certified by the NBS. Standard coals from the ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
1741
13'
PRGMPT I-RAY
1
SPECTRUM OF ELEMENTAL SULFUR
1
I
i
1
1
I
IC01
35
?Z
1:
25
EhERGY (MeV)
Figure 1. A portion of the prompt gamma-ray spectrum from thermal neutron capture by elemental sulfur. The prominent line from hydrogen and the weak line from carbon originate in the sample holder. The nitrogen line is from neutron capture by air in the unevacuated channel
Table I. Sulfur Content of NBS SRM, Percent by Weight Indiv. Material analyses Cement 6 3 3 0.96 0.84 0.88 Cement 635 2.6 2.7 2.7 Orchard Leaves 0.25 1571 0.21 0.24 Coal 1631A 0.58 0.56 0.63 Coal 1631B 1.97 2.06 2.04 Coal 1631C 3.01 2.98 2.96 0.68 Bovine Liver 0.71 1577 0.76 Coal 1632 1.27 1.40 1.30 Fly Ash 1 6 3 3 0.44 0.43 0.44
Av z S.D.
NBS certified value
0.89 0.88 ~ 0 . 0 6 to.01 2.7
io.l
2.8
R"
1.01 0.97
io.05
0.23 i0.02
0.23
1.00
0.59 -0.04
0.546 i0.003
1.07
2.02
2.06
0.98
-0.05
to.01
2.98 i0.02
3.010 t0.008
0.99
0.72 k0.04 1.32 r0.07
1.26 ( 2 0 )
1.05
0.44 20.01
a R is the ratio of S abundance measured in this work to the NBS certified value.
ISGS had been analyzed a t the Survey using x-ray fluorescence a n d wet chemical methods as described by Ruch e t al. (20). T h e accuracy of the method was evaluated by assuming the results from t h e NBS and ISGS to be a true measure of the S content of t h e materials. With this assumption, t h e ratio of the S abundances obtained by measuring prompt gammas t o t h e abundances reported by the originating laboratories ( R ) was taken as a measure of t h e accuracy of the method under investigation. Data in Tables I and I1 show t h a t excellent accuracy is achieved by prompt y-ray analysis. The mean value for R in t h e 13 materials with certified values is 1.0 f 0.04. T h e good agreement between our results on the Orchard Leaves and the NBS provisional value is probably fortuitous since the quantity of S in this SRM is approaching our detection limit. Values for S concentrations in the SRM Bovine Liver are not available from the NBS. There are no S abundances for comparison of the NBS Fly Ash. It is interesting to note t h a t a significant amount of S was retained 1742
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
Table 11. Comparison of S Analyses of Illinois Geological Survey Coals IGS coal No. C 14630
This work ( 7 0 ) Indiv. Av -f values S.D. 1.16 1.06
1.10
1.08
i0.05
IG S value, ( % )
Ra
1.23 i0.05
0.89
4.12 i0.05
1.00
2.78 io.05
1.02
4.77
1.01
C 15231
4.18 3.94 4.1 4.31 t0.2 C 15263 2.94 2.85 2.8 2.70 to. 1 4.89 C 16139 4.40 4.8 i0.4 5.11 C 16408 4.94 4.28 4.6 4.58 to.3 C 17215 3.76 3.97 3.8 3.71 to.1 R is the ratio of the S abundance work t o the ISGS reported value.
t0.05
4.73
0.97
t0.05
3.88 t0.05
0.98
measured in this
Table 111. Reproducibility of Prompt Gamma-Ray Analvsis for Sulfur c?c Std dev of mean Quantity N o . of of sulfur, 841 2390 keV ke V Material analyses mg Element S Element S Cement (NBS N o . 633) Elemental S Coal (NBS 6 3 i A ) Orchard Leaves (NBS No. 1 5 7 1 )
0.8
1.2
(5)
96 9.1 2.3
2.3 5.3
2.7 4.8
(8) (5)
2.1 2.0
3.2 2.8 13.4
13 6.8
(6) (8)
(6)
0.5
in the fly ash after combustion of coal. Since analysis by prompt 7-ray spectrometry is nondestructive, the precision could be measured directly by repeated measurements of the same sample. The ability to reproduce a result was evaluated by calculating the percent standard deviation from the mean value of repetitive analyses. These data are shown in Table 111. Prompt gamma analysis involves so few operations that the sources of variability are limited. T h e statistical uncertainty associated with the interrogation of random events is probably t h e most significant source of
1 1
4333c
--.. ORCHARD L E A F MATRIX
K"
7er
822
ENEQGY
I
34:
86;
ke\
Figure 2. A portion of the prompt gamma-ray spectrum from an orchard leaf sample. The intensity of the 841-keV sulfur line is close to the useful detection limit under the background conditions discussed in the text The satellite peak at - 7 8 5 k e V is unidentified
variability of t h e results, i.e.. t h e precision should vary as a function of t h e intensity of t h e signal. Table 111 shows t h a t in a broad sense this is true. T h e precision for replicate analysis is degraded as the quantity of S being measured decreases. T h e 841-keV line yields a u of 0.8% for 96 mg of S, while 0.5 mg of S in orchard leaves had u of 13%. The latter quantity approaches the practical detection limit of the procedure. T h e precision of results obtained from t h e 841-keV 7 ray is better than t h a t obtained using t h e 2380-keV line. T h e difference in precision between the two lines is small for larger quantities of S b u t becomes increasingly large until t h e 2380-keV line cannot be resolved from background for 0.5 mg of S in the orchard leaf matrix. The 841-keV line still provides reasonably precise results for t h e orchard leaves. There appear t o be real differences in t h e precision of replicate analyses of different materials containing similar quantities of S. T h e differences may be due t o interferences from other elements in the matrix. However if this were the case, it is difficult to explain t h e relatively large standard deviation (13%) obtained using the 2380-keV peak for t h e analysis of 2 mg of elemental S,, a matrix which should be relatively interference free. There appears to be a source of imprecision which is not clearly identifiable. Calcium and K are t h e most likely sources of direct interference in t h e determination of S by this method. Potassium emits a y ray a t 843 keV and Ca has a line at 837 keV. Figure 2 is a portion of the orchard leaf prompt -,-ray spectrum showing the 841-keV S line and the 843-keV gamma from K. T h e weight ratio, K/S, in this matrix is 6.5 which produces about equal intensities for t h e two peaks. Per unit weight, the 841-keV S line is calculated to be >lo0 times more intense t h a n the 837-keV gamma from Ca. Figure 2 shows t h e two peaks from K and S to be well resolved when they are about equally intense. T h e 837-keV Ca peak should also be resolvable from t h e nearby S line. T h e principal effect a t roughly equal intensities is a degradation of the sensitivity due t o a decrease in t h e peak t o background ratio. T h e 841-keV S peak will be unresolvable in matrices with very high K / S or C a / S ratios. In these cases the 2380-keV y ray may be a better choice. T h e Compton continuum produced by the 2223.3-keV photon from H in the polyethylene container is the principal
source of background in the 841-keV region (Figure 1). If we define our detection limit as twice the statistical uncertainty of the background in t h e region of interest, this limit can be determined by counting an empty irradiation container. This detection limit is 100 pg for a 3.5-h irradiation and 300 pg for a 20-min irradiation using t h e 841-keV gamma. In practice the matrix can be expected to degrade the sensitivity to some degree. I t is surprising t h a t the determinations of S in NBS orchard leaves are as good as they appear t o be, considering how close t h e S content is to our calculated detection limit. Since t h e detection limit is established primarily by the gammas from the irradiation container, this limit could be lowered by irradiating t h e material in a container t h a t produced lower gamma backgrounds in the region of interest. In such a configuration, t h e actual limits of detection become more critically dependent on t h e composition of t h e matrix material. T h e 2380-keV peak suffers no interference from the lower energy H gamma, thus the detection limit using this photon is virtually identical to t h a t obtained using the more intense 841-keV line. However, the practically achievable sensitivity does not approach the detection limit because of interferences from the material being measured. This is illustrated by our inability to measure S in Orchard Leaves using the 2380-keV gamma.
ACKNOWLEDGMENT We thank John K u h n , ISGS, for providing the ISGS coal standards for comparison and t h e staff of t h e Omega West Reactor for their assistance with these special irradiations.
LITERATURE C[TED W. Wagner, C. J. Huil, and G. E. Markle, "Advanced Analytical Chemistry", Reinhold, New York, N.Y., 1956, pp 204-207. W. F. Hillebrand, G. E. F. Lundell, H. A. Bright, and J. I. Hoffman, "Applied Inorganic Analysis". John Wiley, New York, N.Y., 1953, pp 711-723. C. M. Johnson and H. Nishita. Anal. Chem., 24, 736 (1952). K . Watanabe, Anal. Chim. Acta, 80, 117 (1975). R. Dams, J. A. Robbins, K . A. Rahn, and J. W. Winchester, Anal. Chem., 42, 861 (1970). S. Shiffman and C. W. Frank, Anal. Chern.. 46, 1804 (1974). G. F. Kirkbright and M. Marshall, Anal. Chem., 44, 1288 (1972). R. Rowland and P. W. West, Anal. Chirn. Acta, 61, 152 (1972). J. E. Hicks, J. E. Fleenor, and H. R . Smith, Anal. Chlrn. Acta, 68, 480 ( 19 74). G. Milazzo and G. Cecchetti, Appl. Spectrosc., 23, 197 (1969). T. B. Johansson, R. E. Van Grieken, J. W Nelson, and J. W. Winchester, Anal. Chem., 47, 855 (1975). R. B. Deshpande, J . Chrornatogr., 2, 117 (1959). K . Schmeiser and 0. Jerchel, Angew. Chern., 65, 366 (1953). R . K . Stevens, J. D. Mulik. A . E. O'Keeffe, and K. J. Krost, Anal. Chern., 43, 827 (1971). E. S.Gladney, E. T. Jurney, and D. B. Curtis, Anal. Chem., 48. 2139 (1976). U.S. Atomic Energy Commission, Califorriium-252 Progress, No. 1, Oct. 1969. A. R. Pouraghabagher and A. E. Profio, Anal. Chern., 46, 1223 (1974). G. A. Bartholornew et al., Nucl. Data, Sect. A , 3 , 430 (1967). E. T. Jurney. H. T. Motz, and S. H. Vegors. Jr., Nucl. Phys. A , 94, 351 (1967). R. R . Ruch, H. J. Gluskoter and N. F. Shimp, Environ. Geol. Notes, 72 65, 92 (1974).
RECEIL-ED for review March 28, 1977. Accepted July 7 , 1977. This work was performed under t h e auspices of t h e U.S. Energy Research a n d Development Administration.
ANALYTICAL CHEMISTRY, VOL. 49, NO. l;!,
OCTOBER 1977
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