Determination of Trace Elements in Glass by Activation Analysis Using Hydrated Antimony Pentoxide for Sodium Removal
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Thomas E. Gills, William F. Marlow,1 and Barbara A. Thompson Analytical Chemistry Division, National Bureau of Standards, Washington, D. C. 20234 concentrations in glass may Determination of trace-element be required in such fields as forensic science (1-3) and archeological chemistry (4). Neutron activation analysis, with its high sensitivity for many elements, should be a suitable method for such analyses and, in fact, has been utilized by Coleman and Wood (1), who describe the determination of about 16 elements at the ppm level in sheet glass. Because of the high sodium content of many glasses, neutron irradiation of even small samples at levels sufficient to detect ppm or ppb concentrations of trace elements can result in the production of 10 mCi or more of 24Na. Thus, in order to measure the traceelement activities, either extensive chemistry must be performed in very high radiation fields, or the sample must be set aside for 1 to 2 weeks until the 24Na activity has decayed away. With this latter procedure, which is the one followed by Coleman and Wood, only long-lived radionuclides can be measured. For some cases, it may be possible to obtain sufficient information in this manner; however, a recent study at NBS required that data be obtained on as many trace elements as possible in a glass matrix. The Office of Standard Reference Materials of the National Bureau of Standards is currently certifying a series of standard glasses containing various trace elements. The trace elements are nominally present at levels of 0.02, 1, 50, and 500 ppm. The base glass, to which the trace elements were added, was prepared from very pure materials. Before beginning detailed analysis of each glass, a survey of the base glass was desired, to learn the expected levels of possible trace elements present before the additions. The base glass is composed of 72% Si02, 12% CaO, 14% Na20, and 2% A1203. Thus, the principal radionuclide produced by thermal-neutron activation will be 24Na. Aluminum-28 can be allowed to decay before counting, and the small amounts of activity from the various Ca radionuclides will not be significant. Small amounts of 31Si remaining after decay are removed during dissolution. Waiting for the 24Na to decay would mean forfeiting information about short-lived radionuclides such as 64Cu, 72Ga, and siMn. The alternative of extensive chemistry at high radiation levels is undesirable, and so a different approach was sought. Girardi, Sabbioni, and Pietra (5-7) have recently reported Guest Worker, NBS, permanent affiliation, Division of Biology and Medicine, USAEC, Germantown, Md. 20545. 1
(1) R. F. Coleman and G. A. Wood, AWRE Report No. 03/68, United Kingdom Atomic Energy Authority, April 1968. (2) R. F. Coleman and N. T. Weston, Forensic Sci. Soc. J., 8 (1). 32(1968). (3) L. T. Atalla and F. W. Lima, Radiochem. Radioanal. Lett., 3 (1), 13 (1970). (4) J. S. Olin, E. C. Miller, and B. A. Thompson, Trans. Amer. Nucí. Soc., 12,492(1969). (5) F. Girardi and E. Sabbioni, J. Radioanal. Chem., 1, 169 (1968). (6) F. Girardi, “Modern Trends in Activation Analysis,” U.S. Dept, of Commerce, National Bureau of Standards. Special Publication 312, Vol. 1, pp 577-616 (1969). (7) F. Girardi, R. Pietra, and E. Sabbioni, ibid., pp 639-641; also Euratom Report, EUR-4287e, Luxembourg, July 1969.
the use of hydrated antimony pentoxide (HAP) for removal of sodium from mixed radiotracer solutions. The high selectivity of the reagent for sodium makes it suitable for use with many types of matrices, and applications to the analysis of biological materials (6, 8), rocks (9), and tobacco (10) have been reported. This paper describes the use of neutron activation analysis in conjunction with HAP for the determination of a number of elements at the ppm and ppb levels
in glass. EXPERIMENTAL
The first experiments were carried out on the base glass to determine the concentrations of the trace elements present initially. The glass was supplied in the form of rods about 1.5 cm in diameter. Samples 1 mm thick were cut from various positions along the rods, using special precautions to avoid contamination during cutting. Each sample disk weighed 0.3-0.8 gram, depending on the rod diameter, and an entire disk was used for analysis in order to eliminate effects of any radial inhomogeneity. Each disk was sealed in medical-grade polyethylene tubing, and a Cu flux monitor was taped to the outside of the polyethylene. The samples were irradiated for 1 hour in pneumatic tube RT-3 of the NBS Reactor, at a power level of 2 MW (~1.2 X 1013 · 2 34567sec-1) (Present operating power level, 10 MW). A decay period of 12-15 hours followed, to
permit reduction of short-lived activities. After this time, a typical sample had a radiation level of 2-5 r/hr at 1 cm due to 24Na.
Behind suitable shielding, the disks were rinsed in alcohol and in 1:1 HNOs to remove surface contamination. They were then placed in Teflon (Du Pont) beakers containing 2-3 ml hot concentrated HC104. A small amount of HF was added, and the mixture was heated until gas evolution ceased and any CaF2 was decomposed. Another small amount of HF was added, and the procedure continued until the glass was all dissolved. This step-wise process was necessary because of the formation of a surface layer of CaF2 which prevented attack of the underlying glass by HF. After the glass was dissolved (45-90 min.), the mixture was taken to fumes of HCIO4 and fumed to incipient dryness to remove SiF4 and residual fluoride. The residue was set aside to cool. During this cooling time, HAP columns were prepared for sodium removal. The HAP, as obtained from the firm of Carlo Erba, Milan, Italy, contained a high proportion of very fine particles, and could not be used directly for column preparation because of plugging. The fines were removed by slurrying 6-10 gram of HAP three times with 12AHC1 and rapidly pouring off the supernate. It is estimated that 73-72 of the HAP was discarded in this procedure. The remaining larger particles were transferred with additional small portions of 12N HC1 to a column assembly consisting of a 2.5-cm diameter glass tube with a coarse fritted glass disk. A glass-wool plug (8) S. Meloni, A. Brandone, and V. Maxia, ibid., pp 642-645. (9) S. F. Peterson, A. Travesi, and G. H. Morrison, ibid., pp 624633.
(10) R. A. Nadkarni and W. D. Ehmann, Radiochem. Radioanal. Lett.,!, 161(1969).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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ENERGYFigure 1.
Gamma-ray spectrum of HAP effluent of irradiated glass sample;
Table I. Mn Au Cu Ga La Sb
Results of Base Glass Analysis, ppma 0.479 ± 0.014 (±2.7%) 0.217 ± 0,013 (±6.0%) 1.035 ± 0.030 (±2.9%) 0.330 ± 0.010 (±3.0%) 0.0291 ± 0.0015 0.0643 ± 0.0041
Limits quoted are ts/ Vñ for the 95 % C.L.:
(±5.1%) (±6.4%)
=
18.
and a glass-fiber filter paper were used to separate the fritted disk from the bulk of the HAP, and a small piece of glass wool was placed on top of the HAP. Two-piece fritted-disk filterchimney assemblies proved very convenient for this purpose because of the ease of removing an old column and cleaning the system for re-use. The column was washed with a small volume of 12A HC1 until the effluent was clear. During this wash, an excessively slow flow rate was occasionally observed, indicating plugging of the column by fine particles, and in these cases the column was discarded and a new one prepared. The columns tended to deteriorate on standing and become plugged, possibly due to attack of the HAP by the strong acid, so that they were not prepared more than 30 minutes ahead of time. The residue from the dissolution step was taken up in 15-20 ml of 12N HC1 and poured through the HAP column. With the heaviest samples, a precipitate of NaCl formed and this was also transferred to the column. The beaker and the column were washed with two 10-ml portions of 12N HC1. The combined effluents were collected in a polyethylene bottle, made up to a standard volume, and counted directly with a 47-cc Ge(Li) detector. The effectiveness of the HAP separation was demonstrated by the fact that a column typically read 2-4 r/hr from 24Na, but no 24Na peaks were observed in the gamma-ray spectrum of the effluent. In the few cases where some 24Na came through into the effluent, it could be removed simply by pouring through a second HAP column. 1832
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100 min counting time;
48 hr after irradiation.
A typical pulse-height spectrum of a sample of the base glass 48 hr after irradiation is shown in Figure 1. The principal radionuclides present are 19SAu, 47Ca, 47Sc, and 72Ga, with smaller contributions from 64Cu, 42K, 140La, and 122Sb. The 47Ca and 47Sc are from the calcium in the glass matrix. Manganese was determined in a separate group of samples,
which were irradiated for 1-2 minutes and dissolved immediately after irradiation. After going through a HAP separation as described above, the 56Mn could have been determined using the same 47-cc Ge(Li) detector, but instead a 3-in. X 3-in. Nal(Tl) crystal was used to obtain a more favorable count rate. Because the base glass contained ~0.3 ppm of Ga, it was necessary to eliminate interference from the 0.835 MeV peak of 72Ga with the 0.847 MeV peak of S6Mn before making the Nal(Tl) measurements. This was done by diluting the HAP effluent to 6-7N HC1 and extracting the gallium chloride complex with isopropyl ether. RESULTS AND DISCUSSION
In all determined by comparison with standards of the pure elements or oxides which were irradiated under the same conditions as the samples, dissolved and made up to the same volume as the samples for counting. The statistical limits shown represent the combined effects of counting statistics and experimental errors due to dissolution, chemical manipulations, positioning, etc. For all elements except Au, the counting statistics probably make the largest contribution to these limits. However, as shown in Figure 1, the principal radionuclide present in the HAP effluents was 198Au. For a typical sample, the net area of the 0.412 MeV 198Au peak was about 106 counts, so that the standard deviation from counting statistics was well below ±1%. Since tracer experiments confirmed Girardi’s report that gold is not retained by the HAP, the scatter in the results may be a reflection of inhomogeneity of gold in the glass. Some of the results obtained are given in Table I.
cases, concentration levels were
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
Tracer experiments for the other elements listed in Table I not carried out, but good reproducibility was obtained regardless of whether the samples were passed through one or two HAP columns. This indicates that these elements are not retained by HAP, supporting Girardi’s results. Very erratic values for potassium were obtained, and it was apparently partially retained on the column at times. This could have been due to co-crystallization with the solid NaCl observed with the heaviest samples. A similar problem was observed for strontium. By examination of the effluent samples after decay of all short-lived radionuclides, small amounts of Co and Ir could be detected, corresponding to about 20 ppb and 1 ppb, respecwere
tively. After decay of 24Na,
some of the HAP columns were counted. 18 2Ta was observed, consistent with Girardi’s report that tantalum is retained by HAP. The Ta level was estimated to be about 0.1 ppm. Scandium-46 was observed in both the effluent and the HAP, n contrast to Girardi’s data, but this is probably due to the
large differences (i.e., high salt concentrations and HC104 residue) between his system and that resulting from the dis-
solution of the glass samples. In conclusion, the use of HAP provides a very powerful tool for the activation analysis of materials with high sodium content. Of the results reported in Table I, only those for gold could have been obtained without sodium removal. The procedure is quite simple and, with a little attention to technique, gives good reproducible results. Received for review July 1, 1970. Accepted September 17, 1970. Presented in part at the 158th National Meeting, ACS, New York, N. Y., September 1969. Certain commercial materials are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material identified is necessarily the best available for the purpose.
Determination of Tin in Copper-Base Alloys by Móssbauer Spectrometry P. A. Pella and J. R. DeVoe Institute for Materials Research, National Bureau of Standards, Washington, D. In a previous report (J), the critical Móssbauer spectral parameters which are pertinent for quantitative analysis were evaluated. Such factors as the source-sample-detector geometry, use of absorption filters, and the background radiation should be considered for the quantitative analytical application of this spectrometry. For absolute quantitative analysis where the sampling is nondestructive, accurate measurements of the Debye-Waller factors of the source and absorber are required. This may be accomplished through careful measurements of the absorption intensity over a wide range of temperatures. However, considering the presently available experimental methodology, these measurements are difficult to make. As an alternative to nondestructive sampling, analysis by this technique can be performed in a manner similar to analytical spectrophotometry. That is, the sample can be dissolved and the analyte incorporated into a reproducible matrix. A calibration curve can then be constructed using a standard in an identical matrix. Utilizing this approach, a simple and selective procedure is described for the determination of tin using NBS copper-base alloys as an example. In order to obtain a convenient concentration range, alloys with a tin content from 1 to 8% were chosen. The method consists of the precipitation of tin as metastannic acid with nitric acid. The precipitate which contains a number of coprecipitated elements is ignited to stannic oxide and a portion mixed with aluminum oxide. The SnCVMOs mixture is then measured for its Sn02 content by comparing the absorption intensity with an appropriate standard. An important feature of this method is (1) P. A. Pella, J. R. DeVoe, D. K. Snediker, and L. May, Anal. Chem., 41, 46 (1969).
C. 20234
that although the Sn02 precipitate contains coprecipitated elements (about 6% by weight), it is possible to measure the Sn02 concentration without interference. The use of 0-Sn as an internal standard in these measurements is also demonstrated for the first time. EXPERIMENTAL used in these experiments was 10 Sn as BaSn03. A 0.05-mm Pd foil was placed over the source to filter the 25-keV Sn X-rays. A cryostat for cooling the samples to 88°K has been previously described (2). The aluminum sample holder for mounting the powdered samples was essentially of the same design reported in a previous paper (3), but with the following modifications. The shim ring which determines the sample thickness was machined from a previous thickness of 3 mm to 0.5 mm. The area of the cell was 1.76 cm,2 34and the Be windows were replaced with two 0.5-mm lucite disks. The spectrometer and associated instrumentation have all been described (4). Treatment of Alloys. Known amounts of NBS alloys SRM 52c, 184, and 37e which contain Cu, Sn, Zn, Pb, Sb, Ni, P, Si, Mn, Al, and Fe were treated with 1:4 nitric acid. The precipitates (about 300 mg each) were digested on a hot plate overnight, filtered, washed with hot 0.1 % HNOs, and dried at 100 °C. The precipitates with filter papers were transferred to Pt crucibles. The filter papers were first charred at low temperature, and then the Pt crucibles were heated in an electric furnace at 1200 °C for 5 minutes. The
Apparatus.
mCi of
The
source
n6m
(2) J. R. DeVoe, ed., Nat. Bur. Stand. (U.S.), Note, 421, 8-15 (1967). (3) L. May and D. K. Snediker, Nucl. Instrum. Methods, 55, 183 (1967). (4) F. C. Ruegg, J. J. Spijkerman, and J. R. DeVoe, Rev. Sci. Instrum., 36, 356 (1965).
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