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Sodium Removal as an Aid to Neutron Activation Analysis H. R . Ralston Bio-Medical Division, Lawrence Radiation Laboratory, Unioersity of California, Livermore, California 94550 Elaine S. Sat0 Radiochemistry Dicision, Lawrence Radiation Laboratory, University of California, Livermore, California 94550 THEFULLY AUTOMATED approach t o neutron activation analysis has been limited to substances with very low sodium content if the sought-for nuclides have half-lives close to, o r less than the 15-hour half-life of Z4Na. The presence of 24Nain the irradiated material makes it difficult if not impossible to detect and quantitate the other gamma peaks. F o r organic materials o r environmental samples such as soil o r rock, the problem of sodium is overwhelming; the instrumental approach is limited t o elements whose activation products have half-lives long enough to allow their measurements after 2 4 N a is lost through decay. Since our activation analysis program is oriented to exploit the instrumental approach, complicated radiochemical separations were excluded from study. A recently developed simple method for selectively removing sodium from irradiated material ( I ) shows promise for extending the instrumental approach to isotopes of shorter half-life. The method is rapid and involves a minimum of radiochemical procedures. First, a series of simple experiments was done to test the adaptability of the method to biological and environmental analyses and to determine the optimum conditions for this application (see Appendix A). Then the method was tested on nonradioactive materials and in a trial analysis of rock samples, liver, and protein. N o effort was made to quantitatively analyze the rock o r liver samples; the object was merely to test the efficiency of sodium removal and to gain experience with the method. The protein sample was analyzed to determine its metal content. This work was done to establish necessary and sufficient conditions for the use of the method for our particular applications. N o attempt was made t o critically evaluate hydrated antimony pentoxide performance under a wide range of experimental conditions.
EXPERIMENTAL Column Preparation. Following the method of Girardi and Sabbioni ( I ) , a quantity of HAP obtained from commercial suppliers was sieved to about 150 to 250 p. A glass column, 6 mm inner diameter, was filled to a height of about 3 cm with HAP and preconditioned with 6 M hydrochloric acid. Irradiation. The samples as finely ground dried powders were sealed in polyethylene vials by welding the snap-type cap in place with a low-wattage soldering iron. The samples were then exposed in a flux of 1013n/cmz sec in the Livermore Pool Type Reactor (LPTR) S-1 irradiation facility. Sodium Separation. After irradiation, each rock sample was transferred from its polyethylene vial to a platinum crucible. Concentrated hydrofluoric and nitric acids were added and the sample was heated to near dryness. Concentrated (1) F. Girardi and E. Sabbioni, J . Radioanal. Chern., 1,169 (1968).
perchloric acid was then added with more hydrofluoric acid, and the sample was again heated to dryness. The residue was dissolved in a small amount of 6 M hydrochloric acid and centrifuged, and the supernate was decanted. This solution procedure was repeated if necessary until no residue remained o r until the radioactivity in the residue was negligible. If the solution was over 10 ml in volume, it was boiled to reduce the volume. Organic samples were dissolved in a solution of concentrated hydrochloric, nitric, and perchloric acids (1 :1 :2) in a glass beaker. This solution was heated to dryness and the residue was dissolved in 2 ml of 6 M hydrochloric acid. One half of each sample was set aside, and the remaining half was then passed through the HAP column with a wash of the beaker. The column was then washed with 1 to 2 ml of 6 M hydrochloric acid. Counting. The process of dissolving the rock sample was begun ninety minutes after the end of the irradiation, and the completely dissolved sample was put into the column three hours after irradiation. The column did not function well during the test on the rock sample, however, and the separation process took considerably longer than anticipated. The separated rock sample was not placed o n the detector until six hours after the end of the irradiation. The liver and protein samples were ready to count within two hours after the irradiation. All samples f o r counting consisted of 6 ml of solution placed in a 90-cc polyethylene jar with a 5-cmdiameter bottom to obtain a thin layer of liquid. The counter was a 12-cc Ge-Li gamma detector with a full width half maximum (FWHM) of 3.5 keV a t 1332 MeV. Counting data were collected on a 2048-channel analyzer equipped with a magnetic tape readout. The magnetic tape was computer-compatible; no further treatment was needed prior t o analysis by a computer program ( 2 ) . I n the fractions of the dissolved samples eluted from the H A P column, the gamma activities were high enough for adequate counting statistics in counting times of 10 minutes without excessive dead times. Both the untreated half of the dissolved sample and the column containing the sodium adsorbed from the separated fraction were too active t o count until a number of half-lives of 24Nahad elapsed. I n all of the samples, the sodium-free fraction was counted as soon as possible and the 24Na-active fraction and column were counted as soon as their activities permitted. For the protein sample, which was not only tested for its sodium reduction factor but also was analyzed for M n Co, Zn, and Cu, standards were irradiated simultaneously. RESULTS Rock Sample. The rock sample (100 mg) was irradiated for 10 hours, then dissolved; half of it was separated o n HAP, (2) H. R. Ralston and G. E. Wilcox, Nat. Bur. Sfand. Spec. Publ. 312,2,1238 (1969).
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Table I. Na Adsorption Capacity of HAP Columna HAP, Na carrier, Na retained on grams rng HAP, rng 0.9 10 9.9 0.9 15 13.6 0.9 20 15.5 0.9 25 14.9 0.9 30 13.9 0.9 35 13.9 0.9 40 14.3 Z2Na tracer with NaN03 carrier was added in 15 rnl of 8M HC1. Washed with 15 rnl of 8MHC1. Table 11. Na Adsorption Capacity of HAP Columna Na carrier, Na retained on HAP, g rng HAP, rng 1 45 14.5 2 36.0 3 45.0 1 30 15.1 2 30.0 a 22Na tracer with NaN03 carrier was added in 15 rnl of 6M HCI. Washed with 15 rnl6M HCI. Table 111. Na Adsorption Capacity of HAP Columna Na carrier, Na retained on HAP, g mg HAP, rng 1 45 21.5 2 45.0 a 22Natracer with NaN03 carrier was added in 10 ml lMHNOa. Washed with 10 ml 1MHN03. ~~
~
Table IV. Potassium Adsorption Capacity of HAP Columna K carrier, K retained on HAP, g mg HAP, rng 0.5 5 4.3 10 6.9 15 7.8 20 7.6 25 9.9 30 10.2 35 11.2 40 11.7 0.5 20 8.4 1 .o 16.1 1.5 20.0 0.5 40 11.7 1 .o 21.8 1.5 28.5 aK-42 tracer with KN03 carrier was added in 10 rnl of 1M HCI. Washed with 10 rnl of 1M HCl.
and both halves were counted for 10.0 minutes. The activity was quite high despite the removal of sodium. This high initial activity was found to be 56Mn. The ratio between the *4Na retained o n the column and that counted in the unseparated fraction was 1.06 which is within the probable error of the experiment. The ratio between the sodium counted in the separated sample and that retained on the column was 0.013. This does not represent as complete a separation as that reported in the literature (I). Liver Sample. The sample of dried human liver (148 mg) was irradiated for 8 hours, then dissolved; half of it was separated o n HAP. The principal activities remaining in the sepaand,42K. rated sample were 56Mn, ~ C U 130
The ratio between the 24Na retained on the column and that in the unseparated fraction was 1.12, which is within the probable errors of the experiment. The ratio between the sodium counted in the separated sample and that retained o n the column was 4.2 X which is somewhat better than the ratio obtained for the rock sample but not as low as was expected. Protein Sample. The sample was 2.67 mg of the enzyme phosphoenolpyruvate carboxylase, which had been extracted from E. coli strain B to a specific enzymatic activity of 90 units/mg (3, 4 ) . It was irradiated for 10 hours. Standards for the determination of Mn, Co, Zn, and Cu were included; the Mn and Co standards were mixed in one vial and the Z n and Cu standards in another. The sodium separation was completed and the first sample was counted 2.0 hours after irradiation. The analysis showed Mn, 3.4 parts per million (0.0091 pg in 2.67 mg); Zn, 288 parts per million (0.77 pg in 2.67 mg); Co, not detected; Cu, not detected. The estimated probable errors for this determination included errors in counting statistics, irradiation geometry, counting geometry, and recovery. The total estimated errors are: Mn, f 23z;Zn, i 2 5 z . The 24Naratio between the treated sample and the column was 3.8 X 10-5. DISCUSSION The sodium separation technique investigated here proved to be effective for the kind of analysis used in the activation analysis program of the Bio-Medical Division, Lawrence Radiation Laboratory. Although our measured decontamination factors were smaller than those reported elsewhere ( I ) , the ratios were sufficient to make an otherwise impossible analysis possible. The observed increase of the decontamination factor when passing from the rock samples to liver tissue t o proteins has been tentatively attributed to the decrease in the mineral content (interfering ions) of the sample. The discrepancy between our measured retention and those previously reported ( I ) is probably due to variations in the HAP material. Differences were noted as a function of material preparation and source. This technique and others of a similar nature promise t o greatly extend our capabilities for meaningful activation analyses. APPENDIX A Adsorption Parameters for Adaptation of the GirardiSabbioni Sodium Removal Procedure. The procedure of Girardi and Sabbioni ( I ) for the removal of sodium from solutions is based o n adsorption of sodium o n hydrated antimony pentoxide (HAP). The adaptability of the technique to analyses at Lawrence Radiation Laboratory was checked by a series of exploratory experiments, from which suitable and/or optimum conditions for the present application were ascertained. Sodium was found to be adsorbed o n HAP from 6 M or 8M HCI, 1M o r 8M H N 0 3 , 6 M HC1/3M "OB, and 8M HC10,. Data for 6M and 8M HC1 and 1M HNOI are presented in Tables I to 111. The adsorption of potassium from 1 M HC1 was also studied (Table IV). (3) T. E. Smith, Arch. Biochem. Biophys., 128,611 (1968). (4) T. E. Smith, Lawrence Radiation Laboratory, University of California, Liverrnore, unpublished work, 1969.
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Cesium and barium were adsorbed from 1M HC1, 1M HNO,, 2 M H C l j l M HNO,, 1M HC104, and 1M HzS04. From solutions of mixed-fission and activation products in 8M HNO,, tantalum was found to be adsorbed along with the sodium. The conditions employed in the present investigation were
selected on the basis of these studies, checked, and found suitable. RECEIVED for review May 21, 1970. Accepted October 5, 1970. This work was performed under the auspices of the United States Atomic Energy Commission.
Solvent Effects in Gel Permeation Chromatography J. G . Bergmann and L. J. Duffy Research and Development Department, American Oil Co., Whiting, Ind.
R. B . Stevenson Research and Development Department, Ammo Chemicals Corp., Whiting, Ind.
WHEREAS HYDRODYNAMIC VOLUME appears to be the effective parameter for the size separation of high-molecular-weight polymers in Gel Permeation Chromatography ( I ) , molar volume is better for low-molecular-weight compounds such as alkanes and aromatics (2). However, some of these compounds show unusual elution behavior in tetrahydrofuran (THF), a common solvent in GPC. For example, alkanes and aromatics with similar molar volumes show large differences in retention volumes, which have been attributed to a n association between the aromatic and the polystyrene divinylbenzene gel (3). Even within the aromatics class, different elution patterns have been observed, not only in T H F ( 4 ) , but also in methylene chloride ( 5 ) ; with increasing molar volume, retention volumes decrease for the catacondensed series of polynuclear aromatics, but there is an anomalous increase for the peri-condensed series. Thus, a G P C study of petroleum resids-complex mixtures of highmolecular-weight compounds including homologs of alkanes and aromatics-is hampered by these varied elution patterns. To simplify the study of elution behavior, we observed the elution of low-molecular-weight model compounds in various solvents and solvent combinations. These studies were supplemented with infrared, ultraviolet, and solubility measurements to determine whether elution order is affected by adsorption, association, and/or differences in the solubilities of the compounds. Conventional GPC equipment (Waters Associates, Models 100 and 200) was used for this work. The results should be useful with more complex systems such as resids. T o date, we have obtained no clear-cut evidence to explain the anomalous behavior of the peri-condensed molecules. However, we have found that the anomalies disappear when 1,2,4-trichlorobenzene (TCB) is used as the GPC solvent. RESULTS AND DISCUSSION
Figure 1 illustrates the complex elution patterns in T H F compared to TCB. Retention volume data for these compounds are presented in Tables I and 11. In THF, five dis(1) Z . Grubisic, P. Rempp, and H. Benoit, J . Polym. Sci., Part B, 5, 753 (1967). ( 2 ) J. Cazes and D. R. Gaskill, Separ. Sci., 2,421 (1967). (3) G. D. Edwards and Q. Y . Ng, J. Polym. Sci., Part C , 21, 105
(1968). (4) T. Edstrom and B. A. Petro, ibid.,p 171. (5) H. Oelert, presented at the API 60 Summer Meeting, Laramie, Wyo., 1969.
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Figure 1. Effect of solvent on elution behavior tinct elution patterns are observed for these six series of compounds (the sulfur-heterocyclics align with the catacondensed aromatics). The series of peri-condensed aromatics shows a n unusual reversal in elution behavior-the larger the molecule, the later it elutes, This reversal seems to be related to peri-condensation. Further support for this observation lies in the elution behavior of the nitrogen compounds. Elution is in the expected order through carbazole, but benzcarbazole-a peri-condensed molecule-elutes later than carbazole. The hydrogenated pyrenes show a maximum in relative retention and, although less clear, this would again appear to be a function of structure. By contrast, in TCB these differences between classes of compounds disappear. With that solvent, the retention
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