Anal. Chem. 1984, 56,761-763
eliminator beams. By virtue of its larger base, this vial is more stable than standard minivials. And finally, its large diameter cap keeps user’s fingers remote from the vial and the vial/cap junction, decreasing personal exposure from the vial contents, and minimizing personal contamination from leaks around the cap.
ACKNOWLEDGMENT The author is indebted to Robert Koch and Glenn Nagel for constructive advice in preparing this publication. Graphs present in this paper were prepared with the help of Richard Deming. Norman Nitzberg provided invaluable technical assistance in construction of the modified vials.
781
Registry No. Tritium, 10028-17-8;polyethylene, 9002-88-4. LITERATURE CITED (1) Horrocks, Donald L. “Applications of Llquid Scintiilatlon Counting”; Academic Press: New York, 1974; Chapter 10. (2) Muse, L. A.; Rao, V. Health Phys. 1976, 37, 457-459. (3) Joseph, S.; Kramer, G. H. AECL-7611; Chalk River Nuclear Laboratorles: Chalk River, Ontarlo, 1982. (4) Ring, J. G.; Relch, A. R.; Marcec, J. J.; Moore, D. H. “The Performance of Liquid Scintillation Counters Using Large and Small Vials”; Packard Instrument Co.: Downers Grove, IL, 1978; p PR-18. (5) Horrocks, D. L. “Liquid Scintillation Counting-Volume Corrected Results”; Beckman Instrument Co.: Irvine, CA, 1983.
RECEIVED for review August 24, 1983. Accepted December 19, 1983.
Determination of Radium in Water by Liquid Scintillation Counting after Preconcentration with Ion-Exchange Resin Hideo Higuchi,* Masaki Uesugi, Kaneaki Satoh, and Naoyuki Ohashi J a p a n Chemical Analysis Center, 295-3 Sanno-cho, Chiba, J a p a n
Masayasu Noguchi Radioisotope a n d Nuclear Engineering School, J a p a n Atomic Energy Research Institute, 2-28-49 Honkomagome, Bunkyo-ku, Tokyo
A method for the determination of radium in envlronmental water by means of a liquid scintillatlon counting technique has been developed. The radium was collected on cation exchange resin with 100% recovery by the batch method. The other radionuclides except uncompiexed elements such as ceslum can be removed by washing the resin with 0.01 M EDTA solution at pH 8. The counting sample was prepared by immersing the resin into emuislon llquld sclntiiiator in a Teflon vial. The chemical procedure took about 2 h and the detectlon limit was 0.03 pCI of 226Ra/L.
has been applied praktically to various surveys (6) of radon in environmental water, and also determination (7,8) of radium combining with the usual chemical procedure based on coprecipitation with barium sulfate. Recently, Prichard et al. (9) have proposed a new analytical technique for n6Ra in water by means of cation exchange resin and LSC without any special procedure for purification of radium. The present paper describes not only an analytical technique of rapid cation exchange separation of radium from other radioactive nuclides but also some information on the migration of radon from an ion exchange resin to a liquid scintillator.
Determination of radium in environmental water has become a matter of interest in public health because radium is one of the most hazardous elements with respect to internal exposure. Therefore it is desirable to develop a method of high sensitivity to analyze a large number of samples rapidly. Analytical methods of conventional use are mostly based on measurements of a rays from radium coprecipitated with barium sulfate (I,2) or of radon gas emanated from radium (3, 4 ) . The former, usually carried out by using a ZnS(Ag) scintillation counter or a proportional counter, requires the time-consuming operation of chemical separation for preparing a thin sample for a ray counting. The latter, usually carried out with an ionization chamber or a radon counter (e.g., Lucas counter), requires complicated operations for handling radon gas. One of the authors (5) has already reported a measurement method for radon in water using a liquid scintillation counting (LSC) technique. Features of the LSC method are easy and efficient extraction of radon from a large quantity of water and simplified measurement with high counting efficiency by a commercial liquid scintillation counter. The LSC method
EXPERIMENTAL SECTION Reagents and Apparatus. Analytical grade reagents and deionized water were used throughout this work. The radioactive solution used for ion exchange procedures contains S4Mn,6oCo, 85Sr,13’Cs, 144Ce,and 226Ra.The solution of 0.01 M EDTA was prepared by dissolving 3.72 g of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) in 990 mL of water, and 1M NaOH solution was added to adjust the pH to 8.0. The cation exchange resin used in this study was Bio-Rad AG50W-X8 (Na’ form, 50-100 mesh) which was preconditioned with sodium chloride solution. The liquid scintillator was Aquasol-I1 emulsion scintillator prepared by New England Nuclear Co. A low background type liquid scintillation counter (Aloka LB-1, Japan) which incorporates anticoincidence guard counters and a 100-mL counting vial was used to measure 222Rnand four daughters (hereafter, called a usual measurement). In addition, a coincidence module specially designed for a+ coincidence was also used. counting of 214Biand 214P0 Procedures. Based upon preliminary experiments, the following analytical procedures were applied to samples of water with pH 1to 8 and salinity less than 0.5%. Two liters of sample was adjusted to pH 1 or above with sodium hydroxide or hy-
0003-2700/84/0356-076 1$01.50/0
0 1984 American Chemical Society
762
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
- 1000r
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.-C LT
80-
- LO = 20 C
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C
;
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0 ._ 0
1
0
CI
2001f
u
0 0 I 2.0
3.0 4.0 001 M
5.0 6.0 7.0 8.0 9.0 EDTA solution IpH 1
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Flgure 1. Relationship between the retention of metal ions on resin and pH of eluate: column, 1.2 cm i.d. X 5.0 cm, 5 g of wet AQ50WX8, 50-100 mesh; flow rate, 1 mL/min.
drochloric acid. Five grams of cation exchange resin was added and continuously stirred for 1 h at 200 rpm. After the resin settled out, the supernatant was discarded by careful decantation. The resin was loaded into a polypropylene column (5 cm x 1.2 cm i.d.) with a quartz wool plug at the bottom and washed with 50 mL of 0.01 M EDTA solution with a flow rate of 2 mL/min followed by 20 mL of water. The wet resin was transferred into a 100-mL Teflon counting vial together with 95 mL of scintillator. After the mixture was allowed to stand for about 14 days for growth of radon, the activities of radon and ita daughters were measured by a liquid scintillation counter. The time required for the chemical procedure, including collection and purification of radium was about 2 h. The standard samples for calibration were prepared by applying the same procedures to water containing known amounts of %h. R E S U L T S AND DISCUSSION Recovery a n d Purification of Radium. The chemical procedures essentially consist of two steps. The first is to concentrate the radium in the sample of water on the resin and the second is to purify the radium against other elements such as calcium, strontium, and cobalt by means of complexing with EDTA. The first step was carried out by the batch method previously described. However, as adsorption of 226Ra on resin usually depends on the concentration of the salt coexisting with radium in the sample solution, it was found that the batch method was not adequate in the case where sodium and calcium contents exceed 2 g and 0.02 g/L, respectively. In such cases for larger quantities of water, the column method should be applied to collect the radium. The volume of resin to be used should be kept to less than 25% of volume of the counting vial to minimize the degradation of counting efficiency by the loss of scintillation light and radiation energy in the resin. Samples of environmental water such as hot spring water sometimes have a wide range of pH (e.g., 2-10). The pH of the solution influences the adsorbtion of radium on the cation exchange resin. According to 226Ratracer experiments, the range of pH a t which radium is firmly adsorbed was found to be 1 or above, since the retention of radium on resin tends to decrease rapidly as the acidity of the solution increases. In this pH range, not only radium but also other cationic elements would be adsorbed on the resin. Radioactive nuclides such as wSr interfere with the quantitative measurement of radium. These interfering elements can be removed by selective complexation of metal ions by EDTA. Good selectivity can be achieved by control of the pH of the eluate. Figure 1 shows the retention of metal ions on the resin ae a function of the pH of eluate. Elements shown in Figure 1 are important radioactive nuclides which were introduced into the environment by nuclear explosions and/or nuclear facilities. These nuclides, except 137Cs,can be eluted from the column at pH 8. Natural radioactive elements such as uranium, thorium,
20 LO 60 80 100 w a t e r c o n t e n t s (%I
Flgure 2. The variation of count rates as a function of water content in the resin: water content of 0% means the resin being dried for 100
min at 80 OC, 100% means the resin saturated with water; counting vial, 20 mL (O),100 mL (0). I
'1 60
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=C 5 0 -
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Channel number Figure 3. Scintillation spectra for dry (upper)and wet (lower)resin containing 228Ra.The countlng vial is made of Teflon, having a volume of 100 mL. The scintillator used Is Aquasol-11.
and their daughters usually have higher constants of complex by EDTA than those of elements examined above, thus these elements would pass through the column (IO). The 13'Cs retained on the resin gives a small interference with radon activity measurement as described later. Also, the a-@ coincidence counting technique serves to determine radon activity without the interference. Activity Measurements. One of features of the present method is to measure radon and its daughters which were released from radium on the resin into the liquid Scintillator. It was found that counting rate strongly depends on water contents in the resin. Figure 2 shows the relationship between counting rate and water content in the resin, which indicates that radon release from the resin into the liquid scintillator is very low for dry resin but steeply increases with water content in the resin around 25% w/w and that it does not depend on water content above 30%. Figure 3 shows scintillation spectra for dry and wet resins obtained at the same time of radon growth. In the spectrum of the wet resin (lower spectrum), two peaks correspond to a rays of 222Rn(5.5 MeV), 21sPo(6.0 MeV), and 214P0(7.7 MeV), while a tailing continuum would be due to rays from 214Pband 214Biin the scintillator and also to a rays from 22sRaretained in the resin which causes loss of scintillation light and a ray energy. On the other hand, in the spectrum for the dry resin no peak was found and this fact suggests that the radon cannot migrate from resin into scintillator in the absence of water. Therefore,
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
763
_Table I. Analytical Results of 226Rafor Spiked Samples and a Hot Spring Water Sample present methoda coprecipitation with BaSO, sample usual CY-P coincidence LSC b 2nP.CC ZnS 9.55 pCi 226Raadded 19.1 pCi 30.6 pCi 40.1 pCi 53.5 pCi
Misasa hot spring
9.23 * 0.10 18.5 * 0.13 30.5 * 0.15 41.3 i: 0.17 55.3 i: 0.20 2.32 * 0.18
9.03 i: 0.35 19.4 ? 0.51 30.0 i: 0.63 39.7 * 0.73 53.1 * 0.84 2.30 * 0.28
1.8 + 0.12a
2.5
i
0.14a
a The error is one standard deviation by the assumption of Poisson’s distribution for counting statistics. ment of solution which is obtained by decomposition of barium sulfate with strong phosphoric acid (8). of barium sulfate precipitate by a 27r gas flow proportional counter or a ZnS scintillation counter.
1.9 CY
i:
0.26a
LSC measureray counting
a+ coincidence method as follows. Another counting technique (11)was employed. zzzRncan
Added Ro-226(pCtJ
Figure 4. Calibration curves of LSC for the usual (0)and the a-P coincidence (0)measurement.
the condition of resin for measurements must be kept wet. The selection of scintillator is also important to obtain better reproducibility of measurement by LSC. A toluene base scintillator which is usually available for radon counting cannot be employed for the water-saturated resin. The wet resin in a toluene scintillator coagulates tightly and sometimes adheres to the inner wall of the counting vial due to the electrostatic charge of the resin itself. This does not give the constant counting efficiency and reproducibility. Prichard et al. (9) treated a water-saturated resin with methanol and then with toluene and, thus, eliminated the above difficulty by using a toluene base scintillator. We followed this procedure and found that the counting efficiency is lower and depends on how thoroughly the resin is rinsed with toluene. It was also observed that a methanol-saturated resin did not coagulate in a toluene base scintillator and gave a counting efficiency as good as the water wet resin in the emulsion scintillator. This fact cannot be explained by the difference of the solubility of radon in these liquids and implies the presence of other mechanism in the radon migration from the resin into the scintillator. A study on this subject is in progress. The usual measurement for routine analysis of 226Rawas performed with a commercial liquid scintillation counter which was designed for low background counting and large volume (100 mL) samples. The typical background and the counting efficiencies are about 15 cpm and 4.5 cps/Bq 222Rn,respectively, for the normal sample and the appropriate pulse height (about 50 keV -1 MeV energy equivalent) of the single channel analyzer. The minimum detectable level is estimated to be 0.3 pCi of 226Ra/Lfor 100 min of counting time, and the further increase of sensitivity may be achieved by processing a larger amount of water with the column method and/or using the
be measured selectively by CY+ coincidence counting of RaC(214Bi)and its daughter RaC’(214Po)having the half-life as short as 162 1.18. In the a-P coincidence circuit, signals of RaC, which were delayed for 0.5 1.1~8, generate long gate pulses of about 500 ps. Thus, the two succesive signals from RaC and RaC’ are counted but single event signals of background radiation and of other radionuclides such as 137Csor zzaRa cannot pass through. The background counting rate in the a+ coincidence measurement is less than 0.01 cpm and the typical counting efficiency is 0.85 cps/Bq 222Rn. In the usual measurement, the contribution of 137Csand zzsRato counting efficiency was found only 2 and 4 % , respectively. The reason of such low counting efficiencies of 137Csand zzsRawould be that these nuclides are retained in the resin and do not migrate into the scintillator. Analytical Results f o r N a t u r a l Water. The accuracy and the precision of this method were examined by applying to natural water samples of 2 L containing the known amount of 226Raand also to the hot spring water sampled near an uranium mining site. The results are shown in Figure 4 and Table I together with those obtained by the conventional method of using CY ray counting of barium sulfate coprecipitate. As shown in Figure 4, the reproducibility and the linearity are good for both the usual and the a-0 coincidence measurement of LSC. The results for spring water are somewhat scattered owing to the counting errors. ACKNOWLEDGMENT The authors cordially thank N. Ikeda, Faculty of Science, Tsukuba University, and T. Hamada, The Japan Radioisotope Association, for their continuous encouragement and advice. Registry No. Ra, 7440-14-4; water, 7732-18-5. LITERATURE C I T E D (1) Goldin, A. S. Anal. Chem. 1981, 33, 406-409. (2) Harley, J. H. EML Procedures Manual (HASL-300). (3) W.H.O. “Method of Radiochemical Analysis”;Tokyo Universlty Internatlonal Edition 23, 1967; pp 117-127. (4) Kametanl, K. Radioisotopes 1975, 24, 193-196. (5) Noguchl, M. Radioisotopes 1964, 13, 362-367. (6) Murakami, Y.; Horluchi, K. J . Radioanal. Chem. 1979, 52, 275-283. (7) Homma, Y.; Murakami, Y. J . Radioanal. Chem. 1975, 2 4 , 173-184. (8) Uesugi, M.; Satoh, K.; Noguchi, M. Proceedings of the 21th Symposium on Radiochemlstry (In Japanese),at Tatsunokuchiv 1977; p 152. (9) Prlchard, H. M.; Gesell, T. F.; Meyer, C. R. Int. J . Appl. Radlat. Isot. 1980, 31, 24. (lo) Fritz, J. S.; Umbreit, G. R. Anal. Chim. Acta 1958, 19, 509-516. (1 1 ) Noguchl, M.; Wakita, H. Ohyobofsuri 1975, 4 4 , 979-983.
RECEIVED for review September 7,1983. Accepted December 12, 1983.
