Influence on the Chemistry Quenching of Low Level Tritium in Natural Water by the Solid Polymer Electrolysis Enrichment Method
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摘要: 固体聚合膜电解浓集法是浓缩氚含量较低（<1 Bq/m3）的天然水样的常用方法，但因水样自身含有杂质离子或电解装置聚合膜带入杂质进入浓集液，使浓集液偏酸性，在测量过程中易产生化学淬灭效应，导致氚的测量值偏低。本文研究了水样自身存在的杂质离子和聚合膜上残留的杂质离子、样品溶液的pH值及其电导率所产生的化学淬灭效应的影响，实验表明，为减少化学淬灭效应，提高测量低含量氚的准确性，需保证水样溶液呈中性，电导率≤1 μS/cm，同时避免杂质沉积在聚合膜上。如果水样溶液的pH值偏酸性、电导率大于1 μS/cm，可采用酸碱混合型离子交换树脂去除水样中自身的杂质；对于聚合膜引入的杂质，可在电解后的水样中加入微量氨水将其pH值调节至中性。Abstract: The Solid Polymer Electrolysis (SPE) enrichment method is a practical way to measure low level tritium (<1 Bq/m3) in natural water samples. However, the concentrated liquid samples are acidified by impurity ions from water samples or the process of water sample pretreatment with a SPE electrolysis device, which results in perturbation when liquid scintillation is added progressively to the concentrated liquid. Consequently, the radioactive activity of tritium will be lower than the reference value because of the chemistry quenching effect occurring in the measurement process. The influence of chemistry quenching caused by impurity ions, pH value and electrical conductivity of the low level tritium samples in the tritium measurement results are discussed in this paper. The results show that, to avoid the chemistry quenching caused by impurity ions deposited in SPE electrolysis device and improve the accuracy of measurement counting, it is necessary to keep the pH value neutral and the electrical conductivity ≤1 μS/cm. Using the ion and acid-base hybrid exchange resin column separates impurity ions for high electrical conductivity (>1 μS/cm) or acidification pH samples. For water samples the impurity ions taken by the polymer film, can be used to adjust the pH value with tiny amounts of ammonia to avoid chemistry quenching and thus reduce the low counting of the tritium natural water samples.
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表 1 低氚样品蒸馏后的电导率和pH值
Table 1. The electrical conductivity and pH value of low-tritium samples treated with distillation
样品编号 电导率(μS/cm) 溶液pH值 加闪烁液后的颜色 样品1 1.3 7 清澈透明 样品2 2.5 7 清澈透明 样品3 16.5 4 乳白色 样品4 4.3 7 清澈透明 样品5 7.3 5 乳白色 样品6 2.6 7 清澈透明 
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表 2 不同电解装置电解后氚测定结果
Table 2. The analytical results of tritium treated with electrolytic device
样品 编号 装置 编号 计数率 (cpm) 电解后氚浓度 的测量值(TU) 氚浓度的 参考值(TU) 相对误差 (%) 加入闪烁液后 溶液颜色 T1 A 14.157 237.33 236.80 0.2 清亮 T2 B 15.804 235.82 236.80 -0.4 清亮 T3 C 9.516 149.69 236.80 -36.8 乳白 T4 D 11.426 183.30 236.80 -22.6 乳白
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表 3 电解装置浓缩后溶液中杂质离子的含量
Table 3. The concentrated of impurity ions in the sample solution enriched with electrolytic device
样品编号 杂质离子的浓度(mg/L) 水样的pH值 Cr2+ Sr2+ Ca2+ Mg2+ T1 - 0.0057 1.511 0.0757 6.67 T2 - 0.0077 2.263 0.0127 6.72 T3 0.2258 0.0106 3.067 0.2852 4.34 T4 0.2606 0.0109 3.291 0.2252 3.87 注：表中仅列出4个样品中含量差异明显的元素。“-”表示测定结果小于仪器检出限。
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