Os水吸收液ICP-MS测量信号强度衰减原因研究

Study on the Causes for ICP-MS Signal Intensity Declination of Os Water Absorption Liquid

蒸馏法能从样品中有效分离出OsO_4,吸收液直接用于ICP-MS测定,是辉钼矿Re-Os定年的关键技术环节。但实验发现,长时间放置的吸收液中Os信号强度显著降低。根据OsO_4化学性质推测原因在于一是挥发使吸收液Os含量减少,二是还原使+8价Os变成低价态,气态比例降低,若雾化效率不变,进入质谱仪的Os减少,两者都会导致信号降低,但具体影响情况不明,需深入研究。本文利用辉钼矿标准物质制备不同放置时间、酸度和温度的吸收液,对比上述条件对ICP-MS信号强度的影响。实验时将残余吸收液与稀释剂、氧化剂封入Carius管加热蒸馏,测定Os含量。结果表明:吸收液放置时间越长,酸度越低,温度越高,信号降幅越大,幅度达到3.2%~68.6%。室温下放置相同时间,低酸度吸收液的Os保存率高于高酸度吸收液,但Os信号强度低于后者,证明了挥发和还原共同导致信号衰减,且还原是主导原因。本文提出,冷冻(-18℃)可抑制OsO_4挥发,提高酸度(约3.5 mol/L)可减弱OsO_4还原,两者结合抑制信号衰减,提高了蒸馏法的灵活性和适用性。

BACKGROUND： Os can be separated from the sample matrix by distillation,and the absorbing liquid can be introduced into ICP-MS directly,which is a key step of the molybdenite Re-Os isotopic age analysis. However,the Os signal intensity declines significantly after long time storage. According to the properties of OsO4,the possible causes may include： 1） volatilization which reduces the Os content in the absorbing liquid,and 2） the reduction in which Os（ Ⅷ） transforms to the low valence state without volatility and OsO4 in the gas state decreases. Although they can both reduce the Os being introduced into ICP-MS ultimately,the extent of their effect is not clear.OBJECTIVES： To quantify the effects of absorbing liquid storage conditions such as duration,acidity and temperature on the Os signal intensity,to identify the cause of this decline and to find out the methods for its suppression.METHODS： The experiment consists of two parts： 1） using the molybdenite Re-Os dating reference materials to construct a different absorbing liquid storage condition series,including duration,acidity and temperature,and measure their Os signal intensities in terms of187 Os/193 Ir relative ratios. 2） sealing the residual absorbing liquid together with the spike and oxidizing agents within the Carius tube with sequent heating and distillation. The187 Os/190 Os ratios were determined to calculate the Os content and the retention rate.RESULTS： After storing,the Os signal intensities of absorbing liquids decline with time from 3. 2% to 68. 6%.The longer time,the lower acidity and the higher temperature for OsO4 absorbing liquid storage,the more obvious Os signal declines. After storing at room temperature for the same time,the Os retention rate of the low-acidity series is higher than that of the high-acidity series,but the former has a lower signal intensity than the latter. This indicates that both volatilization and reduction contribute to the decline of Os signal intensity and are the main factors.CONCLUSIONS： Freezing to-18℃ can inhibit the volatilization of OsO4,whereas increasing the acidity（about3. 5 mol/L） can reduce OsO4 reduction. The combination of the two inhibits signal attenuation,and increases the flexibility and applicability of the distillation method.