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地质论评  2010 

13X分子筛活化及其对氧气转移时吸附/解吸过程中氧同位素分馏效应的研究

Keywords: 13X分子筛,氧同位素分馏,吸附/解吸,分子筛活化,再吸附

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Abstract:

在激光BrF5?法分析硅酸盐和氧化物矿物的真空提取流程中,通常采用氧气作为工作气体来测定样品的氧同位素组成。这需要使用分子筛吸附并转移氧气到质谱进样系统,而分子筛的活化对于有效吸附和解吸氧气至关重要,否则会引起分析过程中的氧同位素分馏。通过采用13X分子筛进行多组实验分析,笔者发现用不同活化程度或中毒失活分子筛沸石转移时可能存在氧同位素分馏。在低温下分子筛是不能够活化的(或活化不完全),吸附能力很低,所测定的?δ18O?值是波动的。在200℃马弗炉中预加热24h,然后迅速转移到温度为100℃、真空度为10-3?Pa系统中加热12h的分子筛,对氧气吸附能力很强,氧同位素分析的绝对误差为?±0.05‰?(1σ),能够满足地球化学氧同位素分析的要求。如果分子筛受BrF5?污染,其吸附能力大为降低,测定的?δ18O?值呈下降趋势,氧同位素分馏高达0.7‰。如果分子筛受水汽污染,质谱接受电压不断地降低,存在解吸温度下的再吸附过程,测定?δ18O?值呈上升趋势。因此,分子筛在使用前要在适当条件下的活化,在发现污染或测量值发生波动变化时要及时更换,否则难以保证氧同位素分析数据的可靠性。

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