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

Activation of 13X Molecular Sieves and Its Effect on Oxygen Isotope Fractionation during Absorption/Desorption in Transferring Oxygen Gas

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10-3Pa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。

For the laser fluorination analysis of oxygen isotopes in silicate and oxide minerals,oxygen gas is commonly used as working gas and molecular sieves are used to absorb and transfer the oxygen gas into the inlet system of IRMS.In doing so,activation of molecular sieves has the influence on the efficiency of gas absorption/desorption and thus on oxygen isotope fractionation in vacuum extraction procedures.A series of experiments were carried out by means of 13X molecular sieves to test the influences.The results show that oxygen isotope fractionation could occur during adsorption and desorption of oxygen gas if the molecular sieves have different degrees of activation.At lower temperatures the molecular sieves are not able to be activated or only incomplete activation.As a result,the absorption capacity of molecular sieves is poor and measured δ^18O values are fluctuated.Nevertheless,if the molecular sieves were heated at 200℃ meffle furnace for 24hrs and then timely transferred to the vacuum system and heated at 100℃ and in 10^-3 Pa for 12hrs,their absorption is much enhanced relative to those untreated and slightly done at the lower temperatures.This leads to an external precision of ±0.05‰ （1σ） for oxygen isotope analysis.If molecular sieves were contaminated by BrF5,their absorption capacity is awfully reduced to cause measured δ^18O values to be lower than the expected ones with the largest fractionation is up to 0.7‰.If molecular sieves were polluted by water vapor,there is re-absorption at the desorption temperature to cause measured δ^18O values to increase with time.Hence,the molecular sieves have to be activated before the vacuum extraction and renewed timely when finding either pollution or the fluctuated δ^18O values.