5A分子筛吸附混合溶剂洗脱-气相色谱-同位素质谱分析土壤中正构烷烃单体碳同位素
Specific Carbon Isotopic Analysis of n-Alkanes in Soils by Gas Chromatography-Isotope Ratio Mass Spectrometry with 5A Molecular Sieve Adsorption and Mixed Solvent Elution
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摘要: 利用5A分子筛吸附,环己烷-正戊烷混合溶剂洗脱分离富集正构烷烃,用气相色谱法测定正构烷烃含量,气相色谱-气体同位素质谱(GC-C-IRMS)测定土壤样品中正构烷烃单体碳同位素。实验优化了5A分子筛用量和洗脱剂的比例,需要络合的正构烷烃的量与分子筛加入量呈线性关系,络合x mg的正构烷烃,需加入2.75x g分子筛,络合环己烷-正戊烷最佳比例为9∶91。探讨了络合过程中5A分子筛对不同链长正构烷烃的络合规律,短链正构烷烃被5A分子筛优先吸附,长链正构烷烃的络合相对滞后。正构烷烃的络合洗脱回收率为44%~72%,精密度(RSD,n＝6)为4%~8%;正构烷烃单体碳同位素分析精度为0.04‰~0.38‰(1σ)。采用5A分子筛净化混合溶剂洗脱方法,分析加油站附近的实际土壤样品,未分峰基本消除,获得良好的净化效果,满足正构烷烃单体碳同位素分析的要求。Abstract: n-alkanes were concentrated after separating by a 5A molecular sieve and extracted by a mixture solvent of cyclohexane and n-pentane. In this study, the amount of 5A molecular sieve and the ratios of cyclohexane and n-pentane were optimized as 2.76 g and 9∶91, respectively. It was discovered that short carbon chain n-alkanes were readily absorbed by a 5A molecular sieve, compared to the long carbon chain n-alkanes. The complexation efficiency is higher for long carbon chain n-alkanes than that of short carbon chain n-alkanes with enough 5A molecular material at higher temperature. In this study, n-alkanes contents were determined by Gas Chromatography (GC) and compound specific carbon isotope ratios of individual n-alkanes in soil were measured by Gas Chromatography-Isotope Ratio Mass Spectrometry (GC-IRMS).The recoveries of n-alkanes ranged from 44% to 72‰ with a precision of 4%-8%. The accuracies of compound specific carbon isotopes of n-alkanes ranged from 0.04‰ to 0.38‰ (1σ). The 5A molecular sieve procedure was used to analyse soil samples around a gas station. The results indicate that almost all of the unseparated and interference peaks were eliminated and the purification of n-alkanes met the requirements of stable carbon isotope analysis.
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