Study on Specific Carbon Isotope Fractionation of n-Alkanes during Column Chromatography Separation-Molecular Sieve Complexation Adsorption
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摘要:
应用气相色谱-气体同位素质谱（GC-C-IRMS）分析正构烷烃单体碳同位素之前，需要对饱和烃样品中正构烷烃和异构烷烃进行预分离、富集，在预分离和富集过程中正构烷烃单体碳同位素是否发生分馏是高精度分析正构烷烃单体碳同位素比值（δ13C）的关键。本文以正构烷烃混合溶液为对象，利用柱色谱、5Å分子筛络合、环己烷-正戊烷混合溶剂两次洗脱，GC-C-IRMS分析正构烷烃单体碳同位素，研究前处理过程中正构烷烃单体碳同位素是否发生分馏。结果表明：使用柱色谱分离前后，多数正构烷烃单体碳同位素比值相差-0.2‰~0.2‰；当5Å分子筛不完全络合时，未络合的正构烷烃单体碳同位素比值偏重约0.7‰，可能发生了微弱的碳同位素分馏，但并未影响洗脱后的正构烷烃单体碳同位素比值；使用环己烷-正戊烷混合溶剂洗脱前后，碳同位素比值相差-0.2‰~0.5‰，以同样方式洗脱第二次，获得的正构烷烃单体碳同位素比值与模拟样品相差-0.3‰~0.2‰。分析不同回收率（>20%）正构烷烃的单体碳同位素比值，处理前后的差值基本在0.3‰以内，可见当正构烷烃回收率低至20%左右时，其单体碳同位素仍未发生明显分馏。柱色谱分离-5Å分子筛络合-混合溶剂洗脱方法适用于回收率大于20%的正构烷烃单体碳同位素分析。
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关键词:
- 正构烷烃单体碳同位素 /
- 碳同位素分馏 /
- 柱色谱分离 /
- 5Å分子筛络合 /
- 混合溶剂洗脱 /
- 气相色谱-气体同位素质谱
Abstract:BACKGROUND Before analyzing the carbon isotopes of n-alkanes by gas chromatography-gas isotope mass spectrometry (GC-IRMS), it is necessary to pre-separate and enrich n-alkanes and isoparaffins in saturated hydrocarbon samples. Whether the carbon isotope fractionation of n-alkanes occurs is the key for the high-precision analysis of the carbon isotope ratio.
OBJECTIVES To determine isotopic fractionation characteristics for n-alkanes during column chromatography and molecular sieve separation.
METHODS Saturated hydrocarbon components were extracted with a 500mg/3mL SPE silica gel column using 2mL n-pentane. After separating by a 5Å molecular sieve and extracting by a mixture solvent of cyclohexane and n-pentane, n-alkanes were concentrated and analyzed by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS).
RESULTS During column chromatographic separation, the carbon isotope ratio of most n-alkanes differed from -0.2‰ to 0.2‰. When n-alkanes was not completely complexed by 5Å molecular sieve, the uncomplexed alkane carbon isotope was about 0.7‰ heavier, and weak carbon isotope fractionation occured, but it did not affect the eluted n-alkanes carbon isotope ratio. Eluted with cyclohexane-n-pentane mixed solvent, the carbon isotope values before and after the elution differed from-0.2‰ to 0.5‰, and eluted in the same way for the second time, the difference between the eluted n-alkanes carbon isotope and simulate sample was between -0.3‰ and 0.2‰. Analyzing the carbon isotope ratios of n-alkanes with different recoveries (>20%), the difference between carbon isotopes of n-alkanes before and after pretreatment was within 0.3‰. It was found that when the recovery was as low as 20%, the carbon isotope ratios did not undergo significant fractionation.
CONCLUSIONS The column chromatography-5Å molecular sieve adsorption and mixed solvent elution method is suitable for the analysis of carbon isotope ratios of normal alkanes, which yields a recovery greater than 20%.
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表 1 工作标准的δ13C值
Table 1. δ13C values of working standards
目标物 工作标准的δ13C (‰) 目标物 工作标准的δ13C (‰) 平均值 SD(n=26) 平均值 SD(n=26) C15 -27.12 0.16 C24 -32.44 0.11 C17 -35.41 0.15 C26 -32.68 0.09 C18 -32.68 0.15 C28 -33.11 0.13 C21 -28.76 0.13 C32 -30.67 0.16 C22 -29.16 0.15 C36 -30.61 0.16 
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