Study on Specific Carbon Isotope Fractionation of n-Alkanes during Column Chromatography Separation-Molecular Sieve Complexation Adsorption
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ABSTRACT
BACKGROUNDBefore 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.OBJECTIVESTo determine isotopic fractionation characteristics for n-alkanes during column chromatography and molecular sieve separation.METHODSSaturated 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).RESULTSDuring 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.CONCLUSIONSThe 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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