Analytical improvements in irm-GC/MS analyses: Advanced techniques in tube furnace design and sample preparation

Peak resolution is an essential requirement for achieving accurate irm-GC/MS compound-specific isotope measurements. Obtaining accurate isotope data from petroleum extracts, in particular, has proven especially difficult due to co-elutions that typically comprise crude oil mixtures. New enhancements in peak resolution in irm-GC/MS analyses have been achieved by substituting a polyimide-coated glass capillary as the furnace reactor. The advantages of the design are significant, and include: (1) incorporation of a zero dead-volume combustion reactor, resulting in almost no loss in gas chromatographic peak resolution, equating to less compound interferences and more accurately measured isotope values and (2) use of direct capillary-to-capillary leak tight, zero dead-volume connections that reduce the incidence of leak problems generally encountered as a result of using fittings of substantially different sizes. In addition, improvements in analytical chromatography using size-exclusion techniques such as urea adduction and molecular sieving reduces the number of components in a sample, and have resulted in the acquisition of more accurate isotope data. Experiments comparing paraffin isotope measurements for whole oil samples to those of urea adducted samples of the same oil have shown that differences of 2‰ are not uncommon. Data from experiments with isotope standards indicate that urea adduction and molecular sieving techniques have no observable fractionation effect on the measurement of isotope values for paraffin components in these mixtures. Considering the intensive effort and expense in generating irm-GC/MS data, the use of relatively inexpensive chromatography techniques such as urea adduction and molecular sieving in isolating components of interest for accurate isotope analysis is justified.