An Improved Urea Adduction Method For Carbon Isotope Ratios Analysis Of Low Concentration Of Nalkanes In Environmental Samples

Both 5 A molecular sieves and urea adduction method are used to separating straight chain from branched/cyclic hydrocarbons. However,the 5A molecular sieves method are easy to be affected by environmental factors, and the urea adduction method cause significant loss of n-alkanes during the long and complex process. The recoveries of the n-alkanes are low in environmental sample with low amount of n-alkanes for the two methods. An improved urea adduction method to separate cyclic/branched organic compounds from the linear ones with high recoveries for low concentration n-alkane samples was developed. The amount of urea, reaction solvent, reaction time of adduction and the extracting solvent during urea adduction procedure were optimized. The optimized process emploies hexane and acetone to dissolve a saturated fraction. The mixture was added to a 2ml urea-saturated methanol solution and allowed to stand for 48 h at 4℃for complete adduction. After the urea crystals were dissolved in distilled water, hexane was added into the mixture to form aquous and organic phases. The straight chain alkanes were recovered by hexane in aquous phase,and by a mixture of hexane and acetone in organic phases. The obtained n-alkanes were analysised by GC and GC-C-IRMS. The method yielded better chromatography, reduced the impact of Unresolved Complex Mixture and co-eluting components. The recoveries of C19-C40 n-alkanes ranged from 79% to 104% with a precision of 2%~6%. The accuracies of compound specific carbon isotopes of n-alkanes ranged from 0.09%-0.63%(1σ). Comparative experiments with 5A molecular sieves were conducted so as to determine the effectiveness and accurate analysis of stable carbon isotope ratios of individual nalkanes of the improved method. The higher recovery and reliably determination of the isotopic ratio shows that the improved method meets the requirement of compound specific carbon isotope analysis of n-alkanes for environmental samples with low concentration of n-alkanes.