CARBON ISOTOPIC ANALYSIS OF INDIVIDUAL N-ALKANES - EVALUATION OF ACCURACY AND APPLICATION TO MARINE PARTICULATE ORGANIC MATERIAL

The accuracy and precision of deltaC-13 analyses for individual n-alkanes determined by gas chromatography/isotope ratio mass spectrometry (GC/IRMS) were determined. A solution of n-alkanes with known deltaC-13 values (ANL-1) was prepared and analyzed by GC/IRMS, and both the precision and accuracy of these measurements were within 0.5 parts per thousand. Silica gel gravity liquid chromatography was carried out on ANL-1 in order to determine if isotopic fractionation occurred during compound class separation. Analyses of the eluates showed that little isotopic fractionation (<0.5 parts per thousand) resulted from this procedure. Two solutions (ANL-2 and EPR-1) were prepared by combining n-alkanes having known deltaC-13 values with natural petroleum backgrounds (unresolved complex mixtures). The GC/IRMS results obtained for these solutions were within 0.5 parts per thousand of the isotope ratio mass spectrometry (IRMS) results for 13 of the 21 n-alkanes measured. Six of the eight n-alkanes with deltaC-13 values that deviated by more than 0.5 parts per thousand from their IRMS results coeluted with other compounds or were present in relatively low abundance. These data indicate that for peaks of adequate size (ion beam current at m/z = 44 > 0.5 nA in this case) which do not coelute with other components, the background can be accurately subtracted using the standard software. Therefore, differences of more than 0.5 parts per thousand in the deltaC-13 values of individual n-alkanes determined by GC/IRMS appear to be significant even in the presence of background. Normal alkanes were extracted and isolated from several samples of particulate organic matter (POM) and a Recent marine sediment. Samples collected during periods of low productivity contained n-C19-n-C24 n-alkanes having higher deltaC-13 values (0.5 parts per thousand or more) than those collected when productivity was high. Normal alkanes (n-C24-n-C28) isolated from sediment samples were depleted in C-13 by more than 0.5 parts per thousand relative to POM taken during the productivity maximum. Similarly, several sediment n-alkanes (n-C24, n-C25 n-C27-n-C29 and n-C31) were depleted in C-13 by 1.0 parts per thousand or more relative to those associated with POM collected when productivity was low. Differences in the deltaC-13 of n-alkanes in these samples can be related to changes in the source and to degradation processes during residence in the water column and in sediments.