Microanalysis of carbon isotope composition in organic matter by secondary ion mass spectrometry

An analytical procedure has been developed for the in situ measurement of carbon isotope composition of organic matter, with a spatial resolution of 20-30 mu m, using a Cameca IMS 1270 ion microprobe. Instrumental mass fractionation (IMF) of carbon isotopes was observed to be independent of primary ion beam intensity and sputtering time, but did depend oil vacuum conditions and oil the chemical composition of the sample. To evaluate such matrix effects, a set of 9 standards representative of the natural chemical variability of organic matter was prepared, with H/C atornic ratios and organic carbon contents (C-org) ranging between 0.04 and 1.74 and between 41 and 100 wt.%, respectively. Under the analytical conditions tested, IMF was not found to be influenced by the presence of silicate mineral impurities in the organic matter, but variations in IMF up to 5 parts per thousand were observed over the set of standards with the magnitude of IMF negatively con-elated to the H/C ratios of samples. Aliphaticity ratios determined using Fourier transform infrared microspectroscopy provided an in situ estimation of H/C ratios with a spatial resolution barely exceeding that of the ion microprobe and permit a correction for matrix effects with a standard error of +/- 0.2 parts per thousand (1 sigma). Taking into account all sources of uncertainty, ion microprobe delta C-13 were accurately determined with a +/- 0.7 parts per thousand (1 sigma) total uncertainty. The mechanism for the matrix effect of H/C ratios upon IMF is still to be determined but it is likely related to the difference in proportion of atomic vs. molecular carbon ions observed between samples of different H/C ratios. (c) 2005 Elsevier B.V. All rights reserved.