Simultaneous In Situ Analysis of Carbon and Nitrogen Isotope Ratios in Organic Matter by Secondary Ion Mass Spectrometry

An insitu measurement method for simultaneous determination of carbon and nitrogen isotope ratios in organic matter was developed by secondary ion mass spectrometry with a spatial resolution of similar to 12m. Secondary ion intensities of C-12(2)-, (CC-)-C-12-C-13, (CN-)-C-12-N-14 and (CN-)-C-12-N-15 were simultaneously measured by three Faraday cups and one electron multiplier. Ions of (C2H-)-C-12 were measured to monitor hydride interferences and to correct for mass bias. The analytical precisions of C-13 and N-15 values of a reference material (UWLA-1 anthracite) were 0.16 parts per thousand and 0.56 parts per thousand, respectively (2s). A negative correlation was observed between the mass bias of carbon and (C2H-)-C-12/C-12(2)- ratios of examined reference materials. In contrast, there was no correlation between mass bias and hydrogen concentration for nitrogen. The C-13(VPDB) and N-15(Air) values of twenty-two individual globules of organic matter, found in carbonate rock of the 1878Ma Gunflint Formation, were determined by the new procedure, ranging from -33.8 parts per thousand to -33.3 parts per thousand and +4.2 parts per thousand to 5.8 parts per thousand, respectively. Means of C-13(VPDB) and N-15(Air) values, -33.5 +/- 0.25 parts per thousand and +5.2 +/- 0.81 parts per thousand, are consistent with reported values from bulk sample analysis within analytical precision.