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

Abstract

An in situ 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 ~ 12 μm. Secondary ion intensities of 12C−2, 12C13C−, 12C14N− and 12C15N− were simultaneously measured by three Faraday cups and one electron multiplier. Ions of 12C2H− were measured to monitor hydride interferences and to correct for mass bias. The analytical precisions of δ13C and δ15N values of a reference material (UWLA‐1 anthracite) were 0.16‰ and 0.56‰, respectively (2s). A negative correlation was observed between the mass bias of carbon and 12C2H−/12C−2 ratios of examined reference materials. In contrast, there was no correlation between mass bias and hydrogen concentration for nitrogen. The δ13CVPDB and δ15NAir values of twenty‐two individual globules of organic matter, found in carbonate rock of the 1878 Ma Gunflint Formation, were determined by the new procedure, ranging from −33.8‰ to −33.3‰ and +4.2‰ to 5.8‰, respectively. Means of δ13CVPDB and δ15NAir values, −33.5 ± 0.25‰ and +5.2 ± 0.81‰, are consistent with reported values from bulk sample analysis within analytical precision.