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 δ15NA ir 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 δ15NA ir values, −33.5 ± 0.25‰ and +5.2 ± 0.81‰, are consistent with reported values from bulk sample analysis within analytical precision.
- carbon isotope ratio
- nitrogen isotope ratio
- organic matter
- secondary ion mass spectrometry