Improving the accuracy and sensitivity of isotope ratio mass spectrometry for d13C and d15N values in very low mass samples for ecological studies

A modified procedure for isotope ratio mass spectrometry (IRMS) of 13C/12C and 15N/14N ratios of very low biomass samples yielded reproducible data and enhanced the resolution of the instrument beyond its typical detection limit. The main modification was to rely only on oxygen from the chromium oxide catalyst allowing greater precision of measurement. Our method was accurate to weights of 8 mg C and 2 mg N with reference standards and test materials. The collembolan could be measured accurately enough for differences between individuals to be detectable. Attaining this level of resolution with ecological samples makes it feasible to use IRMS while retaining sample mass small enough to be functionally useful in studies of micro-organism food webs.