Thermal Heating Induced Fractionation Effect on δ15N Measurements (Using Continuous Flow Isotope Ratio Mass-Spectrometry) for Samples Containing Lower N Contents

Abstract

Simultaneous measurements of stable isotopes of Nitrogen (N), Carbon (C) and Sulfur (S) in a single aliquot of sample using stable isotope mass spectrometry coupled with elemental analyzer (EA) in a continuous flow mode has provided added advantages for stable isotopic data community scientifically as well as economically. For NCS isotopic measurements, certain configurational changes were adapted in the EA setup such as (1) introduction of ‘purge and trap columns’ for adsorbing and desorbing analyte CO2 and SO2, and (2) introduction of in-line ring heater to cover for the entire passage between combustion and reduction reactor to prevent any condensation of analyte gases. These modifications were made to achieve necessary accuracy and precision NCS isotopic measurements. However, the presence of in-line ring heater appears to have an influence on δ15N measurements especially for samples containing lower N amounts (≤2 μmoles). We compared δ15N measurements in both NCS and NC modes by reconfiguring same EA (Vario Pyrocube) connected with Isoprime mass spectrometer with wide range variability in N content. We observed a decreasing pattern in δ15N values with increasing N contents (till the true δ15N value is approached), which is an opposite trend compared to that typically observed with the NC mode of EA setup. We surmise thermal heating of the passage from the combustion and reduction reactors is most likely responsible for the observed pattern. Measured δ15N values for samples containing lower N (≤2 μmoles), however, could be corrected using appropriate correction procedure.