pyisotopomer: A Python package for obtaining intramolecular isotope ratio differences from mass spectrometric analysis of nitrous oxide isotopocules

RATIONALE

Obtaining nitrous oxide isotopocule measurements with isotope ratio mass spectrometry (IRMS) involves analyzing the ion current ratios of the nitrous oxide parent ion (N₂O⁺) as well as those of the NO⁺ fragment ion. The data analysis requires correcting for “scrambling” in the ion source, whereby the NO⁺ fragment ion obtains the outer N atom from the N₂O molecule. While descriptions exist for this correction, and interlaboratory intercalibration efforts have been made, there has yet to be published a package of code for implementing isotopomer calibrations.

METHODS

We developed a user-friendly Python package (pyisotopomer) to determine two coefficients (γ and κ) that describe scrambling in the IRMS ion source, and then to use this calibration to obtain intramolecular isotope deltas in N₂O samples.

RESULTS

With two appropriate reference materials, γ and κ can be determined robustly and accurately for a given IRMS. An additional third reference material is needed to define the zero-point of the delta scale. We show that IRMS scrambling behavior can vary with time, necessitating regular calibrations. Finally, we present an intercalibration between two IRMS laboratories, using pyisotopomer to calculate γ and κ, and to obtain intramolecular N₂O isotope deltas in lake water unknowns.

CONCLUSIONS

Given these considerations, we discuss how to use pyisotopomer to obtain high-quality N₂O isotopocule data from IRMS systems, including the use of appropriate reference materials and frequency of calibration.