Balint, S., M. Schwartz, D. N. Fowler, S. Linnekogel, C. Cromratie, and L Burkemper. 2024. Experimental assessment of elemental analyzer isotope ratio mass spectrometry normalization methodologies for environmental stable isotopes. Rapid Communications in Mass Spectrometry. Volume 38, Issue 17 https://doi.org/10.1002/rcm.9837
Abstract
RATIONALE: In stable isotope mass spectrometry, isotope values are normalized to internationally recognized reference scales using a combination of certified and in-house isotope reference materials. Numerous techniques exist for performing this normalization, but these methodologies need to be experimentally assessed to compare their impact on reproducibility of isotope results.
METHODS: We tested normalization methods by the number of reference materials used, their matrix, their isotopic range, and whether normalization required extrapolating beyond the isotopic range. We analyzed 8 commercially-available isotope reference materials on a ThermoFinnigan Delta-V IRMS and Elementar VisION IRMS for nitrogen and carbon isotope composition via solid combustion with an elemental analyzer and computed every possible isotope normalization (n = 612). Additionally, we assessed how sample matrix impacted linearity effects on both instruments using 5 in-house reference materials.
RESULTS: Normalizations exhibited the best performance when the reference materials spanned an isotopic range of at least 20‰, were matrix matched with the samples, and did not require extrapolation beyond the calibration curve. When these conditions were not met, the number of reference materials used had a significant effect on accuracy, with normalizations composed of two reference materials exhibiting particularly inconsistent performance at isotopic ranges below 20‰. Linearity effects were found to exceed instrument precision by two orders of magnitude irrespective of matrix type and were not predicted by working gas diagnostics.
CONCLUSIONS: Interlaboratory comparability of isotope results are improved when operators of EAIRMS systems select reference materials spanning an isotopic range of at least 20‰. Additionally, using 3 or more isotopic reference materials, avoiding extrapolation beyond the range of the normalization curve, and matching the matrix of the reference materials to the samples improves normalizations.
METHODS: We tested normalization methods by the number of reference materials used, their matrix, their isotopic range, and whether normalization required extrapolating beyond the isotopic range. We analyzed 8 commercially-available isotope reference materials on a ThermoFinnigan Delta-V IRMS and Elementar VisION IRMS for nitrogen and carbon isotope composition via solid combustion with an elemental analyzer and computed every possible isotope normalization (n = 612). Additionally, we assessed how sample matrix impacted linearity effects on both instruments using 5 in-house reference materials.
RESULTS: Normalizations exhibited the best performance when the reference materials spanned an isotopic range of at least 20‰, were matrix matched with the samples, and did not require extrapolation beyond the calibration curve. When these conditions were not met, the number of reference materials used had a significant effect on accuracy, with normalizations composed of two reference materials exhibiting particularly inconsistent performance at isotopic ranges below 20‰. Linearity effects were found to exceed instrument precision by two orders of magnitude irrespective of matrix type and were not predicted by working gas diagnostics.
CONCLUSIONS: Interlaboratory comparability of isotope results are improved when operators of EAIRMS systems select reference materials spanning an isotopic range of at least 20‰. Additionally, using 3 or more isotopic reference materials, avoiding extrapolation beyond the range of the normalization curve, and matching the matrix of the reference materials to the samples improves normalizations.