Experimental assessment of EAIRMS normalization methodologies for
environmental stable isotopes
RATIONALE: In stable isotope mass spectrometry, isotope values
are normalized to internationally recognized reference scales using
certified reference materials and working standards. 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 standards used, their matrix, their isotope range, and
whether normalization required extrapolating beyond the isotope range.
Using 8 certified reference materials and 5 working standards on a
ThermoFinnigan Delta-V IRMS and Elementar VisION IRMS for nitrogen and
carbon isotope composition via solid combustion with an elemental
analyzer, we computed every possible isotope normalization (n=6272).
Additionally, we assessed how sample matrix impacted linearity effects
on both instruments. RESULTS: Normalizations composed of three
or four reference materials had better performance than one-point and
two-point methods, especially when the normalization was matrix-mixed or
extrapolated, and normalizations with an isotope range greater than 15‰
were more accurate under these conditions. Normalizations that were
matrix-matched and were not extrapolated exhibited the highest accuracy.
Linearity effects were found to exceed instrument precision by two
orders of magnitude irrespective of sample matrix and were not predicted
by reference gas diagnostics. CONCLUSIONS: To maximize
interlaboratory comparability of isotope results, operators of EAIRMS
systems should use at least 3 calibration standards to construct their
normalizations, select standards with a large isotope range to avoid
extrapolation, and match the matrix of their standards to their samples
to the best extent possible.