Exploring the Potential of Electrospray-Orbitrap for Stable Isotope Analysis Using Nitrate as a Model

Widely
used isotope ratio mass spectrometers have limited capabilities
to measure metabolites, drugs, or small polyatomic ions without the
loss of structural isotopic information. A new approach has recently
been introduced that uses electrospray ionization Orbitrap to measure
multidimensional isotope signatures of intact polar compounds. Using
nitrate as a model compound, this study aims to establish performance
metrics for comparisons with conventional IRMS at the natural abundance
level. We present a framework on how to convert isotopolog intensities
to δ values that are commonly used in the isotope geochemistry
community. The quantification of seven nitrate isotopologs provides
multiple pathways for obtaining the primary N and O δ values
including non-mass-dependent O isotope variations, as well as opportunities
to explore nonrandom isotopic distributions (i.e., clumping effects)
within molecular nitrate. Using automation and the adaptation of measurement
principles that are specific to isotope ratio analysis, nitrate δ15NAIR, δ18OVSMOW, and
δ17OVSMOW were measured with a long-term
precision of 0.4‰ or better for isotopic reference materials
and purified nitrate from environmental samples. In addition, we demonstrate
promising results for unpurified environmental samples in liquid form.
With these new developments, this study connects the two largely disparate
mass spectrometry fields of bioanalytical MS and isotope ratio MS,
thus providing a route to measure new isotopic signatures in diverse
organic and inorganic solutes.