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

Abstract

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.

Microbial Denitrification: Active Site and Reaction Path Models Predict New Isotopic Fingerprints

“…− to constrain environmental processes. 35,36 We find that the 15 − , is ca. +0.35‰ for all molecular models (Table 1).…”

“…We report15−18 Δ and 18−18 Δ here as it matches the convention of clumped measurements made on other geochemical species, and because we are still developing the best way to measure intact clumped isotope abundances with the least error on electrospray Orbitrap instruments. It is possible that reporting isotopologue abundance ratios directly such as [15 N 18 O]/[ 15 N] or [ 18 O 18 O]/[ 18 O], or more unconventional Orbitrap ratios such as [ 15 N 18 O]/[ 18 O 18 O],35 might be a solution. Developments in this regard will require more studies such as this one which consider the way in which multiple clumped isotopologues behave simultaneously during chemical reactions.…”

Simultaneous, High-Precision Measurements of δ2H and δ13C in Nanomole Quantities of Acetate Using Electrospray Ionization-Quadrupole-Orbitrap Mass Spectrometry

“…More robust technologies with dual-inlet syringe pumps for the sample and standard would avoid human error in sample loading, sample carryover between infusions, and idle instrument time. Such modifications could lower the ER of the measurements even further . Nevertheless, our current measurements achieve internal and external uncertainty (1 SE) in δ 2 H values of less than 3.0‰, equivalent or lower than current NMR and SPME–GC–IRMS methods while requiring 50–1000× less sample. , …”

“…Such modifications could lower the ER of the measurements even further. 24 Nevertheless, our current measurements achieve internal and external uncertainty (1 SE) in δ 2 H values of less than 3.0‰, equivalent or lower than current NMR and SPME−GC−IRMS methods while requiring 50−1000× less sample. 10,12 Measurement Accuracy.…”

Position‐specific carbon isotope analysis of serine by gas chromatography/Orbitrap mass spectrometry, and an application to plant metabolism

“…Looking forward, a related instrument, the Q Exactive HF with an electrospray ionization source, is highly promising for isotope ratio analysis and has been studied in detail for other analytes including pure methionine, acetate, and inorganic ions. 14,[60][61][62] The greatest challenge for amino acid isotope analysis by this method is the lack of efficient fragmentation (e.g. for underivatized serine, the bond between C-2 and C-3 of serine does not fracture during electrospray ionization).…”