Accuracy and precision of ESI-Orbitrap-IRMS observations of hours to tens of hours via reservoir injection

Abstract

Orbitrap isotope ratio mass spectrometry (Orbitrap-IRMS) has recently been applied to high-precision, natural-abundance isotope ratio measurements of a diverse range of compounds, including amino acids, oxyanions, fatty acids, and metals. These measurements can characterize many isotope ratios simultaneously at high (≈1.0‰) precision. In a successful experiment, observed precision will track the shot-noise limit and be limited by experimental time. Some isotope ratios, for example those involving ¹⁷O in organic compounds or multiply-substituted ('clumped') isotopologues, require experimental times of hours to tens of hours to achieve desired precision, while current sample introduction techniques focus on observations on the order of seconds to tens of minutes. In this study, we characterize Orbitrap-IRMS performance for three long duration measurements (individual acquisitions ≥1 h and as long as 24 h) using an automated reservoir injection system coupled to a Q Exactive HF Orbitrap with an electrospray ionization (ESI) source. First, we characterize long-term intra-measurement stability through a 24-h long measurement of acetone. We report the following isotope ratios and precisions (as acquisition errors, errors on the observed ratio within this measurement (σAE)): ¹³C/¹²C (σ_(AE) = 0.07‰), ¹⁷O/¹⁶O (σ_(AE) = 1.1‰), ¹⁸O/¹⁶O (σ_(AE) = 0.3‰), and ¹³C¹³C/¹²C (σ_(AE) = 0.65‰). The σ_(AE) of each tracks the shot noise limit throughout and is limited by the challenging conditions (high resolution and low numbers of ions per scan) required for ¹⁷O/¹⁶O measurement in the presence of ¹³C via Orbitrap. Second, we characterize inter-measurement stability via a sequence of seven 75-min analyses of perchlorate. We observe the following ratios and acquisition errors: ³⁷Cl/³⁵Cl (σ_(AE) = 0.09‰); ¹⁷O/¹⁶O (σ_(AE) = 1.6‰); ¹⁸O/¹⁶O (σ_(AE) = 0.7‰), ³⁷Cl¹⁷O/³⁵Cl¹⁶O (σ_(AE) = 2.7‰), and ³⁷Cl¹⁸O/³⁵Cl¹⁶O (σ_(AE) = 1.2‰). However, we find that inter-measurement drift between acquisitions limits our accuracy and precision for standardized measurements (i.e., error on reported δ values) to ≈1‰ for the ³⁷Cl/³⁵Cl measurement. Hence, the benefits of low σ_(AE) may not be fully realized. Third, we demonstrate accuracy via sample/standard comparisons of a methionine sample with ¹³C enrichment of ≈20‰ relative to a known standard. Using a sequence of seven 60-min analyses, we recover the following isotope ratios and standardized precisions (i.e., error on reported δ values, denoted propagated acquisition errors, σ_(PAE)): ³³S/³²S (σ_(PAE) = 1.0‰), ³⁴S/³²S (σ_(PAE) = 0.7‰), ¹⁵N/¹⁴N (σ_(PAE) = 2.1‰), ²H/¹H (σ_(PAE) = 3.2‰),¹³C/¹²C (σ_(PAE) = 0.4‰), ¹⁸O/¹⁶O (σ_(PAE) = 1.6‰), & ¹³C¹³C/¹²C (σ_(PAE) = 2.8‰) with confirmation of accurate results for the known ¹³C/¹²C and ¹³C¹³C/¹²C enrichments. Together, our results demonstrate the viability of Orbitrap-IRMS for long duration measurements of diverse sample types via an automated reservoir injection system. Inter-measurement stability remains a challenge; we expect our methods to be most applicable to extended measurements of hard-to-observe properties, such as ¹⁷O in organics and clumped isotopologues.

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