High-precision laser spectrometer for multiple greenhouse gas analysis in 1 mL air from ice core samples

Abstract

Abstract. The record of past greenhouse gas composition from ice cores is crucial for our understanding of global climate change. Future ice core projects will aim to extend both the temporal coverage (extending the timescale to 1.5 Myr) and the temporal resolution of existing records. This implies a strongly limited sample availability, increasing demands on analytical accuracy and precision, and the need to reuse air samples extracted from ice cores for multiple gas analyses. To meet these requirements, we designed and developed a new analytical system that combines direct absorption laser spectroscopy in the mid-infrared (mid-IR) with a quantitative sublimation extraction method. Here, we focus on a high-precision dual-laser spectrometer for the simultaneous measurement of CH4, N2O, and CO2 concentrations, as well as δ13C(CO2). Flow-through experiments at 5 mbar gas pressure demonstrate an analytical precision (1 σ) of 0.006 ppm for CO2, 0.02 ‰ for δ13C(CO2), 0.4 ppb for CH4, and 0.1 ppb for N2O, obtained after an integration time of 100 s. Sample–standard repeatabilities (1 σ) of discrete samples of 1 mL STP (Standard Temperature and Pressure) amount to 0.03 ppm, 2.2 ppb, 1 ppb, and 0.04 ‰ for CO2, CH4, N2O, and δ13C(CO2), respectively. The key elements to achieve this performance are a custom-developed multipass absorption cell, custom-made high-performance data acquisition and laser driving electronics, and a robust calibration approach involving multiple reference gases. The assessment of the spectrometer capabilities in repeated measurement cycles of discrete air samples – mimicking the procedure for external samples such as air samples from ice cores – was found to fully meet our performance criteria for future ice core analysis. Finally, this non-consumptive method allows the reuse of the precious gas samples for further analysis, which creates new opportunities in ice core science.

Laser-induced sublimation extraction for cm-resolution multi-species greenhouse gas analysis on ice cores

“…3.2), and to link the CO 2 isotope ratios derived by QCLAS to the international Vienna PeeDee Belemnite (VPDB) scale, we prepared a set of calibration gases (Table 1) with specific mixtures covering the full range from the preindustrial to glacial atmospheric composition in CO 2 , CH 4 , and N 2 O and a range of −3.54 ‰ to −10 ‰ in δ 13 CO 2 . The production procedure and further details are discussed in Bereiter et al (2020).…”

“…The QCLAS system, which performs the actual measurements of greenhouse gas concentrations and the isotopic composition of CO 2 , is described and discussed in detail by Bereiter et al (2020). In this section we provide a short summary of its main characteristics, the measurement procedure, and describe additional modifications that were necessary to ensure the fidelity of the (isotopic) analysis of small air samples derived from ice core sublimation.…”

“…To achieve the necessary resolution for the oldest ice while avoiding all issues outlined above, we developed a novel extraction and analysis method using continuous sublimation extraction of ice core sections coupled to a custom-made dual-quantum cascade laser absorption spectrometer (QCLAS, Bereiter et al, 2020). In particular, by using sublimation, the new method avoids the water phase that leads to in extractu formation of CO 2 and CH 4 and ensures a 100 % extraction efficiency for all gases.…”

“…In particular, by using sublimation, the new method avoids the water phase that leads to in extractu formation of CO 2 and CH 4 and ensures a 100 % extraction efficiency for all gases. Furthermore, the QCLAS is insensitive to drill fluid traces and is designed to simultaneously measure CO 2 , CH 4 and N 2 O concentrations as well as δ 13 CO 2 directly from 1 − 2 mL of extracted air (Bereiter et al, 2020). In the following, we present the technical details of the developed method comprising the laser induced sublimation extraction (LISE) system, the vacuum system, the standard gas inlet, and the calibration procedure.…”

“…In the following, we present the technical details of the developed method comprising the laser induced sublimation extraction (LISE) system, the vacuum system, the standard gas inlet, and the calibration procedure. While the QCLAS is described in detail by Bereiter et al (2020), its particular use and the exact measurement procedure, including modification to the inlet and the calibration/bracketing procedure for the ice core air sample measurements, are described here. A detailed evaluation of the LISE using this measurement setup follows and finally, we analyzed samples from well characterized ice cores (EDC and Talos Dome) to demonstrate the excellent performance of the LISE/QCLAS system.…”