Automated simultaneous measurement of the δ(13) C and δ(2) H values of methane and the δ(13) C and δ(18) O values of carbon dioxide in flask air samples using a new multi cryo-trap/gas chromatography/isotope ratio mass spectrometry system

CleanEx: A Versatile Automated Methane Preconcentration Device for High-Precision Analysis of 13CH4, 12CH3D, and 13CH3D

The relative abundance of methane isotopologues offers key insights into the global methane (CH₄) cycle. Advances in laser spectroscopy enable routine high-precision measurements even for rare deuterated methane isotopologues, ¹²CH₃D and ¹³CH₃D, provided there are sufficiently high methane amount fractions and reproducible measurement conditions, which can be achieved by CH₄ adsorption-desorption techniques. We present a new cryogen-free automated preconcentration device─CleanEx─designed for quantitative extraction of CH₄ from large volumes of sample gas and for cleaning by stepwise temperature-controlled desorption to separate interferant gases. We show that CleanEx has the capability to preconcentrate methane by almost 2000-fold from ∼18 L of air. The performance is demonstrated in a range of methane amount fractions between 2 ppm (μmol mol^-1), which corresponds to the present-day ambient air, up to 1000 ppm, representative for close to source or process conditions. Advantages over existing devices are a significantly larger primary adsorption trap and a secondary cryo-focusing step, which ensures separation of methane from major atmospheric compounds, i.e., O₂, Ar, and CO₂. We have demonstrated quantitative extraction of methane, with no significant isotopic fractionation and high repeatability of 0.2‰, 0.6‰, and 0.8‰ (n = 42) for the studied isotopologue ratios, ¹³CH₄/¹²CH₄, ¹²CH₃D/¹²CH₄, and ¹³CH₃D/¹²CH₄, during cryogenic adsorption-desorption on HayeSep D material. The developed device in combination with a suitable laser spectrometer offers a robust and autonomous method for precise continuous monitoring of δ¹³C-CH₄ and δD-CH₄ in ambient air and optionally Δ¹³CH₃D in process-derived methane.

A new automated method for high-throughput carbon and hydrogen isotope analysis of gaseous and dissolved methane at atmospheric concentrations

RATIONALE

The dual isotope ratio analysis, carbon (δ¹³ C value) and hydrogen (δ² H value), of methane (CH₄ ) is a valuable tracer tool within a range of areas of scientific investigation, not least wetland ecology, microbiology, CH₄ source identification and the tracing of geological leakages of thermogenic CH₄ in groundwater. Traditional methods of collecting, purification, separating and analysing CH₄ for δ¹³ C and δ² H determination are, however, very time consuming, involving offline manual extractions.

METHODS

Here we describe a new gas chromatography, pyrolysis/combustion, isotope ratio mass spectrometry (IRMS) system for the automated analysis of either dissolved or gaseous CH₄ down to ambient atmospheric concentrations (2.0 ppm). Sample introduction is via a traditional XYZ autosampler, allowing either helium (He) purging of gas or sparging of water from a range of suitable, airtight bottles.

RESULTS

The system routinely achieves precision of <0.3‰ for δ¹³ C values and <3.0‰ for δ² H values, based on long-term replicate analysis of an in-house CH₄ /He mix standard (BGS-1), corrected to two externally calibrated reference gases at near atmospheric concentrations of methane. Depending upon CH₄ concentration and therefore bottle size, the system runs between 21 (140-mL bottle) and 200 samples (12-mL exetainer) in an unattended run overnight.

CONCLUSIONS

This represents the first commercially available IRMS system for dual δ¹³ C and δ² H analysis of methane at atmospheric concentrations and a step forward for the routine (and high-volume) analysis of CH₄ in environmental studies.

Oxygen-18 and Carbon-13 isotopes in eCO2and erythrocytes carbonic anhydrase activity of Finnish prediabetic population

Complex human physiological processes create the stable isotopic composition of exhaled carbon dioxide (eCO2), measurable with noninvasive breath tests. Recently, isotope-selective breath tests utilizing natural fluctuation in 18O/16O isotope ratio in eCO2 have been proposed for screening prediabetic (PD) individuals. It has been suggested that 18O/16O fractionation patterns reflect shifts in the activity of carbonic anhydrase (CA), an enzyme involved in the metabolic changes in the PD state. To evaluate the applicability of the breath sampling method in Finnish PD individuals, breath delta values (BDVs, ‰) of 18O/16O (δ18O) were monitored for 120 min in real-time with a high-precision optical isotope ratio spectrometer, both in the fasting state and during a 2-hour oral glucose tolerance test (2h OGTT) with non-labelled glucose. In addition, the BDV of 13C/12C (δ13C) was measured, and total erythrocyte CA activity was determined. δ18O and CA did not demonstrate any statistically significant differences between PD and non-diabetic control (NDC) participants. Instead, δ13C was significantly lower in PD patients in comparison to NDCs in the fasting state and at time points 90 and 120 min of the 2h OGTT, thus indicating slightly better potential in identifying Finnish PD individuals. However, overlapping values were measured in PD participants and NDCs, and therefore, δ13C cannot be applied as a sole measure in screening prediabetes at an individual level. Thus, because the combination of environmental and lifestyle factors and anthropometric parameters has a greater effect on glucose metabolism and CA activity in comparison to the PD state, 18O/16O and 13C/12C fractionations or CA activity did not prove to be reliable biomarkers for impaired glucose tolerance in Finnish subjects. This study was conducted under the clinicaltrials.gov ID NCT03156478.

Metrology for stable isotope reference materials: 13C/12C and 18O/16O isotope ratio value assignment of pure carbon dioxide gas samples on the Vienna PeeDee Belemnite-CO2 scale using dual-inlet mass spectrometry

Isotope ratio measurements have been conducted on a series of isotopically distinct pure CO₂ gas samples using the technique of dual-inlet isotope ratio mass spectrometry (DI-IRMS). The influence of instrumental parameters, data normalization schemes on the metrological traceability and uncertainty of the sample isotope composition have been characterized. Traceability to the Vienna PeeDee Belemnite(VPDB)-CO₂ scale was realized using the pure CO₂ isotope reference materials(IRMs) 8562, 8563, and 8564. The uncertainty analyses include contributions associated with the values of iRMs and the repeatability and reproducibility of our measurements. Our DI-IRMS measurement system is demonstrated to have high long-term stability, approaching a precision of 0.001 parts-per-thousand for the 45/44 and 46/44 ion signal ratios. The single- and two-point normalization bias for the iRMs were found to be within their published standard uncertainty values. The values of ¹³C/¹²C and ¹⁸O/¹⁶O isotope ratios are expressed relative to VPDB-CO₂ using the [Formula: see text] and [Formula: see text] notation, respectively, in parts-per-thousand (‰ or per mil). For the samples, value assignments between (-25 to +2) ‰ and (-33 to -1) ‰ with nominal combined standard uncertainties of (0.05, 0.3) ‰ for [Formula: see text] and [Formula: see text], respectively were obtained. These samples are used as laboratory reference to provide anchor points for value assignment of isotope ratios (with VPDB traceability) to pure CO₂ samples. Additionally, they serve as potential parent isotopic source material required for the development of gravimetric based iRMs of CO₂ in CO₂-free dry air in high pressure gas cylinder packages at desired abundance levels and isotopic composition values. Graphical abstract CO₂ gas isotope ratio metrology.

Determining Biogenic Content of Biogas by Measuring Stable Isotopologues 12CH₄, 13CH₄, and CH₃D with a Mid-Infrared Direct Absorption Laser Spectrometer

A tunable laser absorption spectrometer (TLAS) was developed for the simultaneous measurement of δ¹³C and δD values of methane (CH₄). A mid-infrared interband cascade laser (ICL) emitting around 3.27 µm was used to measure the absorption of the three most abundant isotopologues in CH₄ with a single, mode-hop free current sweep. The instrument was validated against methane samples of fossil and biogenic origin with known isotopic composition. Three blended mixtures with varied biogenic content were prepared volumetrically, and their δ¹³C and δD values were determined. Analysis demonstrated that, provided the isotopic composition of the source materials was known, the δ¹³C and δD values alone were sufficient to determine the biogenic content of the blended samples to within 1.5%.