Chemical and Isotopic Characterization of Industrial Gases: An Integrated and Robust Approach Combining Sampling and Analytical Measurements

Abstract. Recent studies have examined temporal fluctuations in the amount and carbon isotope content (δ13C) of CO2 produced by the respiration of roots and soil organisms. These changes have been correlated with diel cycles of environmental forcing (e.g., sunlight and soil temperature) and with synoptic-scale atmospheric motion (e.g., rain events and pressure-induced ventilation). We used an extensive suite of measurements to examine soil respiration over 2 months in a subalpine forest in Colorado, USA (the Niwot Ridge AmeriFlux forest). Observations included automated measurements of CO2 and δ13C of CO2 in the soil efflux, the soil gas profile, and forest air. There was strong diel variability in soil efflux but no diel change in the δ13C of the soil efflux (δR) or the CO2 produced by biological activity in the soil (δJ). Following rain, soil efflux increased significantly, but δR and δJ did not change. Temporal variation in the δ13C of the soil efflux was unrelated to measured environmental variables, and we failed to find an explanation for this unexpected result. Measurements of the δ13C of the soil efflux with chambers agreed closely with independent observations of the isotopic composition of soil CO2 production derived from soil gas well measurements. Deeper in the soil profile and at the soil surface, results confirmed established theory regarding diffusive soil gas transport and isotopic fractionation. Deviation from best-fit diffusion model results at the shallower depths illuminated a pump-induced ventilation artifact that should be anticipated and avoided in future studies. There was no evidence of natural pressure-induced ventilation of the deep soil. However, higher variability in δ13C of the soil efflux relative to δ13C of production derived from soil profile measurements was likely caused by transient pressure-induced transport with small horizontal length scales.

A new perspective on the δ13C signal preserved in speleothems using LC–IRMS analysis of bulk organic matter and compound specific stable isotope analysis

Temperature-responsive liquid chromatography (TRLC) enables the possibility of retention modulation under fully aqueous isocratic conditions through incorporation of smart polymers such as poly(N-isopropylacrylamide) (PNIPAAm) into the stationary phase. This feature seems to be particularly advantageous for liquid chromatography isotope ratio mass spectrometry (LC-IRMS), which is constrained by its requirement for fully aqueous eluents and limited stationary phase options. In this study, we demonstrate for the first time the coupling of TRLC with LC-IRMS for compound-specific carbon isotope analysis. Using testosterone and related steroids as a model substance class, we show that PNIPAAm-based TRLC columns provide stable δ13C measurements across isothermal and temperature gradient programs without introducing isotopic bias (σ ≤ 0.5‰ across all measurements). One-dimensional TRLC-IRMS enabled separation of several steroid standards under fully aqueous conditions and the determination of testosterone in an ethanol-based gel. A heart-cut two-dimensional (2D) LC-IRMS configuration, combining reversed phase LC in the first dimension with TRLC in the second dimension, achieves baseline separation of otherwise coeluting steroids and accurate δ13C determination in complex, lipid-rich pharmaceutical matrices. This work establishes TRLC as a powerful addition to the LC-IRMS toolkit, expanding method flexibility and selectivity without sacrificing isotopic accuracy and opening new avenues for compound specific isotope analysis via LC-IRMS.

We developed an experimental method for precise determination of carbon stable isotope ratio (δ13C) of soil-respired CO2 under natural condition. We devised a flask sampling system optimized for collecting soil-respired CO2 to minimize the measurement artifacts related to pressure anomaly. The δ13C of soil-respired CO2 was estimated from relationship between change rates of the CO2 mole fraction and the δ13C of the CO2 in a closed chamber at the soil surface by using two end-member simple mixing model. We tested the influence of CO2 enrichment in the soil-chamber headspace on the estimates of the δ13C of soil respired CO2 by using high-precision measurements of CO2 mole fraction and δ13C. To our results, the estimates of the δ13C of soil respired CO2 was rather insusceptible to the influence of the CO2 enrichment in the chamber as compared with the soil CO2 efflux. Improvement of analytical precision of δ13C is preferred approach to reduce the error in the estimates of δ13C of soil respired CO2. On the other hand, extending the sampling range of CO2 mole fraction in the chamber can be cost-effective means for the error-reduction practically.

Measurements of the stable carbon isotope ratios (δ13C) of microbial metabolic end products presents a promising method for monitoring in situ bioremediation of petroleum hydrocarbons. Differences between the δ13C values of hydrocarbons and indigenous carbon sources (e.g., plant matter, soil carbonates) can be exploited to trace the origins of metabolic end products. However, in zones of methanogenesis and/or where the δ13C values of endogenous plant matter overlap those of hydrocarbons, δ13C measurements can produce ambiguous results. In such cases, simultaneous measurement of the radiocarbon (14C) contents of metabolic end products can be used to determine their sources. This method was applied at a gasoline station spill site where hydrocarbons were the only source of 14C-free carbon. Combined δ13C and 14C measurements of soil gas CO2 and dissolved inorganic carbon in groundwater enabled quantification of carbon inputs. Furthermore, low 14C contents of high δ13C CO2 were crucial in establishing that the soil gas CO2 was derived from methanogenesis of hydrocarbons and not shell dissolution. In addition, low 14C content coupled with a 16‰ drop in the δ13C values of CO2 across a semipermeable layer beneath the gas station building confirmed that microbial oxidation of methane was occurring within this zone.

Compound-specific stable isotope analysis of hydrogen (δD) and carbon (δ13C) in organic compounds is a valuable tool in biogeochemical research. A key limitation of this method is the relatively large amount of sample required to achieve desirable precision. We developed a large-volume (20 μL) injection method that allows for high throughput analysis of less concentrated samples and tested it for δ13C and δD measurements of n-alkanes. We also conducted a comparison of reference standards and assessed several methods to normalize and correct n-alkane δD and δ13C measurements. The mean precision of the δD method based on 233 environmental n-alkane samples (two to three replications per sample) is 4.0‰ (1σ, estimated from the weighted mean of the pooled unbiased standard deviations) and 0.46‰ (1σ) for δ13C from 37 environmental samples (two to three replications per sample). The evaluation of reference standards shows that the use of n-alkane standards with large offsets in δD values in adjacent n-alkane chains can lead to biases in measurement correction. The large-volume injection method shows good reproducibility of δ13C and δD measurements of n-alkanes and reduces the required sample concentration by about 80%. We propose that for δD measurements, a reference standard set should be used in which each reference standard has a limited range of δD values and no adjacent n-alkane chains, to minimize memory effects.

Cavity ring-down spectroscopy (CRDS) is becoming increasingly popular for δ13 C-CO2 analysis of air. However, little is known about the effect of high 13 C abundances on the performance of CRDS. Overlap between 12 CO2 and 13 CO2 spectral lines may adversely affect isotopic-CO2 CRDS measurements of 13 C-enriched samples. Resolving this issue is important so that CRDS analysers can be used in CO2 flux studies involving 13 C-labelled tracers. We tested a Picarro G2131-i CRDS isotopic-CO2 gas analyser with specialty gravimetric standards of widely varying 13 C abundance (from natural to 20.1 atom%) and CO2 mole fraction (xCO2 : <0.1 to 2116ppm) in synthetic air. The presence of spectroscopic interference between 12 CO2 and 13 CO2 bands was assessed by analysing errors in measurements of the standards. A multi-component calibration strategy was adopted, incorporating isotope ratio and mole fraction data to ensure accuracy and consistency in corrected values of δ13 C-CO2 , x12 CO2 , and x13 CO2 . CRDS measurements of x13 CO2 were found to be accurate throughout the tested range (<0.005 to 100ppm). On the other hand, spectral cross-talk in x12 CO2 measurements of standards containing elevated levels of 13 CO2 led to inaccuracy in x12 CO2 , total-xCO2 (x12 CO2 +x13 CO2 ), and δ13 C-CO2 data. An empirical relationship for x12 CO2 measurements that incorporated the 13 C/12 C isotope ratio (i.e. 13 CO2 /12 CO2 , RCO2) as a secondary (non-linear) variable was found to compensate for the perturbations, and enabled accurate instrument calibration for all CO2 compositions covered by our standard gases. 13 C-enrichement in CO2 leads to minor errors in CRDS measurements of x12 CO2 . We propose an empirical correction for measurements of 13 C-enriched CO2 in air by CRDS instruments such as the Picarro G2131-i.

Do pesticides degrade in surface water receiving runoff from agricultural catchments? Combining passive samplers (POCIS) and compound-specific isotope analysis

Pesticide degradation and export losses at the catchment scale: Insights from compound-specific isotope analysis (CSIA)

Abstract. The measurement of ammonia (NH3) in ambient air is a sensitive and priority topic due to its impact on ecosystems. NH3 emissions have continuously increased over the last century in Europe because of intensive livestock practices and the enhanced use of nitrogen-based fertilizers. European air quality monitoring networks monitor atmospheric NH3 amount-of-substance fractions. However, the lack of stable reference gas mixtures (RGMs) of atmospheric amount-of-substance fractions of ammonia to calibrate NH3 analyzers is a common issue of the networks, which results in data that are not accurate, traceable, or, thus, geographically comparable. In order to cover this lack, LNE (Laboratoire National de Métrologie et d'Essais) developed, in close collaboration with the company 2M PROCESS, a gas reference generator to dynamically generate NH3 RGMs in air. The method is based on gas permeation and a further dynamic dilution to obtain an amount-of-substance fractions ranging between 1 and 400 nmol mol−1 (also well known as ppb or parts per billion; 1 ppb (NH3) to ≈ 0.7 µg m−3) to cover the amount-of-substance fractions of ammonia measured in ambient air (emissions) and the operating range of the NH3 analyzers used by the monitoring networks. The calibration of the elements of the generator against the LNE primary standards ensures the traceability of the RGMs to the international system of units. Furthermore, the highly accurate flow and oven temperature measurements of the reference generator, together with the associated calibration procedure defined by LNE, guarantee relative expanded uncertainties of the calibration of the NH3 analyzers that are lower than 2 % (coverage factor = 2). This result is very satisfactory, considering the low NH3 amount-of-substance fraction levels (1 to 400 nmol mol−1) and the phenomena of adsorption and desorption, especially in the presence of traces of water on contact surfaces. A bilateral comparison was organized between METAS (Swiss Federal Institute of Metrology) and LNE, which consisted of the calibration of a Picarro G2103 gas analyzer by both national metrology institutes (NMIs). The results highlighted the good agreement between the NH3 reference generators developed by the two institutes and allowed the validation of both LNE's reference generator and calibration procedure. Since the end of 2020, LNE has calibrated several NH3 analyzers from the French air quality monitoring networks (Associations Agréées de Surveillance de la Qualité de l'Air – AASQA) using the newly developed SI-traceable RGMs. The enhanced number of calibrations provided may increase the comparability, accuracy, and traceability of the NH3 measurements carried out on French territory.

Since 1992, the National Oceanic and Atmospheric Administration (NOAA), Climate Monitoring and Diagnostics Laboratory of the United States and the Commonwealth Scientific and Industrial Research Organization (CSIRO), Division of Atmospheric Research of Australia have routinely analyzed the same air sample collected biweekly at Cape Grim, Tasmania. Comparisons of measurements of atmospheric CO2, CH4, CO, H2, and the stable isotopes of CO2 (δ13C, δ18O) are used to assess the consistency of observations independently made by the two laboratories. Results demonstrate that conventional intercomparison strategies based on occasional exchange of high‐pressure cylinders are not sufficient to ensure adequate comparability between flask data records. The NOAA/CSIRO flask air intercomparison experiment has higher time resolution and captures artifacts that are specific to flask measurements. This ongoing experiment provides a stringent quality control test of individual laboratories' experimental methods and internal calibration schemes, and is a valuable tool in working toward reliable integration of atmospheric data from independent laboratories. Results from the first 7 years of this intercomparison show that NOAA and CSIRO Cape Grim measurement records of CH4 are consistent to within 1 nmol mol−1 (0.04%) and the CO2 records to ∼0.2 μmol mol−1 (0.06%). CH4 and CO2 observations from the two programs may be combined into more extensive data sets for specific purposes. Several causes of the observed differences in the measurements of δ13C and δ18O Of CO2 have been identified and independently corroborated; combined data sets of these isotopomers appear possible. Possible explanations are provided for the observed differences in measurements of CO and H2 from the same sample, which show significant variability with time.

Carbon dioxide (CO2) is one of the most important long-lived anthropogenic greenhouse gases. Ocean, land and biosphere contribute to take up CO2 emissions, but approximately half of fossil fuel CO2 accumulates in the atmosphere. The study of isotopic composition of CO2 can give useful information for assessing and quantifying the uptake of CO2 in the environmental compartments, as well as for distinguishing natural from anthropogenic carbon in the atmosphere. In this work, an activity for the development of a Fourier Transform Infrared spectroscopy (FTIR) based method for δ13C-CO2 determination in CO2 in air mixtures is presented. The FTIR can be calibrated by a classical approach based on primary calibration gas standards, but an alternative calibration can be based on the generation of synthetic spectra, by means of radiative transfer calculation codes such as the Multiple Atmospheric Layer Transmission (MALT University of Wollongong, Australia). Another software (B-FOS) developed at the Bureau International des Poids et Mesures (BIPM) allows to interface MALT and the FTIR management software. This calibration approach is fast and reliable and can be used when the classical calibration based on reference gas mixtures might be demanding. The uncertainty obtained for δ13C-CO2 measurements is around 0.1 ‰, at a nominal CO2 mole fraction of 400 μmol mol-1 in air.

Progress in the development of pure CO2 gas standards for δ13C, δ18O and Δ47 measurements as well as CO2 in air gas standards (with mole fractions in the range 350 µmol/mol to 800 µmol/mol) for δ13C, δ18O measurements are described. Initial results indicate the potential to produce standards with internal consistencies at the 0.005 ‰ level for δ13C and standard uncertainties of 0.015 ‰ in relation to the VPDB scale, with the magnitude of the latter principally limited by the homogeneity of primary carbonate reference materials.An initial driver for standards development was the requirement for appropriate calibration strategies and standards [1] to support commercially developed laser-based instruments that have grown in number over the last decade. These analysers can measure real-time isotopic ratio variations of greenhouse gases, and notably CO2, allowing their application across a wide range of scientific and technical disciplines. The development of appropriate standards and calibration methods has required the links and traceability to primary carbonate materials via the IRMS dual inlet reference method to be re-examined.Outputs of the project so far include:Establishment of a facility to produce stable pure CO2 gas standards in 6L cylinders at 2 bar with δ13C values from -1 ‰ to +45 ‰ vs VPDB, with internal consistency approaching the 0.005 ‰ level, and an effective calibration option for dual inlet IRMS systems as demonstrated in the international comparison CCQM-P204 completed in 2021 [2];Studies of Δ47 values of mixtures of different pure CO2 gas, and the reproducibility and stability of these and their potential to act as reference standards for clumped isotope ratio measurements with IRMS systems;The development and validation of a cryogenic Air Trapping system to extract CO2 from air for determination of δ13C and δ18O-CO2 with IRMS, including a correction for the N2O present in samples. The facility is currently being used for another international comparison (CCQM-P239) of CO2 in in air standards from 15 institutes containing CO2 over the range of 380 μmol mol−1 to 800 μmol mol−1 and δ13C and δ18O-CO2 values from 1 ‰ to -43 ‰ and -7 ‰ to -35 ‰, respectively. The method demonstrates excellent reproducibility, with standard deviations of 0.005% and 0.05% for δ13C and δ18O-CO2, respectively, and will demonstrate the level of equivalence of new CO2 in air isotope ratio standards currently being produced.[1] Flores, E., Viallon, J., Moussay, P., Griffith, D. W. T. &amp; Wielgosz, R. I. Calibration strategies for FT-IR and other isotope ratio infrared spectrometer instruments for accurate δ13C and δ18O measurements of CO2 in air. Anal. Chem. 89, 3648–3655 (2017).[2] J Viallon et a, Final report of CCQM-P204, comparison on CO2 isotope ratios in pure CO2, 2023 Metrologia 60 08026 DOI 10.1088/0026-1394/60/1A/0802

Monitoring indoor environmental quality is a key requirement for addressing challenges related to smart cities and human health, including indoor air quality. Traditionally, indoor air monitoring has relied on high-end instrumentation, limiting large-scale and widespread deployment. The increasing availability of low-cost sensors offers new opportunities for scalable monitoring systems, provided their metrological performance is properly characterised. In this context, the Italian PRIN project MIRABLE (Measurement Infrastructure for Research on Healthy and Zero Energy Buildings in Novel Living Lab Ecosystems), involving the Italian National Metrology Institute (INRiM) and the Politecnico di Torino, aims to develop a multidomain measurement infrastructure for indoor environments using low-cost sensors in a full-scale living laboratory. At INRiM, a dedicated calibration system was developed to ensure the metrological traceability of CO 2 and CO measurements obtained from low-cost sensors. Reference gas mixtures were prepared gravimetrically according to the International Standard ISO 6142-1 and dynamically diluted to reach low concentration levels. Calibration was carried out using a primary non-dispersive infrared (NDIR) reference analyser and a specially designed calibration chamber. This study presents calibration results for selected CO 2 low-cost sensors and a preliminary evaluation of measurement uncertainty. The same methodology will be extended to low-cost CO and NO x sensors in future work.

We present a novel technique in which the carbon isotope ratio (delta(13)C) of soil CO(2) is measured from small gas samples (<5 mL) injected into a stream of CO(2)-free air flowing into a tunable diode laser absorption spectrometer (TDL). This new method extends the dynamic range of the TDL to measure CO(2) mole fractions ranging from ambient to pure CO(2), reduces the volume of sample required to a few mL, and does not require field deployment of the instrument. The measurement precision of samples stored for up to 60 days was 0.23 per thousand. The new TDL method was applied with a simple gas well sampling technique to obtain and measure gas samples from shallow soil depth increments for CO(2) mole fraction and delta(13)C analysis, and subsequent determination of the delta(13)C of soil-respired CO(2). The method was tested using an artificial soil system containing a controlled CO(2) source and compared with an independent method using the TDL and an open soil chamber. The profile and chamber estimates of delta(13)C of an artificially produced CO(2) flux were consistent and converged to the delta(13)C of the CO(2) source at steady state, indicating the accuracy of both methods under controlled conditions. The new TDL method, in which a small pulse of sample is measured on a carrier gas stream, is analogous for the TDL technique to the development of continuous-flow configurations for isotope ratio mass spectrometry. While the applications presented here are focused on soil CO(2), this new TDL method could be applied in a number of situations requiring measurement of delta(13)C of CO(2) in small gas samples with ambient to high CO(2) mole fractions.