Use of the optogalvanic effect (OGE) for isotope ratio spectrometry of 13CO2 and 14CO2

The use of isotopic carbon dioxide lasers for determination of carbon (and oxygen) isotope ratios was first demonstrated in 1994. Since then a commercial device called LARA^?, has been manufactured and used for Helicobacter pylori breath tests using ¹³C-labelled urea. The major advantages of the optogalvanic effect compared with other infrared absorption isotope ratio measurement techniques are its lack of optical background and its high sensitivity resulting from a signal gain proportional to laser power. Continuous normalisation using two cells, a standard and sample, lead to high accuracy as well as precision. Recent advances in continuous flow measurement of ¹³C/¹²C ratios of CO₂ in air and extensions of the technique to ¹⁴C, which can be analysed as a stable isotope, are described.     

Partitioning of atmospheric carbon dioxide over Central Europe: insights from combined measurements of CO2 mixing ratios and their carbon isotope composition

Regular measurements of atmospheric CO (2) mixing ratios and their carbon isotope composition ((13)C/(12)C and (14)C/(12)C ratios) performed between 2005 and 2009 at two sites of contrasting characteristics (Krakow and the remote mountain site Kasprowy Wierch) located in southern Poland were used to derive fossil fuel-related and biogenic contributions to the total CO (2) load measured at both sites. Carbon dioxide present in the atmosphere, not coming from fossil fuel and biogenic sources, was considered 'background' CO (2). In Krakow, the average contribution of fossil fuel CO (2) was approximately 3.4%. The biogenic component was of the same magnitude. Both components revealed a distinct seasonality, with the fossil fuel component reaching maximum values during winter months and the biogenic component shifted in phase by approximately 6 months. The partitioning of the local CO (2) budget for the Kasprowy Wierch site revealed large differences in the derived components: the fossil fuel component was approximately five times lower than that derived for Krakow, whereas the biogenic component was negative in summer, pointing to the importance of photosynthetic sink associated with extensive forests in the neighbourhood of the station. While the presented study has demonstrated the strength of combined measurements of CO (2) mixing ratios and their carbon isotope signature as efficient tools for elucidating the partitioning of local atmospheric CO (2) loads, it also showed the important role of the land cover and the presence of the soil in the footprint of the measurement location, which control the net biogenic surface CO (2) fluxes.     

Using a laser-based CO2 carbon isotope analyser to investigate gas transfer in geological media

CO₂ stable carbon isotopes are very attractive in environmental research to investigate both natural and anthropogenic carbon sources. Laser-based CO₂ carbon isotope analysis provides continuous measurement at high temporal resolution and is a promising alternative to isotope ratio mass spectrometry (IRMS). We performed a thorough assessment of a commercially available CO₂ Carbon Isotope Analyser (CCIA DLT-100, Los Gatos Research) that allows in situ measurement of δ ¹³C in CO₂. Using a set of reference gases of known CO₂ concentration and carbon isotopic composition, we evaluated the precision, long-term stability, temperature sensitivity and concentration dependence of the analyser. Despite good precision calculated from Allan variance (5.0 ppm for CO₂ concentration, and 0.05 ‰ for δ ¹³C at 60 s averaging), real performances are altered by two main sources of error: temperature sensitivity and dependence of δ ¹³C on CO₂ concentration. Data processing is required to correct for these errors. Following application of these corrections, we achieve an accuracy of 8.7 ppm for CO₂ concentration and 1.3 ‰ for δ ¹³C, which is worse compared to mass spectrometry performance, but still allowing field applications. With this portable analyser we measured CO₂ flux degassed from rock in an underground tunnel. The obtained carbon isotopic composition agrees with IRMS measurement, and can be used to identify the carbon source.     

Biomedical Application of an Isotope Selective Nondispersive Infrared Spectrometer for 13CO2 and 12CO2 Concentration Measurements in Breath Samples

A newly developed isotope selective nondispersive infrared (NDIR) spectrometer for the measurement of ¹³CO₂ and ¹²CO₂ concentrations in breath samples was applied as a low cost and very simple to operate alternative to isotope ratio mass spectrometry (IRMS). We used this device for several biomedical applications ([¹³C]urea breath test, [¹³C]leucine metabolism, [¹³C]methacetin catabolism of rats) and found that the results agree very well with IRMS.     

Factors influencing stable isotope ratios in CH4 and CO2 within subenvironments of freshwater wetlands: implications for delta-signatures of emissions

Much uncertainty still exists regarding spatial and temporal variability of stable isotope ratios (13C/12C and D/H) in different CH4-emission sources. Such variability is especially prevalent in freshwater wetlands where a range of processes can influence stable isotope compositions, resulting in variations of up to approximately 50% for delta13C-CH4 and approximately 50% for deltaD-CH4 values. Within a temperate-zone bog and marsh situated in southwestern Ontario, Canada, gas bubbles in pond sediments exhibit only minor seasonal and spatial variation in delta13C-CH4, deltaD-CH4 and delta13C-CO2 values. In pond sediments, CO2 appears to be the main source of carbon during methanogenesis either directly via CO2 reduction or indirectly through dissimilation of autotrophic acetate. In contrast, CH4 production occurs primarily via acetate fermentation at shallow depths in peat soils adjacent to ponds at each wetland. At greater depths within soils, sigmaCO2 and H2O increasingly exert an influence on delta13C- and deltaD-CH4 values. Secondary alteration processes (e.g., methanotrophy or diffusive transport) are unlikely to be responsible for depth-related changes in stable isotope values of CH4. Recent models that attempt to predict deltaD-CH4 values in freshwater environments from D/H ratios in local precipitation do not adequately account for such changes with depth. Subenvironments (i.e., soil-forming and open water areas) in wetlands should be considered separately with respect to stable isotope signatures in CH4 emission models.     

Stable isotope determination of ester and ether methyl moieties in plant methoxyl groups

Plant methoxyl groups of lignin and pectin have both distinct stable hydrogen isotope (δ(2)H) and carbon isotope (δ(13)C) values that can be used for studying environmental processes and for investigating the origin and authenticity of biomaterials. Up to now, the reported methods have been applied only to determine isotope values of the bulk plant methoxyl pool. In this work, we have applied several methods to distinguish between stable isotope ratios of methoxyl groups of pectin and the bulk plant methoxyl pool. Our results demonstrate that by applying alkaline hydrolysis to specifically cleave off the ester methyl moiety (pectin-like), we can distinguish δ(2)H and δ(13)C values of the pectin methoxyl pool from the bulk methoxyl pool. No measureable isotope discrimination was observed either during sample preparation or during analytical measurement. Furthermore, using this method, no major isotope difference in either the hydrogen or carbon isotope signature of the methoxyl groups of plant pectin and bulk matter from plant species such as leaves from trees, apples, carrots and potatoes was noted. We show the methanol released during alkaline hydrolysis of plant material and subsequently treated with hydriodic acid to be an excellent procedure to measure specifically and precisely the δ(13)C and δ(2)H isotope values of plant pectin-like methoxyl groups. This method is particularly advantageous when plant matter with a low methoxyl content has to be analysed.     

Continuous measurements of stable carbon isotopes in CO2 with a near-IR laser absorption spectrometer

A near-IR laser absorption spectrometer using a technique of wavelength modulation spectroscopy is used to measure stable carbon isotope ratios of ambient CO₂ (δ¹³C) via the absorption lines ¹²CO₂ R(17) (2ν₁ + ν¹₂ − ν¹₂ + ν₃) at 4978.205 cm⁻¹ and ¹³CO₂ P(16) (ν₁ + 2ν₂ + ν₃) at 4978.023 cm⁻¹. The isotope ratios are measured with a reproducibility of 0.02‰ (1σ) in a 130-s integration time over a 12-h period. The humidity effect on δ¹³C values has been evaluated in laboratory experiments. The δ¹³C values of CO₂ in ambient air were measured continuously over 8 days and agreed well with those from isotope ratio mass spectrometry of canister samples. The spectrometer is thus capable of real-time, in situ measurements of stable carbon isotope ratios of CO₂ under ambient conditions.     

Determination of (13)CO(2)/(12)CO(2) Ratio by IRMS and NDIRS

Abstract Breath tests using (13)C-labelled substrates require the measurement of (13)CO(2)/(12)CO(2) ratio in breath gas samples. Next to isotope ratio mass spectrometry (IRMS), which is very sensitive but also complex and expensive, alternatively isotope selective nondispersive infrared spectrometry (NDIRS) can be used to determine the (13)CO(2)/(12)CO(2) ratio in expired breath. In this study we compared NDIRS- with IRMS-results to investigate whether the less expensive and very simply to operate NDIRS works as reliable as IRMS. For this purpose we applicated 1-(13)C-Phenylalanine to patients with advanced liver cirrhosis and healthy volunteers and took duplicated breath samples for IRMS and NDIRS at selected time points. Our data show a good correlation between these two methods for a small number of samples as required for simple breath tests. Longer series, where repeated measurements are required on the NDIRS instrument lead to a decreasing correlation. This indicates the superiority of IRMS concerning (13)CO(2)-kinetics over longer time periods.     

The effect of internal CO2 conductance on leaf carbon isotope ratio

We estimated internal CO2 conductance (gi) for C3 plant species of different life forms (annual herbs, deciduous trees and evergreen trees) grown in a variety of environments to examine the effect of gi on their leaf carbon isotope ratio (delta13C). The purpose of this study was to test the validity of using delta13C as an index of photosynthetic water use efficiency (WUE). When comparing deciduous tree species grown in contrasting light environments, there was a strong positive relationship between delta13C and WUE. Similarly, delta13C was positively correlated to WUE when comparing the different species of evergreen trees. However, the difference in WUE between evergreen and deciduous tree species did not relate to that in leaf delta13C. In addition, WUE was similar between highland and lowland herbaceous plants, although the former had a much higher delta13C. The positive relationship between delta13C and WUE did not hold across different life forms and different altitudes when differences in gi did not relate to those in stomatal conductance, resulting in independence of chloroplast CO2 partial pressure from intercellular CO2 partial pressure.     

Stable isotope determination of ester and ether methyl moieties in plant methoxyl groups

Plant methoxyl groups of lignin and pectin have both distinct stable hydrogen isotope (δ²H) and carbon isotope (δ¹³C) values that can be used for studying environmental processes and for investigating the origin and authenticity of biomaterials. Up to now, the reported methods have been applied only to determine isotope values of the bulk plant methoxyl pool. In this work, we have applied several methods to distinguish between stable isotope ratios of methoxyl groups of pectin and the bulk plant methoxyl pool. Our results demonstrate that by applying alkaline hydrolysis to specifically cleave off the ester methyl moiety (pectin-like), we can distinguish δ²H and δ¹³C values of the pectin methoxyl pool from the bulk methoxyl pool. No measureable isotope discrimination was observed either during sample preparation or during analytical measurement. Furthermore, using this method, no major isotope difference in either the hydrogen or carbon isotope signature of the methoxyl groups of plant pectin and bulk matter from plant species such as leaves from trees, apples, carrots and potatoes was noted. We show the methanol released during alkaline hydrolysis of plant material and subsequently treated with hydriodic acid to be an excellent procedure to measure specifically and precisely the δ¹³C and δ²H isotope values of plant pectin-like methoxyl groups. This method is particularly advantageous when plant matter with a low methoxyl content has to be analysed.     

Partitioning of atmospheric carbon dioxide over Central Europe: insights from combined measurements of CO 2 mixing ratios and their carbon isotope composition

Regular measurements of atmospheric CO ₂ mixing ratios and their carbon isotope composition (¹³C/¹²C and ¹⁴C/¹²C ratios) performed between 2005 and 2009 at two sites of contrasting characteristics (Krakow and the remote mountain site Kasprowy Wierch) located in southern Poland were used to derive fossil fuel-related and biogenic contributions to the total CO ₂ load measured at both sites. Carbon dioxide present in the atmosphere, not coming from fossil fuel and biogenic sources, was considered ‘background’ CO ₂. In Krakow, the average contribution of fossil fuel CO ₂ was approximately 3.4%. The biogenic component was of the same magnitude. Both components revealed a distinct seasonality, with the fossil fuel component reaching maximum values during winter months and the biogenic component shifted in phase by approximately 6 months. The partitioning of the local CO ₂ budget for the Kasprowy Wierch site revealed large differences in the derived components: the fossil fuel component was approximately five times lower than that derived for Krakow, whereas the biogenic component was negative in summer, pointing to the importance of photosynthetic sink associated with extensive forests in the neighbourhood of the station. While the presented study has demonstrated the strength of combined measurements of CO ₂ mixing ratios and their carbon isotope signature as efficient tools for elucidating the partitioning of local atmospheric CO ₂ loads, it also showed the important role of the land cover and the presence of the soil in the footprint of the measurement location, which control the net biogenic surface CO ₂ fluxes.     

Effect of different lipid extraction methods on delta13C of lipid and lipid-free fractions of fish and different fish feeds

For many ecological applications of stable carbon isotope techniques, it is necessary to separate the lipid and lipid-free fractions. The effect of different lipid extraction methods on the isotope signature of the remaining lipid-free matter as well as the lipid fraction was tested. A hot extraction form of the Soxhlet method using petrol-ether was compared with two liquid-liquid extraction methods for lipid determination described by Bligh and Dyer and Smedes. Solid samples of fish and different natural food items were subjected to extraction and the carbon isotope ratios in lipid and lipid-free matter determined by IRMS. All methods were suitable for lipid extraction from all samples analysed here and did not cause biologically relevant differences (> 1%) in carbon isotopic ratios, except the Bligh and Dyer extraction method using chloroform which caused systematic errors for delta13C when applied to diatoms.     

Stable isotopic analysis of pyrogenic organic matter in soils by liquid chromatography-isotope-ratio mass spectrometry of benzene polycarboxylic acids

Pyrogenic organic matter (PyOM), the incomplete combustion product of organic materials, is considered stable in soils and represents a potentially important terrestrial sink for atmospheric carbon dioxide. One well-established method of measuring PyOM in the environment is as benzene polycarboxylic acids (BPCAs), a compound-specific method, which allows both qualitative and quantitative estimation of PyOM. Until now, stable isotope measurement of PyOM carbon involved measurement of the trimethylsilyl (TMS) or methyl (Me) polycarboxylic acid derivatives by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). However, BPCA derivatives can contain as much as 150% derivative carbon, necessitating post-analysis correction for the accurate measurement of δ13C values, leading to increased measurement error. Here, we describe a method for δ13C isotope ratio measurement and quantification of BPCAs from soil-derived PyOM, based on ion-exchange chromatography (IEC-IRMS). The reproducibility of the δ13C measurement of individual BPCAs by IEC-IRMS was better than 0.35‰ (1σ). The δ13C-BPCA analysis of PyOM in soils, including at natural and artificially enriched 13C-abundance, produced accurate and precise δ13C measurements. Analysis of samples that differed in δ13C by as much as 900‰ revealed carryover of <1‰ between samples. The weighted sum of individual δ13C-BPCA measurements was correlated with previous isotopic measurements of whole PyOM, providing complementary information for bulk isotopic measurements. We discuss potential applications of δ13C-BPCA measurements, including the study of turnover rates of PyOM in soils and the partitioning of PyOM sources based on photosynthetic pathways.     

Determination of 13CO2/12CO2 Ratio by IRMS and NDIRS

Abstract

Breath tests using ¹³C-labelled substrates require the measurement of ¹³CO₂/¹²CO₂ ratio in breath gas samples. Next to isotope ratio mass spectrometry (IRMS), which is very sensitive but also complex and expensive, alternatively isotope selective nondispersive infrared spectrometry (NDIRS) can be used to determine the ¹³CO₂/¹²CO₂ ratio in expired breath. In this study we compared NDIRS- with IRMS-results to investigate whether the less expensive and very simply to operate NDIRS works as reliable as IRMS. For this purpose we applicated 1-¹³C-Phenylalanine to patients with advanced liver cirrhosis and healthy volunteers and took duplicated breath samples for IRMS and NDIRS at selected time points. Our data show a good correlation between these two methods for a small number of samples as required for simple breath tests. Longer series, where repeated measurements are required on the NDIRS instrument lead to a decreasing correlation. This indicates the superiority of IRMS concerning ¹³CO₂-kinetics over longer time periods.     

Isotopic composition and concentration measurements of atmospheric CO2 with a diode laser making use of correlations between non-equivalent absorption cells

Correlations between a sample and a sealed reference cell of a tunable diode laser spectrometer for the measurement of the isotopic composition (δ ¹³C) and the concentration of atmospheric carbon dioxide in air have been investigated. Likely due to fluctuations of the laser emission profile, these correlations have been used to improve the performance of the instrument. In a comparison with isotope ratio mass spectrometer and gas chromatographic measurements, an accuracy of 0.15‰ for δ ¹³C and 0.05 ppmv for the CO₂ concentration is demonstrated for 40 s integration time. Long-term stability and field deployment of the instrument have been investigated during a few days measurement campaign in Paris.     

Stable isotope ratio method for the characterisation of the poultry house environment

Stable isotope analysis was applied to describe the poultry house environment. The poultry house indoor environment was selected for this study due to the relevant health problems in animals and their caretakers. Air quality parameters including temperature, relative humidity, airflow rate, NH₃, CO₂ and total suspended particles, as well as mean levels of total airborne bacteria and fungi count, were measured. Carbon isotope ratios (¹³C/¹²C) were obtained in size-segregated aerosol particles. The carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) isotope ratios were measured in feed, litter, scrapings from the ventilation system, feathers and eggs. Additionally, the distribution of δ¹³C and δ¹⁵N values in different tissues of the chicken was examined. The airborne bacteria and fungi extracted from the air filters collected from poultry farms were grown in the laboratory in media with known isotope values and measured for stable isotope ratios. Analysis of isotope fractionation between microorganisms and their media indicated the applicability of stable isotope analysis in bulk samples for the identification of source material. The analysed examples imply that stable isotope analysis can be used to examine the indoor environment along with its biology and ecology, and serve as an informative bioanalytical tool.     

Carbon isotope analysis in urea at high 13C-abundances using the 13/12CO2-breath test device FANci2

The increasing application of 13C-labelled urea in medicine requires simple and reasonable methods for measuring highly enriched C in urea. The combination: ultimate organic analysis--mass spectrometry so far prescribed is complicated and expensive. For medical diagnosis, however, isotope selective nondispersive infrared spectrometers (NDIRS) have been available for many years. One of these tools is FANci2 which is very reasonable and easily to be operated. By means of such devices also urea highly enriched in 13C can be analysed, provided that the samples are first diluted with a defined amount of urea of natural isotopic composition and then transformed into carbon dioxide by means of urease. The relative abundance of 13C in this carbon dioxide, measured by nondispersive infrared spectrometry, is then a measure of the 13C abundance in the initial urea sample. Comparison of results of such measurements with those attained by mass spectrometry proves that this procedure is feasible and yields precis results.     

Carbon stable isotope composition of modern and archaeological Cornelian cherry fruit stones: a pilot study

The carbon stable isotope content of Cornelian cherry stones collected from wild tree stands in Serbia, SE Europe, was measured using elemental analyser-isotope ratio mass spectrometry, with the aim of recording natural carbon isotope composition of the fruit stones and its possible variation. The results show a significant variation in the carbon isotope values; we identified several environmental factors that, along with a number of other possible determinants, likely contributed to this variation. The obtained data are compared with the measurement of carbon isotope content of an archaeological specimen of Cornelian cherry stone discovered at the Neolithic site of Vin?a (ca. 5600–4500 BC) in Serbia. Notwithstanding the limitedness of the data and the complexity surrounding carbon fractionation and the isotopic variation, it is suggested that the differences/similarities in carbon isotope ratios between modern and archaeological Cornelian cherry stones, when measured for much larger assemblages, could potentially offer a glimpse into growing conditions of Cornelian cherry trees in the past.     

Effect of different lipid extraction methods on δ13C of lipid and lipid-free fractions of fish and different fish feeds

For many ecological applications of stable carbon isotope techniques, it is necessary to separate the lipid and lipid-free fractions. The effect of different lipid extraction methods on the isotope signature of the remaining lipid-free matter as well as the lipid fraction was tested. A hot extraction form of the Soxhlet method using petrol-ether was compared with two liquid-liquid extraction methods for lipid determination described by Bligh and Dyer and Smedes. Solid samples of fish and different natural food items were subjected to extraction and the carbon isotope ratios in lipid and lipid-free matter determined by IRMS. All methods were suitable for lipid extraction from all samples analysed here and did not cause biologically relevant differences (>1) in carbon isotopic ratios, except the Bligh and Dyer extraction method using chloroform which caused systematic errors for δ ¹³C when applied to diatoms.     

Stable isotope mass balances versus concentration differences of dissolved inorganic carbon – implications for tracing carbon turnover in reservoirs

The aim of this study was to identify sources of carbon turnover using stable isotope mass balances. For this purpose, two pre-reservoirs in the Harz Mountains (Germany) were investigated for their dissolved and particulate carbon contents (dissolved inorganic carbon (DIC), dissolved organic carbon, particulate organic carbon) together with their stable carbon isotope ratios. DIC concentration depth profiles from March 2012 had an average of 0.33 mmol L^–1. Increases in DIC concentrations later on in the year often corresponded with decreases in its carbon isotope composition (δ¹³C_DIC) with the most negative value of –18.4?‰ in September. This led to a carbon isotope mass balance with carbon isotope inputs of ?28.5?‰ from DOC and ?23.4, ?31.8 and ?30.7?‰ from algae, terrestrial and sedimentary matter, respectively. Best matches between calculated and measured DIC gains were achieved when using the isotope composition of algae. This shows that this type of organic material is most likely responsible for carbon additions to the DIC pool when its concentrations and δ¹³C_DIC values correlate negatively. The presented isotope mass balance is transferable to other surface water and groundwater systems for quantification of organic matter turnover.