Accuracy and precision of a laser-spectroscopy approach to the analysis of δ2H and δ18O in human urine

The isotope ratio analysis of body water often involves large sample numbers and lengthy sample processing. Here we demonstrate the ability of isotope ratio infrared spectroscopy (IRIS) to rapidly and accurately analyse the isotope ratios of water in urine. We analysed water extracted from human urine using traditional isotope ratio mass spectrometry (IRMS) and compared those values with IRIS-analysed extracted water and un-extracted urine. Regression analyses for δ²H and δ¹⁸O values between (1) extracted water analysed via IRMS and IRIS and (2) urine and extracted water analysed via IRIS were significant (R ²=0.99). These results indicate that cryogenic distillation of urine was not required for an accurate estimate of the isotopic composition of urine when using IRIS.     

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.     

Microcombustion of ng Amounts of Carbon in Non-Volatile Materials for isotope Ratio Evaluation

Abstract

A novel microcombustion technique for carbon isotopic analysis of nanogram amounts of carbon in non-volatile materials based on isotope ratio monitoring (irm) mass spectrometry is described. Liquid or solid samples placed in a quartz sleeve are combusted at 1000°C in a continuous stream of helium and oxygen. CO₂ removed from the carrier gas stream by cryogenic trapping is transferred onto a GC column. Following GC separation, the CO₂ is transferred via an open split to the ion source of a gas isotope ratio mass spectrometer. Reproducibility for samples >25 nmol carbon is <1‰. Problems associated with blanks from various sources and with reproducible deposition of small sample amounts led to variable accuracy, which was dependent on the compound class being analysed. Minimum sample size was in the range from 5 to 10 nmol carbon. Measurements of dissolved organic carbon (DOC) of groundwater from Germany yielded consistent values of δ¹³C = -28.8‰.     

基于傅里叶变换红外光谱法CO2气体碳同位素比检测研究

介绍了基于傅里叶变换红外技术检测CO₂气体碳同位素比的新方法, 详细介绍了如何从HITRAN红外数据库中提取气体标准吸收截面; 介绍了基于非线性最小二乘法反演CO₂气体碳同位素比和整套实验装置的组成及实验步骤. 从理论和实验分析两方面讨论了温度和气压变化对δ¹³CO₂值的影响规律. 对于同一CO₂标准气体, 采用FTIR和同位素质谱法两种技术进行了δ¹³CO₂值对比检测, 两种测量技术的平均值差异仅为0.25%. 从实验结果可以看出, FTIR技术可以实现对CO₂气体碳同位素比的检测.     

Carbon Isotope Analysis in Urea at High 13C-Abundances Using the 13/12CO2-Breath Test Device Fanci2

Abstract

The increasing application of ¹³C-labelled urea in medicine requires simple and reasonable methods for measuring highly enriched ¹³C 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 precise results.     

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.     

Measurement of δ13C values of soil amino acids by GC–C–IRMS using trimethylsilylation: a critical assessment

In this study, we evaluated trimethylsilyl (TMS) derivatives as derivatization reagents for the compound-specific stable carbon isotope analysis of soil amino acids by gas chromatography–combustion–isotope ratio mass spectrometry (GC–C–IRMS). We used non-proteinogenic amino acids to show that the extraction–derivatization–analysis procedure provides a reliable method to measure δ¹³C values of amino acids extracted from soil. However, we found a number of drawbacks that significantly increase the final total uncertainty. These include the following:

• production of multiple peaks for each amino acid, identified as di-, tri- and tetra-TMS derivatives;

• a number of TMS-carbon (TMS-C) atoms added lower than the stoichiometric one, possibly due to incomplete combustion;

• different TMS-C δ¹³C for di-, tri- and tetra-TMS derivatives.

For soil samples, only four amino acids (leucine, valine, threonine and serine) provide reliable δ¹³C values with a total average uncertainty of 1.3?‰. We conclude that trimethylsilyl derivatives are only suitable for determining the ¹³C incorporation in amino acids within experiments using ¹³C-labelled tracers but cannot be applied for amino acids with natural carbon isotope abundance until the drawbacks described here are overcome and the measured total uncertainty significantly decreased.     

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.     

Isotopic fingerprint of the middle Olt River basin,Romania

One of the most important tributaries of the Danube River in Romania, the Olt River, was characterized in its middle catchment in terms of the isotopic composition using continuous flow–isotope ratio mass spectrometry (CF–IRMS). Throughout a period of 10 months, from November 2010 to August 2011, water samples from the Olt River and its more important tributaries were collected in order to investigate the seasonal and spatial isotope patterns of the basin waters. The results revealed a significant difference between the Olt River and its tributaries, by the fact that the Olt River waters show smaller seasonal variations in the stable isotopic composition and are more depleted in ¹⁸O and ²H. The waters present an overall enrichment in heavy isotopes during the warm seasons.     

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.     

A novel methodological approach for δ18O analysis of sugars using gas chromatography-pyrolysis-isotope ratio mass spectrometry

Although the instrumental coupling of gas chromatography-pyrolysis-isotope ratio mass spectrometry (GC-Py-IRMS) for compound-specific δ¹⁸O analysis has been commercially available for more than a decade, this method has been hardly applied so far. Here we present the first GC-Py-IRMS δ¹⁸O results for trimethylsilyl-derivatives of plant sap-relevant sugars and a polyalcohol (glucose, fructose, sucrose, raffinose and pinitol). Particularly, we focus on sucrose, which is assimilated in leaves and which is the most important transport sugar in plants and hence of utmost relevance in plant physiology and paleoclimate studies. Replication measurements of sucrose standards and concentration series indicate that the GC-Py-IRMS δ¹⁸O measurements are not stable over time and that they are amount (area) dependent. We, therefore, suggest running sample batch replication measurements in alternation with standard concentration series of reference material. This allows for carrying out (i) a drift correction, (ii) a calibration against reference material and (iii) an amount (area) correction. Tests with ¹⁸O-enriched water do not provide any evidence for oxygen isotope exchange reactions affecting sucrose and raffinose. We present the first application of GC-Py-IRMS δ¹⁸O analysis for sucrose from needle extract (soluble carbohydrate) samples. The obtained δ¹⁸O_{sucrose/ Vienna Standard Mean Ocean Water (VSMOW)} values are more positive and vary in a wider range (32.1–40.1 ‰) than the δ¹⁸O_{bulk/ VSMOW} values (24.6–27.2 ‰). Furthermore, they are shown to depend on the climate parameters maximum day temperature, relative air humidity and cloud cover. These findings suggest that δ¹⁸O_sucrose of the investigated needles very sensitively reflects the climatically controlled evaporative ¹⁸O enrichment of leaf water and thus highlights the great potential of GC-Py-IRMS δ¹⁸O_sucrose analysis for plant physiology and paleoclimate studies.     

Observation of 18O/16O isotope effects at the cathode of a polymer electrolyte membrane fuel cell

¹⁸O/¹⁶O isotope effects were observed at the cathode of a polymer electrolyte membrane fuel cell at 25 and 35°C. Results of experiments in which the ¹⁸O/¹⁶O isotope ratios of the oxygen gases supplied to and exhausted from the cell were measured revealed that the lighter isotope ¹⁶O reacted more preferentially to form water molecules at the cathode than the heavier one, ¹⁸O. The value of the oxygen isotope separation factor, S₁, defined as the ratio of the ¹⁸O/¹⁶O isotope ratios of the oxygen gases supplied to and exhausted from the cell, ranged from 1.0030 to 1.0139, and tended to decrease with decreasing rate of oxygen utilisation (θ) and with increasing flow rate of the feed oxygen gas (D_F). The value of another separation factor, S₂, defined as the ratio of the ¹⁸O/¹⁶O isotope ratios of the exhausted oxygen gas and oxygen having reacted to form water molecules at the cathode, ranged from 1.0049 to 1.0304. The S₂ value was much less affected by the change in θ and D_F than the S₁ value with the majority of the S₂ value being in the range of 1.0240–1.0304.     

Gas Forming Processes within Lignite Mining Dumps

Abstract

Gas analyses of the soil atmosphere of lignite mining dumps yielded increased contents of carbon dioxide. To get information about the potential sources and the carbon dioxide releasing capacity of the dumps, samples of dump material were investigated for their contents and isotopic compositions of organic and inorganic carbon as well as the carbon dioxide in the soil atmosphere. The contents of organic and inorganic carbon were found to vary depending on type of dump material. The isotopic composition of the organic carbon ranges between ?24.5 and ?26.5‰, which is typical for humous materials. The carbonates are found to be of marine origin (δ¹³C: +0.5 to ?1.1‰). By means of the isotope investigations it could be shown that the carbon dioxide in the lignite mining dump arises from these two different sources. Mixing ratios can be calculated using the isotope balance equation. Both reaction paths are associated with oxygen consumption and do not result in an increased gas pressure within the dump.     

AgF desalination procedure for the routine determination of oxygen and hydrogen isotopic composition of saline waters and brines

The routine methods for stable oxygen and hydrogen isotope analysis of water involve water–CO₂ gas equilibration and water reduction on hot metal (e.g. Zn, Cr, U) and subsequent mass spectrometric analysis of the evolved gases of CO₂ and H₂ for ¹⁸O/¹⁶O and ²H/¹H ratios, respectively. Precise determination of the isotopic composition of water in brines with application of these standard methods is still problematic and technically often impossible due to detrimental influence of dissolved salts. The new method of brine desalination presented in this study overcomes the problem of the isotope salt effects encountered during the application of the routine techniques for the determination of the isotopic composition of high saline waters. The procedure combines two technical steps: (i) the chemical precipitation of Mg and Ca ions as insoluble non-hydroscopic fluorides, and (ii) the vacuum distillation of water from solution–precipitate mixture. The application of simple vacuum distillation allows full extraction of water and dehydration of remaining salts in a temperature range from 300 to 350?°C without hydrogen and oxygen isotope fractionation. The precision and accuracy of δ¹⁸O and δ²H determination of saline waters and brines with prior application of AgF desalination procedure is comparable with that usually obtained for fresh waters.     

A new 15N tracer method to determine N turnover and denitrification of Pseudomonas stutzeri

Although denitrification is one of the key processes of ecosystem N turnover, the understanding of the regulation of the denitrification pathway is still limited due to the lack of feasible methods for the quantification of N₂ formation. Based on the previously developed isotope pairing method, we present a new in vitro ¹⁵N tracer method for the quantification of N₂ released from denitrification by bacterial cultures. The application of the new method was enabled by replacing the background air in the sample flasks with a gas mixture of He and O₂ with an approximately 50-fold reduced N₂ background (1.7% v/v), allowing for a direct and sensitive quantification of N₂ formation with isotope-ratio mass spectrometry after ¹⁵N-labelling on the one hand, but leaving the method relatively insensitive to intrusion of ambient N₂ on the other hand. The method was tested on bacterial cultures of Pseudomonas stutzeri grown at different oxygen levels. Additionally, NO and N₂O formation were determined with a chemoluminescence analyser and a gas chromatograph, respectively. Following labelling with ¹⁵N-ammonium and ¹⁵N-nitrate, it could be shown that P. stutzeri used ammonium preferably for biomass build-up, and nitrate preferably as electron acceptor. Between 84–107% of the total available N could be recovered. Due to the high sensitivity of the new method only low levels of ¹⁵N tracer were necessary, minimising substrate-induced effects and making this method also an appropriate tool for the use on soil cores. By that it offers a new method for studying denitrification in terrestrial ecosystems.     

Isotope ratio monitoring gas chromatography/Mass spectrometry of D/H by high temperature conversion isotope ratio mass spectrometry

Of all the elements, hydrogen has the largest naturally occurring variations in the ratio of its stable isotopes (D/H). It is for this reason that there has been a strong desire to add hydrogen to the list of elements amenable to isotope ratio monitoring gas chromatography/mass spectrometry (irm-GC/MS). In irm-GC/MS the sample is entrained in helium as the carrier gas, which is also ionized and separated in the isotope ratio mass spectrometer (IRMS). Because of the low abundance of deuterium in nature, precise and accurate on-line monitoring of D/H ratios with an IRMS requires that low energy helium ions be kept out of the m/z 3 collector, which requires the use of an energy filter. A clean mass 3 (HD(+.)) signal which is independent of a large helium load in the electron impact ion source is essential in order to reach the sensitivity required for D/H analysis of capillary GC peaks. A new IRMS system, the DELTA(plus)XL(trade mark), has been designed for high precision, high accuracy measurements of transient signals of hydrogen gas. It incorporates a retardation lens integrated into the m/z 3 Faraday cup collector. Following GC separation, the hydrogen bound in organic compounds must be quantitatively converted into H(2) gas prior to analysis in the IRMS. Quantitative conversion is achieved by high temperature conversion (TC) at temperatures >1400 degrees C. Measurements of D/H ratios of individual organic compounds in complicated natural mixtures can now be made to a precision of 2 per thousand (delta notation) or, better, with typical sample amounts of approximately 200 ng per compound. Initial applications have focused on compounds of interest to petroleum research (biomarkers and natural gas components), food and flavor control (vanillin and ethanol), and metabolic studies (fatty acids and steroids). Copyright 1999 John Wiley & Sons, Ltd.     

Simultaneous thermogravimetry - mass spectrometry for a solid oxide fuel cell cathode: (La0.7Sr0.3)0.9MnO3

A SOFC cathode related perovskite material, (La_0.7Sr_0.3)_0.9MnO₃, has been investigated by simultaneous thermogravimetry - mass spectrometry from room temperature to 1770 K. Water, carbon dioxide and oxygen were detected by mass spectrometry. Water and carbon dioxide evolution can be interpreted by assuming that prior to the thermogravimetry-mass spectrometry measurement about 0.5 % of the lanthanum component had reacted with carbon dioxide and water to form La₂(CO₃)₃*8H₂O, which dehydrated and decomposed via La₂O₂CO₃ into La₂O₃ and evolving H₂O and CO₂ during the present experiment. The observation that the lanthanum strontium manganite emitted oxygen in two stages can be ascribed to the two different oxygen sites in the perovskite lattice, that is, the oxygen excess and deficient regions.     

Isotopic Analysis of Oxygen by Optical Spectroscopy

The isotopic analysis of oxygen (range of concentration: 1 to 31 at. %O¹⁸) by emission spectroscopy (O 1 8446 Å line) by using a Fabry-Pérpt spectrometer with photoelectric recording of the spectral lines is described. The measuring error is within ± 8% (for 1 at. %O¹⁸). The results are compared with those obtained by mass spectrometry.     

Position-specific carbon isotope analysis of trichloroacetic acid by gas chromatography/isotope ratio mass spectrometry

Trichloroacetic acid (TCAA) is an important environmental contaminant present in soils, water and plants. A method for determining the carbon isotope signature of the trichloromethyl position in TCAA using gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) was developed and tested with TCAA from different origins. Position-specific isotope analysis (PSIA) can provide direct information on the kinetic isotope effect for isotope substitution at a specific position in the molecule and/or help to distinguish different sources of a compound. The method is based on the degradation of TCAA into chloroform (CF) and CO? by thermal decarboxylation. Since thermal decarboxylation is associated with strong carbon isotope fractionation (ε = -34.6 ± 0.2‰) the reaction conditions were optimized to ensure full conversion. The combined isotope ratio of CF and CO? at the end of the reaction corresponded well to the isotope ratio of TCAA, confirming the reliability of the method. A method quantification limit (MQL) for TCAA of 18.6 μg/L was determined. Samples of TCAA produced by enzymatic and non-enzymatic chlorination of natural organic matter (NOM) and some industrially produced TCAA were used as exemplary sources. Significant different PSIA isotope ratios were observed between industrial TCAA and TCAA samples produced by chlorination of NOM. This highlights the potential of the method to study the origin and the fate of TCAA in the environment.