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 CO₂ 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 CO₂ 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. Copyright © 2011 John Wiley & Sons, Ltd.     

Reconstructing bulk isotope ratios from compound‐specific isotope ratios

Carbon isotope analysis by bulk elemental analysis coupled with isotope ratio mass spectrometry has been the mainstay of δ¹³C analyses both at natural abundance and in tracer studies. More recently, compound‐specific isotope analysis (CSIA) has become established, whereby organic constituents are separated online by gas or liquid chromatography before oxidation and analysis of CO₂ for constituent δ¹³C. Theoretically, there should be concordance between bulk δ¹³C measurements and carbon‐weighted δ¹³C measurements of carbon‐containing constituents. To test the concordance between the bulk and CSIA, fish oil was chosen because the majority of carbon in fish oil is in the triacylglycerol form and ～95% of this carbon is amenable to CSIA in the form of fatty acids. Bulk isotope analysis was carried out on aliquots of oil extracted from 55 fish samples and δ¹³C values were obtained. Free fatty acids (FFAs) were produced from the oil samples by saponification and derivatised to fatty acid methyl esters (FAMEs) for CSIA by gas chromatography/combustion/isotope ratio mass spectrometry. A known amount of an internal standard (C15:0 FAME) was added to allow analyte quantitation. This internal standard was also isotopically calibrated in both its FFA (δ¹³C = ?34.30‰) and FAME (δ¹³C = ?34.94‰) form. This allowed reporting of FFA δ¹³C from measured FAME δ¹³C values. The bulk δ¹³C was reconstructed from CSIA data based on each FFA δ¹³C and the relative amount of CO₂ produced by each analyte. The measured bulk mean δ¹³C (SD) was ?23.75‰ (1.57‰) compared with the reconstructed bulk mean δ¹³C of ?23.76 (1.44‰) from CSIA and was not significantly different. Further analysis of the data by the Bland‐Altman method did not show particular bias in the data relative to the magnitude of the measurement. Good agreement between the methods was observed with the mean difference between methods (range) of 0.01‰ (?1.50 to 1.30). Copyright © 2010 John Wiley & Sons, Ltd.     

Membrane permeation continuous‐flow isotope ratio mass spectrometry for on‐line carbon isotope ratio determination

Gaseous membrane permeation (MP) technologies have been combined with continuous‐flow isotope ratio mass spectrometry for on‐line δ¹³C measurements. The experimental setup of membrane permeation‐gas chromatography/combustion/isotope ratio mass spectrometry (MP‐GC/C/IRMS) quantitatively traps gas streams in membrane permeation experiments under steady‐state conditions and performs on‐line gas transfer into a GC/C/IRMS system. A commercial polydimethylsiloxane (PDMS) membrane sheet was used for the experiments. Laboratory tests using CO₂ demonstrate that the whole process does not fractionate the C isotopes of CO₂. Moreover, the δ¹³C values of CO₂ permeated on‐line give the same isotopic results as off‐line static dual‐inlet IRMS δ¹³C measurements. Formaldehyde generated from aqueous formaldehyde solutions has also been used as the feed gas for permeation experiments and on‐line δ¹³C determination. The feed‐formaldehyde δ¹³C value was pre‐determined by sampling the headspace of the thermostated aqueous formaldehyde solution. Comparison of the results obtained by headspace with those from direct aqueous formaldehyde injection confirms that the headspace sampling does not generate isotopic fractionation, but the permeated formaldehyde analyzed on‐line yields a ¹³C enrichment relative to the feed δ¹³C value, the isotopic fractionation being 1.0026 ± 0.0003. The δ¹³C values have been normalized using an adapted two‐point isotopic calibration for δ¹³C values ranging from ?42 to ?10‰. The MP‐GC/C/IRMS system allows the δ¹³C determination of formaldehyde without chemical derivatization or additional analytical imprecision. Copyright © 2009 John Wiley & Sons, Ltd.     

Testing the use of septum‐capped vials for 13C‐isotope abundance analysis of carbon dioxide

Studying ecosystem processes in the context of carbon cycling and climate change has never been more important. Stable carbon isotope studies of gas exchange within terrestrial ecosystems are commonly undertaken to determine sources and rates of carbon cycling. To this end, septum‐capped vials (‘Exetainers’) are often used to store samples of CO₂ prior to mass spectrometric analysis. To evaluate the performance of such vials for preserving the isotopic integrity (δ¹³C) and concentration of stored CO₂ we performed a rigorous suite of tests. Septum‐capped vials were filled with standard gases of varying CO₂ concentrations (～700 to 4000 ppm), δ¹³C values (approx. ?26.5 to +1.8‰_V‐PDB) and pressures (33 and 67% above ambient), and analysed after a storage period of between 7 and 28 days. The vials performed well, with the vast majority of both isotope and CO₂ concentration results falling within the analytical uncertainty of chamber standard gas values. Although the study supports the use of septum‐capped vials for storing samples prior to mass spectrometric analysis, it does highlight the need to ensure that sampling chamber construction is robust (air‐tight). Copyright © 2010 John Wiley & Sons, Ltd.     

Stable isotope ratio analysis to differentiate temporal diets of a free‐ranging herbivore

Stable isotope ratio analysis (SIRA) of carbon (δ¹³C) and nitrogen (δ¹⁵N) in tissue samples of herbivores can identify photosynthetic pathways (C₃ vs. C₄) of plants consumed. We present results from free‐ranging Rocky Mountain elk (Cervus elaphus) that highlight the ability to differentiate diets using tissue δ¹³C and δ¹⁵N. The signatures of δ¹³C and δ¹⁵N differed in tissues of varying metabolic activity: muscle, a short‐term dietary indicator (i.e., 1–2 months) and hoof, a long‐term dietary indicator (i.e., 3–12 months). We also documented that δ¹³C and δ¹⁵N values along elk hooves (proximal, middle, distal sections) elucidated temporal shifts in dietary selection. The carbon isotopes of the composite hoof were similar to those of the middle section, but the composite hoof differed in δ¹³C from the distal and proximal sections. The δ¹³C and δ¹⁵N signatures also differed among elk populations, indicating temporal dietary shifts of individuals occupying disparate native range and human‐derived agricultural landscapes. Analyses of stable isotopes in various tissues highlighted carbon and nitrogen assimilation through time and differences in the foraging ecology of a rangeland herbivore. Published in 2009 by John Wiley & Sons, Ltd.     

Methods and limitations of ‘clumped’ CO2 isotope (Δ47) analysis by gas‐source isotope ratio mass spectrometry

The geochemistry of multiply substituted isotopologues (‘clumped‐isotope’ geochemistry) examines the abundances in natural materials of molecules, formula units or moieties that contain more than one rare isotope (e.g. ¹³C¹⁸O¹⁶O, ¹⁸O¹⁸O, ¹⁵N₂, ¹³C¹⁸O¹⁶O₂^2?). Such species form the basis of carbonate clumped‐isotope thermometry and undergo distinctive fractionations during a variety of natural processes, but initial reports have provided few details of their analysis. In this study, we present detailed data and arguments regarding the theoretical and practical limits of precision, methods of standardization, instrument linearity and related issues for clumped‐isotope analysis by dual‐inlet gas‐source isotope ratio mass spectrometry (IRMS). We demonstrate long‐term stability and subtenth per mil precision in 47/44 ratios for counting systems consisting of a Faraday cup registered through a 10¹² Ω resistor on three Thermo‐Finnigan 253 IRMS systems. Based on the analyses of heated CO₂ gases, which have a stochastic distribution of isotopes among possible isotopologues, we document and correct for (1) isotopic exchange among analyte CO₂ molecules and (2) subtle nonlinearity in the relationship between actual and measured 47/44 ratios. External precisions of ～0.01‰ are routinely achieved for measurements of the mass‐47 anomaly (a measure mostly of the abundance anomaly of ¹³C‐¹⁸O bonds) and follow counting statistics. The present technical limit to precision intrinsic to our methods and instrumentation is ～5 parts per million (ppm), whereas precisions of measurements of heterogeneous natural materials are more typically ～10 ppm (both 1 s.e.). These correspond to errors in carbonate clumped‐isotope thermometry of ±1.2 °C and ±2.4 °C, respectively. Copyright © 2009 John Wiley & Sons, Ltd.