Performance of the Wet Oxidation Unit of the HPLC Isotope Ratio Mass Spectrometry System for Halogenated Compounds

The performance of liquid chromatography–isotope ratio mass spectrometry (LC-IRMS) for polar halogenated compounds was evaluated. Oxidation capacity of the system was tested with halogenated acetic acids and halogenated aromatic compounds. Acetic acid (AA) was selected as a reference compound for complete oxidation and compared on the molar basis to the oxidation of other analytes. The isotope values were proofed with calibrated δ¹³C values obtained with an elemental analyzer (EA). Correct isotope values were obtained for mono- and dichlorinated, fluorinated, and tribrominated acetic acids and also for aniline, phenol, benzene, bromobenzene, chlorobenzene, 1,2-dichlorobenzene, 2,4,6-trichlorophenol, pentafluorophenol, and nitrobenzene. Incomplete oxidation of trichloroacetic acid (TCA) and trifluoroacetic acid (TFA) resulted in lower recovery compared to AA (37% and 24%, respectively) and in isotopic shift compared to values obtained with EA (TCA Δδ¹³C_EA/LC‑IRMS = 8.8‰, TFA Δδ¹³C_EA/LC‑IRMS = 6.0‰). Improvement of oxidation by longer reaction time in the reactor and increase in the concentration of sulfate radicals did not lead to complete combustion of TCA and TFA needed for δ¹³C analysis. To the best of our knowledge, this is the first time such highly chlorinated compounds were studied with the LC-IRMS system. This work provides information for method development of LC-IRMS methods for halogenated contaminants that are known as potential threats to public health and the environment

Compound Specific Stable Chlorine Isotopic Analysis of Volatile Aliphatic Compounds Using Gas Chromatography Hyphenated with Multiple Collector Inductively Coupled Plasma Mass Spectrometry

Stable chlorine isotope analysis is increasingly used to characterize sources, transformation pathways, and sinks of organic aliphatic compounds, many of them being priority pollutants in groundwater and the atmosphere. A wider use of chlorine isotopes in environmental studies is still inhibited by limitations of the different analytical techniques such as high sample needs, offline preparation, confinement to few compounds and mediocre precision, respectively. Here we present a method for the δ³⁷Cl determination in volatile aliphatic compounds using gas chromatography coupled with multiple-collector inductively coupled plasma mass spectrometry (GC-MC-ICPMS), which overcomes these limitations. The method was evaluated by using a suite of five previously offline characterized in-house standards and eight chlorinated methanes, ethanes, and ethenes. Other than in previous approaches using ICP methods for chlorine isotopes, isobaric interference of the ³⁶ArH dimer with ³⁷Cl was minimized by employing dry plasma conditions. Samples containing 2–3 nmol Cl injected on-column were sufficient to achieve a precision (σ) of 0.1 mUr (1 milliurey = 0.001 = 1‰) or better. Long-term reproducibility and accuracy was always better than 0.3 mUr if organics were analyzed in compound mixtures. Standardization is carried out by using a two-point calibration approach. Drift, even though very small in this study, is corrected by referencing versus an internal standard. The presented method offers a direct, universal, and compound-specific procedure to measure the δ³⁷Cl of a wide array of organic compounds overcoming limitations of previous techniques with the benefits of high sensitivity and accuracy comparable to the best existing approaches

Coupling of a Headspace Autosampler with a Programmed Temperature Vaporizer for Stable Carbon and Hydrogen Isotope Analysis of Volatile Organic Compounds at Microgram per Liter Concentrations

One major challenge for the environmental application of compound-specific stable isotope analysis (CSIA) is the necessity of efficient sample treatment methods, allowing isolation of a sufficient mass of organic contaminants needed for accurate measurement of the isotope ratios. Here, we present a novel preconcentration techniquethe coupling of a headspace (HS) autosampler with a programmed temperature vaporizer (PTV)for carbon (δ¹³C) and hydrogen (δ²H) isotope analysis of volatile organic compounds in water at concentrations of tens of micrograms per liter. The technique permits large-volume injection of headspace samples, maintaining the principle of simple static HS extraction. We developed the method for multielement isotope analysis (δ¹³C and δ²H) of methyl <i>tert</i>-butyl ether (MTBE), benzene, toluene, ethylbenzene, and <i>o</i>-xylene (BTEX), and analysis of δ¹³C for chlorinated benzenes and ethenes. Extraction and injection conditions were optimized for maximum sensitivity and minimum isotope effects. Injection of up to 5 mL of headspace sample from a 20 mL vial containing 13 mL of aqueous solution and 5 g of NaCl (10 min of incubation at 90 °C) resulted in accurate δ¹³C and δ²H values. The method detection limits (MDLs) for δ¹³C were from 2 to 60 μg/L (MTBE, BTEX, chlorinated ethenes, and benzenes) and 60–97 μg/L for δ²H (MTBE and BTEX). Overall, the HS–PTV technique is faster, simpler, isotope effect-free, and requires fewer treatment steps and less sample volume than other extraction techniques used for CSIA. The environmental applicability was proved by the analysis of groundwater samples containing BTEX and chlorinated contaminants at microgram per liter concentrations

Compound-Specific Hydrogen Isotope Analysis of Heteroatom-Bearing Compounds via Gas Chromatography–Chromium-Based High-Temperature Conversion (Cr/HTC)–Isotope Ratio Mass Spectrometry

The traditional high-temperature conversion (HTC) approach toward compound-specific stable isotope analysis (CSIA) of hydrogen for heteroatom-bearing (i.e., N, Cl, S) compounds has been afflicted by fractionation bias due to formation of byproducts HCN, HCl, and H₂S. This study presents a chromium-based high-temperature conversion (Cr/HTC) approach for organic compounds containing nitrogen, chlorine, and sulfur. Following peak separation along a gas chromatographic (GC) column, the use of thermally stable ceramic Cr/HTC reactors at 1100–1500 °C and chemical sequestration of N, Cl, and S by chromium result in quantitative conversion of compound-specific organic hydrogen to H₂ analyte gas. The overall hydrogen isotope analysis via GC–Cr/HTC–isotope ratio mass spectrometry (IRMS) achieved a precision of better than ± 5 mUr along the VSMOW-SLAP scale. The accuracy of GC–Cr/HTC–IRMS was validated with organic reference materials (RM) in comparison with online EA–Cr/HTC–IRMS and offline dual-inlet IRMS. The utility and reliability of the GC–Cr/HTC–IRMS system were documented during the routine measurement of more than 500 heteroatom-bearing organic samples spanning a δ²H range of −181 mUr to 629 mUr

Organic Reference Materials for Hydrogen, Carbon, and Nitrogen Stable Isotope-Ratio Measurements: Caffeines, <i>n</i>‑Alkanes, Fatty Acid Methyl Esters, Glycines, l‑Valines, Polyethylenes, and Oils

An international project developed, quality-tested, and determined isotope−δ values of 19 new organic reference materials (RMs) for hydrogen, carbon, and nitrogen stable isotope-ratio measurements, in addition to analyzing pre-existing RMs NBS 22 (oil), IAEA-CH-7 (polyethylene foil), and IAEA-600 (caffeine). These new RMs enable users to normalize measurements of samples to isotope−δ scales. The RMs span a range of δ²H_VSMOW‑SLAP values from −210.8 to +397.0 mUr or ‰, for δ¹³C_VPDB‑LSVEC from −40.81 to +0.49 mUr and for δ¹⁵N_Air from −5.21 to +61.53 mUr. Many of the new RMs are amenable to gas and liquid chromatography. The RMs include triads of isotopically contrasting caffeines, C₁₆ <i>n</i>-alkanes, <i>n</i>-C₂₀-fatty acid methyl esters (FAMEs), glycines, and l-valines, together with polyethylene powder and string, one <i>n</i>-C₁₇-FAME, a vacuum oil (NBS 22a) to replace NBS 22 oil, and a ²H-enriched vacuum oil. A total of 11 laboratories from 7 countries used multiple analytical approaches and instrumentation for 2-point isotopic normalization against international primary measurement standards. The use of reference waters in silver tubes allowed direct normalization of δ²H values of organic materials against isotopic reference waters following the principle of identical treatment. Bayesian statistical analysis yielded the mean values reported here. New RMs are numbered from USGS61 through USGS78, in addition to NBS 22a. Because of exchangeable hydrogen, amino acid RMs currently are recommended only for carbon- and nitrogen-isotope measurements. Some amino acids contain ¹³C and carbon-bound organic ²H-enrichments at different molecular sites to provide RMs for potential site-specific isotopic analysis in future studies