The influence of different referencing methods on the accuracy of δ(13) C value measurement of ethanol fuel by gas chromatography/combustion/isotope ratio mass spectrometry

Stable Carbon Isotope Analysis of Hexachlorocyclohexanes by Liquid–Liquid Extraction Gas Chromatography Isotope Ratio Mass Spectrometry: Method Evaluation and Applications

Compound specific isotope analysis (CSIA) and enantiomer specific isotope analysis (ESIA) are powerful tools for assessing the fate of hexachlorocyclohexanes (HCHs) in the environment. However, there is no systematic study on the CSIA and ESIA analysis test methods of the carbon isotopes of HCHs in water and soil environments, in particular the isotope fractionation in the pre-concentration process. We endeavored to test the compatibility of CSIA and ESIA with the liquid–liquid extraction method of HCHs in water. The results showed that there were negligible changes in the δ13C of HCHs after extraction, indicating that liquid–liquid extraction can be used as a pre-concentration method for the determination of δ13C of HCHs in water. The optimized method was validated and then applied to differentiate three HCHs from different manufacturers, to identify in situ degradation of HCHs of groundwater from a contaminated site and to resolve the carbon isotope fractionation occurring in the α-HCH oxidation by CaO2/Fe(II) Fenton system. The results showed that the same reagents from different manufacturers have different carbon isotope compositions, and different isomers from the same manufacturer also have different isotope compositions, showing useful evidence in identifying the source of HCHs. The more enriched δ13C in the down-gradient wells indicated that HCHs have undergone biodegradation or/and chemical reactions in the groundwater system of the site. Carbon isotopic enrichment factors (εC) of −1.90 ± 0.10‰ were obtained in the oxidation process. Hence, the method validated in this study has great potential as a method for identifying the degradation of HCHs in a water environment.

Compound-specific carbon and hydrogen isotope analysis of volatile organic compounds using headspace solid-phase microextraction

Natural flavouring materials are in high demand, and a premium price is paid for all-natural flavourings, making them vulnerable to fraud. At present, compound-specific isotope analysis (CSIA) is perhaps the most sophisticated tool for determining flavour authenticity. Despite promising results, the method is not widely used, and the results are limited to the most common volatile organic compounds (VOCs). This paper describes a robust protocol for on-line measurements of δ¹³C and δ²H using HS-SPME coupled with GC-C-IRMS and GC-HTC-IRMS for common fruit VOCs. To achieve reproducible and accurate results, a combination of a peak size/linearity correction with drift correction were used. Finally, the results were normalised by multiple point linear regression using the known and measured values of reference materials. Special care was taken to avoid irreproducible isotopic fractionation and the effects of equilibration, adsorption, desorption times and temperatures on δ¹³C or δ²H values were examined. Method validation was performed, and the average combined measurement uncertainty (MU) was 0.42‰. All the δ¹³C_VPDB values were below ±3*MU, regardless of analytical conditions. In contrast, for δ²H_VSMOW-SLAP values, only low temperature (30 °C) with equilibration time (15 min) and shorter adsorption time (between 10 and 20 min) can produce an isotopic difference of <10‰. Therefore, method optimisation can minimise MU, and data normalisation and method validation are essential for obtaining meaningful data for use in flavour authenticity studies.

Compound-specific stable carbon isotope analysis of hexabromocyclododecane diastereoisomers using gas chromatography/isotope ratio mass spectrometry

RATIONALE

Compound-specific stable isotope analysis (CSIA) is a powerful tool for the source apportionment and characterization of environmental transformation processes, especially for new emerging contaminants. In this study, we have developed an effective method for determination of the stable carbon isotope ratios of hexabromocyclododecane diastereoisomers.

METHODS

Three diastereoisomers of hexabromocyclododecane (HBCD), α-, β-, and γ-HBCD, were separated on a preparative high-performance liquid chromatography (HPLC) system. Their carbon isotope ratios were determined using gas chromatography/isotope ratio mass spectrometry (GC/IRMS), and compared with data obtained by elemental analyzer/isotope ratio mass spectrometry (EA/IRMS).

RESULTS

α-, β-, and γ-HBCD were well separated by the preparative HPLC system. Method validation results indicated excellent precision and reproducibility. For a series of injection volumes (0.5 to 3 μL), the average carbon isotope ratios for α-HBCD, β-HBCD, and γ-HBCD were -26.42‰, -26.88‰, and -26.43‰, respectively, and their deviations from those of the HBCD standard (-26.52‰) were all lower than the analytical uncertainty of 0.5‰. Relative standard deviations of intra-day and inter-day injections of HBCD were in the ranges 0.35-0.64% and 0.37-0.76%, respectively. Comparison with EA/IRMS further verified the accuracy of the HBCD stable carbon isotope ratio measured by GC/IRMS.

CONCLUSIONS

This work offers a novel approach to separate and concentrate the three major isomers of HBCD and to determine their stable carbon isotope ratios. This permits analysis of their carbon isotope ratios in environmental samples in order to elucidate the sources and abiotic or biological transformation processes of HBCD in the environment.

A high throughput method for total alcohol determination in fermentation broths

Background

The potassium dichromate oxidation method used in determination of alcohols in fermentation has two major disadvantages. This method cannot be used to determine alcohols in raw fermentation broth samples, which often contain various reducing sugars. The method is not environment friendly due to the carcinogenicity of Cr (VI) used.

Results

A new method for determination of reducing sugars and total alcohols in raw fermentation broths was developed. The fermentation broth was pretreated to remove proteins, polysaccharides, glycerol and organic acids. The colorimetric change from both total alcohols and reducing sugars by potassium permanganate oxidation was measured. The portion of colorimetric change from oxidation of reducing sugars was determined by DNS test and subtracted. The remaining portion of colorimetric change was then used to calculate the total alcohol concentration in the sample.

Conclusions

Using this method, total alcohol concentration can be easily and accurately determined in both distilled samples and raw fermentation broth samples. It is fast and environmental friendly.