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Large-volume Injection Coupled to Comprehensive Gas Chromatography – Time-of-Flight Mass Spectrometry for the Analysis of 3-MCPD Esters in Edible Oils
In 2006 Zelinková et al. reported the detection of 3-chloropropane-1,2-diol fatty acid esters (3-MCPD-ester) in edible oils. In native or unrefined fats and oils, no or only traces of 3-MCPD-esters were detectable but in nearly all refined fats and oils, concentrations of 3-MCPD-esters in the range of 0.2–20 mg/kg are present. There are several methods available for the determination of 3-MCPD-esters, from which gas chromatography coupled to mass spectrometry (GC–MS) is the most common technique. However, one of the greatest challenges when using chromatographic separation here is the coelution of compounds of interest with large amounts of matrix constituents.
The current methods for 3-MCPD-ester analysis in edible oils and fats actually measure the total 3-MCPD content of the oil or fat after hydrolysis. The procedures consist of a number of subsequent steps starting with the hydrolysis, removal of the fatty acids (as their FAMEs), extraction of the free 3-MCPD with salting out, derivatization with phenylboronic acid, pre-concentration by solvent evaporation and finally GC–MS analysis. Deuterium labelled 3-MCPD-d5 or esters thereof, are used as internal standards. Potential problems in the procedure are degradation of the 3-MCPDs during (alkaline) hydrolysis resulting in higher detection limits. Furthermore, formation of additional 3-MCPDs is possible if chloride salts are used in the salting out extraction steps.
Several studies have been reported in which large-volume injection (LVI) methods were used for the GC determination of trace pollutants. The LVI technique enables significant improvement of sensitivity of the analytical methods. Rather than using splitless injections of 1–2 µL, with LVI it is possible to inject sample volumes of over 100 µL. Another reason to use LVI can be to simplify sample preparation (e.g., by taking out concentration steps such as solvent evaporation or salting out.)
About a decade ago, a new chromatographic technique for the characterization of complex samples became commercially available: comprehensive two-dimensional gas chromatography (GC×GC), first reported by Phillips et al. GC×GC has a much increased peak capacity offering significantly improved detection limits through chromatographic refocusing. Due to the high peak capacity and the numerous compounds that are resolved in a GC×GC separation, the use of a mass spectrometer is highly desirable for identification and confirmation purposes. Dallüge et al. reported that only MS instruments that can acquire a minimum of 50 full spectra per second allow reliable identification, and subsequent quantification, of the classical narrow peaks in the two-dimensional chromatogram. At present, the time-of-flight mass spectrometer (ToF MS) is the instrument of choice to achieve this since providing full mass range spectra along with high data acquisition rates.
In this work a feasibility study is presented that focuses on the use of LVI coupled to GC×GC–TOF-MS for efficient, more reliable and more sensitive 3-MCPD ester analysis edible in oils and fats.