Intube headspace extraction for the determination of the diacetyl offflavor in beer

In the quality control of beer, several VOCs are monitored. These compounds include C3-C5 alcohols, C2-C5 esters, dimethyl sulfide and 1,2-diketones (diacetyl, 2,3-pentanedione). These compounds are present at different concentration levels ranging from tens of ppm (alcohols) to ppb level (diacetyl).

Beer samples are normally analyzed by SHS in combination with GC. In order to cover all solutes and concentration levels, often several runs are needed per sample. Alternatively, the analysis is performed using effluent splitting to three detectors: FID for alcohols and esters, selective sulfur detection (FPD, PFPD) for dimethyl sulfide (DMS) and ECD for diketones. The three detectors allow sufficient sensitivity and selectivity, but this set-up is rather complicated and problems with splitters and thus robustness are encountered. With mass spectroscopic detection, all solutes can be detected using the simultaneous scan and selected ion-monitoring (SIM) acquisition of state-of-the-art MS systems. However, for some solutes, the sensitivity of mass spectroscopic detection is at the limit, especially in combination with SHS.

The total ion chromatograms obtained for a beer sample using classical SHS sampling and ITEX sampling are compared in Figure 3A and 3B, respectively. Both chromatograms represent the datafiles obtained in scan acquisition mode. Ethanol, the most abundant peak, elutes at 6 min. The peak at 4 min corresponds to the air peak (MS scan from m/e 29). It is clear that more peaks are detected using the ITEX sampling. The following solutes could be identified using the mass spectra: 1-propanol (peak 1), ethyl acetate (peak 2), 2-methyl-1-propanol

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Figure 3 Total ion chromatograms for beer by SHS (A) and ITEX (B) sampling, and SIM chromatograms at m/z 86 for diacetyl by SHS (C) and HSSE (D). Belgian lager beer (10 mL) was placed in a 20-mL headspace vial and the samples were analysed as such. The SHS conditions were as follows: sample conditioning at 80°C for 15 min, 2.5 mL headspace; the ITEX conditions were sample conditioning at 80°C for 15 min, 10 extraction strokes of 1 mL at 50 mL/s and desorption at 250°C with 1 mL headspace at 50 mL/s. The analyses were performed on an Agilent 6890 GC — 5975 MSD combination equipped with a 20-m L x 0.18-mm ID x 1 mm df DB-VRX column. The carrier gas was helium at 200 kPa constant pressure. Injection was split at 1/25. The oven was programmed from 40°C, 5 min, at 10°C/min to 250°C, 10 min. The MSD transfer line was set at 250°C. The MS was operated in the scan/SIM mode with 50 ms dwell times.

(peak 3), ethyl propanoate (peak 4), 3-methyl-1-butanol (peak 5), 2-methyl-1-butanol (peak 6), 2-methyl propyl acetate (peak 7), ethyl butyrate (peak 8), 3-methyl butyl acetate (peak 9) and 2-methyl butyl acetate (peak 10).

Also DMS could be detected at 7.7 min (Figure 3B). Using an extracted ion chromatogram (EIC), the peak can be quantified without problem in the beer sample. The signal-to-noise, measured on ion m/z 62 was 70. The concentration of DMS in this sample was 8 ppb. In the chromatogram obtained by SHS, DMS was difficult to detect (Figure 3A). Only in an extracted ion trace, a small peak with signal-to-noise of 5 could be detected, but no library search confirmation was obtained. The sensitivity was thus increased by a factor of more than 10 for this compound using ITEX sampling. The EICs for ion m/z 86, typical for diacetyl, obtained by GC-MS in SIM mode are compared in Figure 3C and 3D. At 13.1 min, diacetyl can be detected in the chromatogram obtained by SHS only as a trace (S/N = 8 in Figure 3C). Using ITEX, the peak can be detected more easily (Figure 3D) and confirmation of the identity through the relative ratios of target and qualifier ions is possible. The S/N value obtained by ITEX was 44 or 6 times higher than with SHS. The concentration of diacetyl was in the order of 10 ppb.

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