Fast Detection of VOCs in Beer Aroma

The detection and resolution of low concentration species linked with beer aroma analysis depend on the sensitivity and speed of the Vocus PTR-TOF.

Terpenes and other aroma compounds regulate the distinctive flavors that influence the beer and wine preferences of consumers. Proton transfer reaction mass spectrometry (PTR-MS) is a rapid and sensitive methodology used to analyze volatile organic compounds (VOCs), such as these beer aroma compounds.

Unfortunately, the analysis of alcoholic beverages with PTR-MS typically presents challenges, as the high-concentration of ethanol titrates the instrument, precluding the precise quantification of lower concentration VOCs which exist in the liquid or headspace. Furthermore, PTR-MS cannot differentiate between isomers, such as different monoterpenes or sesquiterpenes, which embody an identical elemental composition but divergent molecular structures.

Measurement of Low Concentration Beer Aroma VOC Isomers

To measure low concentration VOC isomers, a rapid gas chromatogram (GC) – which can segregate isomers – was coupled in line alongside a TOFWERK Vocus 2R PTR-TOF. Quantitative flavor analysis of alcoholic beverages is rendered possible by the fact that ethanol elutes from the GC well ahead of the majority of aroma compounds.

Figure 1. GC-PTR-MS chromatograms of terpenes in the headspace of a beer sample. Data were acquired with a Fast GC coupled to a Vocus 2R PTR-TOF. The overall analysis time was approximately 15 min. Beer headspace was sampled for 30 s before injection into the Fast GC column that was then ramped from room temperature to 225 °C.

Figure 1 exhibits analysis of beer headspace with the Vocus GC-PTR-TOF. By implementing exact mass analysis, mass spectral peaks at mass/charge 137.1325 Th and 205.1951 Th were established as monoterpenes and sesquiterpenes, respectively.

Figure 2. Mass spectral separation of terpenes from isobaric species. In addition to the need for chromatographic separation of isomers, accurate characterization of terpenes (or other compound classes) by GC-PTR-MS requires that the mass analyzer can resolve any co-eluting compounds that have identical nominal mass and accurately identify species of interest based on exact mass. For example, these data show that at retention time 640 seconds the Vocus 2R resolves multiple compounds at nominal mass 137 Th and treats only the peak at 137.1325 Th as monoterpene signal.

Figure 2 demonstrates a sample of this analysis, wherein the TOF resolves terpenes from co-eluting VOCs embodying equivalent mass/charge. In Figure 1, the multiple chromatographic peaks detected at each of these mass/charge values are isomeric monoterpenes (red) and sesquiterpenes (blue).

Various isomers of each compound class are baseline separated, which permits straightforward measurement. The detection and resolution of these low concentration species with rapid GC depends on the high speed (10 mass spectra per second) and sensitivity (10000 cps/ppbv) of the Vocus PTR-TOF.

Authors

Luca Cappellin¹, Felipe Lopez-Hilfiker¹, Megan Claflin²
1. TOFWERK, Thun, Switzerland 2. Aerodyne Research, Billerica, MA, USA

This information has been sourced, reviewed and adapted from materials provided by TOFWERK.

For more information on this source, please visit TOFWERK.