Proton Transfer Reaction Mass Spectrometry (PTR-MS) is an ultra-sensitive method used for detecting diverse volatile organic compounds (VOCs) in real time. The breadth of compounds detectable by PTR-MS can impede examination of samples comprising of large numbers of VOCs. If isobars, which possess the same nominal mass, are not resolved by the mass analyzer, users cannot assuredly identify unfamiliar compounds or measure specific VOCs of interest.
Identification of Isobaric VOCs with High Resolution PTR-MS
The Vocus PTR-TOF 2R has the highest PTR-MS resolving power available. To highlight the benefits this offers, the head space of a hot cup of coffee was examined by direct sampling into the inlet of a Vocus 2R, without any chromatographic separation or sample pre-treatment. Coffee is known to possess a complex blend of VOCs that influence aroma, flavor, and color, and PTR-MS is frequently used for online monitoring of roasting and direct aroma examination.
The recorded mass spectrum contained numerous unique peaks, with numerous isobaric peaks at very nominal masses. Figure 1 provides data for 143 Th, where six isobars are seen. Four of these could be recognized based on exact mass. It also illustrates the data reprocessed to mimic measurement with lower mass resolving power. With each step down in resolving power, analysis of the spectrum becomes more problematic and less definite — showing that confident examination of VOCs in compound mixtures necessitates high resolving power.
Figure 1. The headspace of a hot cup of coffee—which is known to contain a rich mixture of VOCs—was directly analyzed using a Vocus PTR-TOF 2R operated with a mass resolving power of 13000. Thousands of compounds were observed in the mass range up to 400 Th, with multiple isobaric peaks at most nominal masses. For example, six VOCs were distinctly resolved at 143 Th. Four of these were identified as known coffee constituents based on exact mass. To emphasize the analytical importance of high mass resolution analysis, the Vocus 2R data were reprocessed to simulate measurement at lower resolving powers. At a resolving power of 6000 multiple peaks, including Kojic acid, are no longer identifiable. At mass resolving power 1000, the data appear to be a single peak.
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