Identification of Isobaric VOCs in Coffee Aroma with High Resolution PTR-MS

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Analysis of the chemically complex headspace of coffee demonstrates how the high resolving power of the Vocus PTR-TOF 2R enables confident analysis of isobaric VOCs.

Luca Cappellin, Jessica Rodriguez, Michael Cubison, and Felipe Lopez
TOFWERK, Thun, Switzerland

Identification of Isobaric VOCs with High Resolution PTR-MS

Proton Transfer Reaction Mass Spectrometry (PTR-MS) is an ultra-sensitive technique for real-time detection of diverse volatile organic compounds (VOCs).  The breadth of compounds detectable by PTR-MS can hinder analysis of samples containing large numbers of VOCs.  If isobars, which have the same nominal mass, are not resolved by the mass analyzer one cannot confidently identify unknown compounds or quantify specific VOCs of interest.  

The Vocus PTR-TOF 2R has the highest available PTR-MS resolving power. To demonstrate the advantages this brings, the head space of a hot cup of coffee was analyzed by direct sampling into the inlet of a Vocus 2R, without any sample pre-treatment or chromatographic separation.  Coffee is known to contain a complex mixture of VOCs that affect flavor, aroma, and color, and PTR-MS is often used for online monitoring of roasting and direct aroma analysis.    

The recorded mass spectrum contained thousands of unique peaks, with multiple isobaric peaks at most nominal masses.  The figure to the right shows data for 143 Th, where six isobars are observed.  Four of these could be identified based on exact mass.   The figure also shows the data reprocessed to simulate measurement with lower mass resolving power.   With each step down in resolving power, interpretation of the spectrum becomes more difficult and less certain –  demonstrating that confident analysis of VOCs in complex mixtures demands high resolving power.       

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.