ABSTRACT
Atmospheric pressure ionization mass spectrometry (APIMS) was developed for the direct analysis of volatile compounds in the gas phase [1], and the technology was adapted in our laboratory for the measurement of flavor release in vivo on a breath-bybreath basis [2]. Proton-transfer-reaction mass spectrometry (PTRMS) uses the same basic principles as API and has also found applications in flavor analysis [3, 4]. Both techniques sample volatile compounds directly from the gas phase into the ionization source with no prior chromatographic separation. Ionization conditions are set so that formation of the protonated molecular ions is favored, as extensive fragmentation makes interpretation of the resulting ion trace extremely difficult. Maintaining consistent ionization in the source so that all compounds are ionized is a potential problem with all charge transfer ionization processes. Providing the analyte concentration is within the linear range, reliable quantification can be achieved [2], but there are certain conditions that can lead to selective suppression of ionization, leading to inconsistent and nonquantitative results. One example is the analysis of beer and wine samples in which the high ethanol content (about 4% and 12% v/v, respectively) interferes with the ionization of aroma compounds present at trace levels (typically milligrams per kilogram or micrograms per kilogram).
