Determining specific food volatiles contributing to PTR-MS ion profiles using GC-EI-MS

This work focused on developing a method to determine the volatile compounds that contribute to individual masses observed by PTR-MS in the headspace of a food product (e.g., cheese crackers). The process of interfacing a PTR-MS with a GC-MS (electron impact) through an existing sniffing port is outlined, and the problems faced in doing so are discussed. For the interface developed, linearity for both detectors working online for a wide range of concentrations of a selected compound (hexanal) was good (R-2 = 0.88). There was also a good correlation between the responses for both instruments (confidence interval for the slope between 0.56 and 1.18) over a range in concentrations despite the different ionization processes taking place. The application of our system (PTR-MS/GC-MS interface) to a real food system (cheese crackers) in which volatiles were isolated via purge and trap allowed the assignments of most of the PTR-MS masses to major volatile compounds in the samples. However, in this interface it is important to consider some limitations related to GC resolution, compound identification by EI-MS, PTR-MS sensitivity (and overloading), PTR-MS inlet requirements (ca. 20 mL/min), ion chemistry in the PTR-MS, and potentially changing sample composition over time, altering the contribution of a given compound to a specific ion. These issues are discussed.