Atmospheric pressure chemical ionisation mass spectrometry for in vivo analysis of volatile flavour release

To follow volatile flavour release in the expired air of people during eating, several physiological and analytical constraints must be observed to obtain good quality data. An interface has been developed to sample air from the nose and ionise the volatile compounds contained therein by atmospheric pressure chemical ionisation. The ions formed are detected in a quadrupole mass spectrometer. The interface design overcomes many of the constraints and allows volatile detection in the 100–10 ppbv range, depending on the nature of the volatile compound to be analysed. The principles and operating parameters of the interface are described. Control of the ionisation process is effected through the operating conditions within the interface. Since many compounds are introduced simultaneously into the interface, it is preferable to minimise fragmentation otherwise the spectra are extremely complex and it is difficult to assign ions to compounds unequivocally, with the result that quantification is imprecise. The ionisation parameters are set to favour formation of the protonated molecular (MH+) ion from the compounds and minimise formation of cluster ions, which can also confuse the spectra. The sensitivity and linearity of the technique is demonstrated for a range of flavour volatiles with a dynamic range of three orders of magnitude. The lower limit of sensitivity is determined by the signal to noise ratio whereas the upper limit occurs when all the available charge is exhausted. The technique was designed primarily to monitor real-time changes in the concentration of known volatiles during eating and has limited identification power. However, for reliable quantification, it is necessary to assign the ions monitored in-nose with the volatile compounds present in the food. Besides assignments made on nominal mass, isotopic ratio analysis and accurate mass measurements have also been used to determine the contribution of certain compounds to a particular ion intensity.