Trace gas detection from fermentation processes in apples; an intercomparison study between proton-transfer-reaction mass spectrometry and laser photoacoustics

Abstract

A custom-built proton-transfer-reaction mass spectrometry (PTR-MS) instrument was used to monitor the emission of various compounds (aldehydes, alcohols, acids, acetates and C-6 compounds) related to fermentation, aroma and flavour, released by four apple cultivars (Elstar, Jonaglod, Granny Smith and Pink Lady) under short anaerobic (24 h) and post-anaerobic conditions. The novel feature of our instrument is the new design of the collisional dissociation chamber, which separates the high pressure in the drift tube (2 mbar) from the high vacuum pressure in the detection region (10 −6 mbar). The geometry of this chamber was changed and a second turbo pump was added to reduce the influence of collisional loss of ions, background signals and cluster ions, which facilitates the interpretation of the mass spectra and increases the signal intensity at the mass of the original protonated compound. With this system, detection limits of similar magnitude to the ones reported in literature are reached. An intercomparison study between PTR-MS and a CO laser-based photoacoustic trace gas detector is presented. The alcoholic fermentation products (acetaldehyde and ethanol) from young rice plants were simultaneously monitored by both methods. A very good agreement was observed for acetaldehyde production. The photoacoustic detector showed about two times lower ethanol concentration as compared to PTR-MS, caused by memory effects due to sticking of compounds to the walls of the nylon tube used to transport the trace gases to the detector.