Effects of carbon dioxide in breath gas on proton transfer reaction-mass spectrometry (PTR-MS) measurements

Abstract

PTR-MS is becoming a common method for the analysis of volatile organic compounds (VOCs) in human breath. Breath gas contains substantial and, particularly for bag samples, highly variable concentrations of water vapour (up to ∼6.3%) and carbon dioxide (up to ∼6.5%). The goal of this study was to investigate the effects of carbon dioxide on PTR-MS measurements; such effects can be expected in view of the already well known effects of water vapour. Carbon dioxide caused an increase of the pressure in the PTR-MS drift tube (∼1% increase for 5% CO 2), and this effect was used to assess the CO 2 concentration of breath gas samples along the way with the analysis of VOCs. Carbon dioxide enhanced the concentration ratio of protonated water clusters (H 3O +H 2O) to protonated water (H 3O +) in the drift tube. Using the observed increase, being ∼60% for 5% CO 2, it is estimated that the mobility of water cluster ions in pure CO 2 is almost 65% lower than in air. Carbon dioxide had a significant effect on the mass spectra of the main breath gas components methanol, ethanol, 1-propanol, 2-propanol, acetone, and isoprene. Carbon dioxide caused a small increase (<10% for 5% CO 2) of the normalised main signals for the non-fragmenting molecules methanol and acetone. The increase can be much higher for the fragmenting VOCs (ethanol, propanol, and isoprene) and was, for 5% CO 2, up to ∼60% for ethanol. This effect of CO 2 on fragment patterns is mainly a consequence of the increased abundance of protonated water clusters, which undergo softer reactions with VOCs than the hydronium ions. Breath gas samples stored in Teflon bags lost ∼80% of CO 2 during 3 days, the decrease of VOC signals, however, is mainly attributed to decreasing VOC concentrations and to the loss of humidity from the bags.