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

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% CO2), and this effect was used to assess the CO2 concentration of breath gas samples along the way with the analysis of VOCs. Carbon dioxide enhanced the concentration ratio of protonated water clusters (H3O+H2O) to protonated water (H3O+) in the drift tube. Using the observed increase, being ∼60% for 5% CO2, it is estimated that the mobility of water cluster ions in pure CO2 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% CO2) 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% CO2, up to ∼60% for ethanol. This effect of CO2 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 CO2 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.