The Quantification of Ethanol in Exhaled Breath by Proton-Transfer-Reaction Mass Spectrometry

Proton transfer reaction-time-of-flight-mass spectrometry (PTR-ToF-MS) is used for the real-time, highly sensitive detection and monitoring of volatiles in exhaled breath. However, reliable interpretation of breath volatilome data requires an understanding of the ion-molecule chemistry occurring within the instrument’s drift tube. This is demonstrated in this paper for ethanol, for which quantification based on the observed total product ions to reagent ion signal intensities will be inaccurate owing to the generation of a product ion that is not directly observed, resulting in reduced total product ion signal intensities. In this study, we report the product ion distributions of ethanol as a function of reduced electric field strength under both normal and humid buffer gas operating conditions, with the high humid conditions being more representative for breath analysis. The primary objective is to provide an improved approach for the quantitative determination of ethanol in breath samples. Density functional theory calculations have been used to provide mechanistic insight into the underlying reaction pathways and comparative PTR-ToF-MS measurements are provided of isotopically labelled ethanol isotopologues together with two other alcohols that are commonly present in exhaled breath, namely 1-propanol and 2-propanol. A significant complication in the isotopic studies arises from extensive deuterium-hydrogen exchange between deuterated product ions and H2O, which alters the observed ion distributions under both normal and humid conditions. Collectively, the elements of this study establishes a comprehensive mechanistic framework for ethanol PTR-ToF-MS ion chemistry and demonstrate that accurate quantification of ethanol in exhaled breath requires consideration of primary and secondary product-ion channels that remains experimentally unobserved for ethanol. The findings provide the foundation for the more reliable determination of exhaled ethanol and other primary alcohol concentrations by PTR-ToF-MS that may ultimately reduce, or even eliminate, the need for ethanol-specific instrument calibration which is problematic in clinical settings.