Mayhew, Chris A.Chris A.MayhewHerbig, JensJensHerbigBeauchamp, Jonathan D.Jonathan D.Beauchamp2025-12-192025-12-19202097801281996719780128223970https://publica.fraunhofer.de/handle/publica/50231110.1016/B978-0-12-819967-1.00010-42-s2.0-85143312105Breath analysis was among the earliest applications explored for proton transfer reaction-mass spectrometry (PTR-MS) when it was first developed just over a quarter of a century ago. PTR-MS retains a strong presence in breath research due to its high sensitivity, online detection capabilities, and broad dynamic range, allowing for direct real-time breath-by-breath analysis of multiple analytes simultaneously. Technological developments, particularly the introduction of a time-of-flight (TOF) mass analyzer (PTR-TOF-MS), as well as greatly improved sensitivity and ultratrace level detection limits, have pushed the boundaries of real-time breath analysis applications. This chapter introduces PTR-MS and reviews key applications and discoveries in breath and related topics, with technical highlights and prospective future developments.enfalseBuffered end-tidal samplingPharmacokineticsPTR-MSReal-time breath analysisTime-of-flightbook article