An exploratory study on online quantification of isoprene in human breath using cavity ringdown spectroscopy in the ultraviolet

Breath analysis offers a promising method of noninvasive analyses of volatile metabolites and xenobiotics present in human body. Isoprene is one of the highest abundant volatile organic compounds (VOCs) present in human exhaled breath. Breath isoprene (50-200 part per billion by volume (ppbv) or higher) can be analyzed by using mass spectroscopy-based methods, yet laser absorption spectral detection of breath isoprene has not been much reported, partially due to its ultraviolet (UV) absorption wavelength and the spectral overlap with other breath VOCs such as acetone in the same wavelength region. These facts make it challenging to develop a spectroscopy-based breath isoprene analyzer for a potential portable instrument. Here we report on the development of a cavity ringdown spectroscopy (CRDS) system for detection of breath isoprene in the UV region near 226 nm. First, we investigated spectral absorption interferences near 226 nm and selected an optimal detection wavelength at 226.56 nm with minimum to no spectral interference. We then measured absorption cross-sections of isoprene at 225.5-227.4 nm under controlled cavity pressures, and the measured absorption cross-section 1.93 × 10^-17 cm²/molecule at 226.56 nm was used to quantify isoprene in different cases including human breath gas samples. Finally, we validated the CRDS system by measuring breath gas samples from 19 human subjects using proton transfer reaction mass spectrometry (PTR-MS). The CRDS system shows good linear response (R² = 0.999), high stability (0.2%), and high accuracy (R² = 0.906 with PTR-MS). The limit of detection of the system was 0.47 ppbv, with average over 100 ringdown events (equivalent to 5 s). This work represents the first exploratory study of the detection of breath isoprene using CRDS. The results demonstrate the potential of developing a CRDS-based breath analyzer for online, near-real time, sensitive analysis of breath isoprene for further research that would help to elucidate its physiological and clinical significance.

Keywords: Absorption cross-sections; Breath isoprene; Cavity ringdown spectroscopy; Proton transfer reaction mass spectrometry; Ultraviolet.