Abstract
Broadly tunable external cavity quantum cascade lasers (EC-QCLs) in combination with off-axis integrated cavity enhanced spectroscopy (OA-ICOS) provide high molecular gas sensitivity and selectivity. We used an EC-QCL in the region of 1150–1300 cm−1 in both broadband scan mode, as well as narrow scanning mode around 1216 cm−1, respectively, for detection of acetone in exhaled breath. This wavelength region is essential for accurate determination of breath acetone due to the relative low spectral influence of other endogenous molecules like water, carbon dioxide or methane. We demonstrated that ethanol has a strong spectroscopic influence on the acetone concentration in exhaled breath, an important detail that has been overlooked so far. An ethanol correction is proposed and validated with the reference measurements from a proton-transfer reaction mass spectrometer (PTR-MS) for the same breath samples from ten persons. With the ethanol correction, both broadband and narrowband molecular spectroscopy represent an attractive way to accurately assess the exhaled breath acetone. The importance of considering spectroscopic ethanol influence is essential, especially for the narrowband scans, (e.g., 1216 cm−1), for which the error in determining the acetone concentrations can rise up to 39% if it is not considered.
Highlights
Accurate determination of concentration levels of metabolites in human breath offers a valuable non-invasive way for disease diagnosis and metabolic status monitoring [1]
Normal human breath contains common atmospheric gases such as H2 O, CO2, N2 and O2 in relatively high concentrations, and hundreds of volatile organic compounds (VOCs), among which acetone and ethanol are two examples of the most abundant ones with levels varying from tens of parts-per-billion volume to parts-per-million volume [1,2]
Examples of external cavity quantum cascade lasers (EC-QCLs) measuring broadband absorption spectra include the detection of ethane using wavelength modulation spectroscopy with a sensitivity of 100 ppbv [22], Freon 125 with quartz enhanced photoacoustic spectroscopy (QEPAS) [23] and fluorocarbons at low ppbv levels in real-time [24]
Summary
Accurate determination of concentration levels of metabolites in human breath offers a valuable non-invasive way for disease diagnosis and metabolic status monitoring [1]. Examples of EC-QCLs measuring broadband absorption spectra include the detection of ethane using wavelength modulation spectroscopy with a sensitivity of 100 ppbv [22], Freon 125 with quartz enhanced photoacoustic spectroscopy (QEPAS) [23] and fluorocarbons at low ppbv levels in real-time [24] In both near-IR and mid-IR wavelength regions, acetone has strong absorption bands which overlap with those of ethanol. The present work combines a continuous-wave (cw) external cavity quantum cascade laser, described in detail elsewhere [17], operating between 1120 and 1450 cm with off-axis integrated cavity output spectroscopy (OA-ICOS) [25,26,27] This region is well suited for monitoring breath acetone given the relative low spectral influence of other substances in exhaled human breath, such as carbon dioxide, water vapor, methane, and ethanol at endogenous levels. The EC-QCL has been used in a broadband-scan mode for the region of 1150–1300 cm , as well as in narrowband-scan mode around 1216 cm
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