Abstract
Carbon monoxide (CO) is an important biomarker as it originates in the human body from the heme (component of hemoglobin) degradation. Tunable laser absorption spectroscopy in the mid-infrared wavelength region is used for sensitive trace gas sensing of exhaled carbon monoxide (CO). Based on a quantum cascade laser emitting at 4.61 µm, two different spectroscopic methods are investigated: off-axis integrated cavity output spectroscopy (OA-ICOS) and wavelength modulation 2f/1f spectroscopy (WMS). The optical sensors integrate a slow feedback system to correct for wavelength drifts improving their stability over days. Both approaches demonstrate a high reproducibility and sensitivity during online measurements of exhaled human breath. Considering the detection limit to be the equal to the standard deviation of the background fluctuations, the noise-equivalent detection limit for both OA-ICOS and WMS is 7 ppbv (1-s averaging time), leading to a noise-equivalent absorption sensitivity of 3.1 × 10−7 cm−1 Hz−1/2, which is sufficient for measurements of exhaled CO (eCO). Collection and measurements of eCO samples were investigated, and different exhalation flow rates and breath-holding time were explored, to provide a reliable sampling method for future medical investigations.
Highlights
The sampling of biomarkers in exhaled breath is of interest for the clinical practice, because of its noninvasive character as compared to other sampling methods
Continuouswave quantum cascade lasers (QCL) in combination with absorption spectroscopy are a good choice because QC lasers are compact, have a narrow linewidth and milliWatt power levels in the mid-infrared wavelength region
In off-axis integrated cavity output spectroscopy (ICOS) (OA-ICOS), the light is injected into the high fines cavity in such a way that the free spectral range (FSR) of the cavity is less than the laser linewidth; as such, many cavity modes couple to the laser light [29, 30]
Summary
The sampling of biomarkers in exhaled breath is of interest for the clinical practice, because of its noninvasive character as compared to other sampling methods. Major research efforts are directed toward simple, portable sensors with ppbv (parts-per-billion, 1:109) sensitivity and subsecond time resolution For such an approach, continuouswave quantum cascade lasers (QCL) in combination with absorption spectroscopy are a good choice because QC lasers are compact, have a narrow linewidth and milliWatt power levels in the mid-infrared wavelength region. CRDS involves coupling the laser beam with a high-finesse optical cavity This method requires precise alignment and a good mode-matching between the laser frequency and the free spectral range (FSR) of the cavity. In off-axis ICOS (OA-ICOS), the light is injected into the high fines cavity in such a way that the FSR of the cavity is less than the laser linewidth; as such, many cavity modes couple to the laser light [29, 30] In such a setup, the alignment is robust as the laser beam does not optically require strong mode-matching to the cavity. The sensors have been validated with online measurements of exhaled human breath
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