Laser-spectrometric methods to derive absolute and traceable carbon monoxide (CO) amount fractions in exhaled human breath could be of advantage for early disease detection as well as for treatment monitoring. As proof-of-principle laboratory experiment, we employed intra-pulse and continuous wave (cw) quantum cascade laser spectroscopy (QCLAS), both at 4.6？μm. Additional experiments were carried out applying cw cavity ring-down spectroscopy (CRDS) with a CO sideband laser and a QCL. We emphasize metrological data quality objectives, thatis, traceability and uncertainty, which could serve as essential benefits to exhaled breath measurements. The results were evaluated and compared on a 100？μmol/mol CO level using the two QCLAS spectrometers, and the cw CO sideband laser CRDS setup. The relative standard uncertainties of the pulsed and the cw QCLAS CO amount fraction results were ±4.8 and ±2.8%, respectively, that from the CO sideband laser CRDS was ±2.7%. Sensitivities down to a 3 nmol/mol CO level were finally demonstrated and quantified by means of cw CRDS equipped with a QCL yielding standard uncertainties of about ±2.5 that are exclusively limited by the available line strength figure quality. With this study we demonstrate the achieved comparability of CO quantifications, adhering metrological principles. 1. Introduction The last years exhibited the development of new laser sources such as quantum cascade lasers [1] operating in the mid infrared where the fundamental bands of most infrared active molecules are located. In breath analysis for instance, mid infrared light sources have been used to measure the amount fraction of biomarkers such as acetone or carbon monoxide (CO) found in exhaled human breath using different laser spectroscopic techniques with detection limits down to the pmol/mol level [2]. Carbon monoxide, which we focus on in this work, is in air a pollutant resulting from the incomplete burning of carbon-containing fuels. As a process product it can reach quite large concentration levels of several tens of μmol/mol, expressed as amount fractions. CO in breath of smokers could be found in a range of up to a few μmol/mol, which is well above atmospheric levels (typically 100？nmol/mol). Patients suffering from anaemias, oxidative stress, and respiratory infections have been found with abnormal levels of carbon monoxide. Therefore, CO is discussed as being a biomarker for these diseases. However, because of the low μmol/mol level of CO reported to be present in exhaled breath and because of the presence of several molecules at the same
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