M. Brandstetter, C. Koch, A. Genner, B. Lendl:
"Measures for optimizing pulsed EC-QC laser spectroscopy of liquids and application to multi-analyte blood analysis";
Proc. SPIE-The International Society for Optical Engineering (eingeladen), Quantum Sensing and Nanophotonic Devices XI (2014), 8993, 89931U; 11 S.

Kurzfassung englisch:
We employed a broadly tunable pulsed external cavity (EC)-QC laser with a spectral tuning range from 1030 cm-1 to
1230 cm-1 and a tuning speed of 166 cm-1/s for direct absorption spectroscopy of aqueous solutions. The laser offered
spectral power densities of up to four orders of magnitude higher than available with a conventional FTIR spectrometer.
Therefore, a portable demonstration system with a large optical path length transmission flow cell (165 μm) could be
realized preventing clogging of the flow cell. In pulsed mode an EC-QC laser provides significantly higher peak power
levels than in continuous-wave mode, but pulse-to-pulse intensity variations, intra-pulse mode hops and mechanical
imperfections of the scanning mechanism significantly impair the quality of resulting absorbance spectra. This article
reports on measures which we found appropriate to reduce the initially high noise level of EC-QC laser absorbance
spectra. These measures include a spectral self-referencing algorithm that makes use of the inherent structure of the
EC-QC laser's gain curve to correct laser instabilities, as well as Fourier filtering, among others. This enabled us to
derive infrared spectra which were finally useful for quantitative analysis in blood plasma samples. Finally, with the
appropriate measures in place and using partial least squares regression analysis it was possible to simultaneously
quantify 6 blood analytes from a single physical measurement of a 200 μL blood sample. This proves the potential of
EC-QC lasers for practical application in clinical point of care analysis.

Quantum cascade laser, External cavity, Pulsed operation, Liquid, Clinical diagnostics, Blood, Point of Care

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