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M. Brandstetter, A. Genner, L. Volgger, B. Lendl:
"Liquid phase analysis using a pulsed external cavity quantum cascade laser for mid-IR spectroscopy";
Hauptvortrag: Sixth International Conference on Advanced Vibrational Spectroscopy. ICAVS VI, Sonoma County. California; 12.06.2011 - 17.06.2011; in: "Icavs Vi", aus Umweltschutzgründen nicht gedruckt, einsehbar unter: http://www.icavs6.org/attendees/presentingauthor/list/icavs/, (2011).

Liquid phase laser absorption spectroscopy of aqueous solutions in the mid-IR regime mainly relies on high tunability and sufficient emission power. Both qualities are provided by External Cavity - Quantum Cascade Lasers (EC-QCL) which makes them the first choice. The typical absorption bandwidths in liquids are in the range of tens of wavenumbers. Other than for gases it is therefore not strictly necessary to use mode-hop free continuous-wave laser sources [1]. In fact, pulsed EC-QCLs, which offer higher peak pulse power levels and broader tuning ranges can be employed. However, in order to achieve low noise measurements a high wavelength accuracy as well as repeatability in the emission power is crucial.

In our contribution we report on a comprehensive characterization of a pulsed EC-QCL for application in liquid phase absorption measurements from a spectroscopist´s point of view. The performance of broadband pulsed mode EC-QCLs is mainly affected by two factors: The pulse-to-pulse power fluctuations caused by mode-hopping and spectral non-linearities caused by mechanical instabilities of the laser grating. The correction of these effects is usually done by pulse averaging and by setting up a reference channel including an etalon to achieve wavelength-calibration. In our work, however, we aimed on the realization of a single-channel setup with regard to portable sensor applications which allows to avoid the use of etalons and additional moving parts, such as beam modulators. Our laser source was a pulsed EC-QCL (Daylight Solutions Inc.) which offered a tuning range between 1030 cm 1 and 1230 cm 1 with a peak pulse power of 350 mW.

The emission of the available EC-QCL was characterized by time resolved step-scan FTIR spectroscopy at a spectral resolution of 0.1 cm 1. For measuring the emission characteristics during a laser pulse step-scan operation of the FTIR spectrometer was triggered by single pulses of the laser reaching a time resolution of 2 ns. Full spectral scans were recorded again using step-scan FTIR spectroscopy. Here the spectrometer was triggered by the start of each scan. From the obtained data a novel sample evaluation protocol was developed which allowed to significantly improve the repeatability of the recorded EC-QCL spectra. Signal stabilities better than 0.5 mAU for a single spectral scan without reference channel were achieved when measuring through 140 µm of water. This lead to 3 mg/dL (30 ppm) limit of detection [2] for glucose in water, which is sufficient for clinical glucose monitoring where the sensitivity should be better than 10 mg/dL. The result was a simple room-temperature operated sensor concept consisting of only the EC QCL, a flow cell, a mirror and a thermoelectrically-cooled MCT-detector.

[1] G. Wysocki, R.F. Curl, F. Tittel, R. Maulini, J.M. Bulliard, J. Faist "Widely tunable mode-hop free external cavity quantum cascade laser for high resolution spectroscopic applications ", Applied Physics B 81, pp. 769-777 (2005). [2] M. Brandstetter, A. Genner, K. Anic, B. Lendl "Tunable external cavity quantum cascade laser for the simultaneous determination of glucose and lactate in aqueous phase" Analyst 135., pp. 3260-3265 (2010).