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B. Lendl, M. Brandstetter:
"External Cavity Quantum Cascade Lasers for Measurement of Liquids";
Hauptvortrag: The Great Scientific Exchange, Kansas City, MO, USA; 17.10.2010 - 21.10.2010.

Widely tunable External Cavity Quantum Cascade Lasers (EC-QCL)
open a broad field of potential applications for absorption
spectroscopy of liquids, e.g. in medical diagnosis and process
analytical chemistry. There, both tunability and high emission power
are crucial, especially when measuring in highly absorbing matrices,
e.g. water. The high emission power facilitates large optical
pathlengths which brings advantages in terms of robustness (no
clogging of transmission cells) and higher signals according to Beer´s
law. Since the absorption bands in liquids are relatively broad (in the
range of tens of wavenumbers) the lasers can be operated in pulsed
mode where they offer higher peak pulse power levels and broader
tuning ranges compared to mode-hop free operation. Over decades
the gold standard in mid-IR spectroscopy has been FT-IR
spectrometry. Given the several magnitudes lower spectral power
density of the thermal light sources used there EC-QCLs should come
along with several spectroscopic benefits. In this context the
achievable signal-to-noise ratios (SNR) and the parameters
influencing the SNR will be discussed. Furthermore, suitable
methods for a comprehensive characterization of EC-QCLs are
presented. Two field applications of an EC-QCL based sensor system
(200 cm-1 tuning range) are presented: Firstly, a mid-IR absorption
sensor for routine point-of-care blood analysis was developed and
applied in an intensive care environment. Combined with
multivariate data analysis the sensor could simultaneously quantify
several physiologically relevant blood parameters in human blood
plasma, including glucose, triglycerides and albumin. Secondly, the
sensor was applied as a realtime analyzer for monitoring the degree
of contamination of cleaning water in a cleaning in place (CIP)
system. Compared to standard analyzers which are measuring the
total organic carbon the QCL-based system not only offered
significantely improved time resolution but also information about
the type of contamination present in the CIP equipment. Finally, a
short outlook on potential new applications of EC-QCLs for
measurements in the liquid phase will be given, including the
combination of EC-QCLs and evanescent field sensors.