[Zurück]

S Schaden:
"Application of Mid-Infrared Quantum Cascade Lasers";
Betreuer/in(nen), Begutachter/in(nen): B. Lendl, H. Hoffmann; 164, 2006.

The aim of this thesis was to explore the potential of Quantum Cascade Lasers as light
source for infrared spectroscopy in aqueous phase for new applications. The advantages
of Quantum Cascade Lasers (QCL) compared to fourier transform infrared techniques
(FTIR) concerning the significantly higher light power and the simplicity of the optical
design were used to develop compact and powerful analytical methods. Different kinds
of flow systems for sample preparation as well as for filling the sample cell were
investigated. Additionally innovative concepts for QCL based sampling techniques for
aqueous systems such as simultaneous measuring with two QCLs or wavelength
modulation were performed.
The research results of this thesis are present in 5 papers, which already have been
published (paper I and II), are accepted for publication (paper III) or have been
submitted for publication (paper IV and V) in international scientific journals.
Paper I reports for the first time the on-line hyphenation of a QCL based IR detection to
a capillary electrophoresis (CE) system. In order to succeed with this hyphenation a
dedicated IR-transparent flow cell was constructed. The assessable optical path length
could be increased, from the until now published 10-15 μm in CE-FTIR experiments to
60 μm by use of this powerful mid-infrared laser. Functional group detection i.e.
carbohydrate-detection, was accomplished for quantifying the analytes.
The first simultaneous measurements of two analytes in aqueous solution employing two
quantum cascade lasers is presented in paper II. For the realization an optical set-up was
developed allowing sampling of one flow cell with two QCL beams (Dual-QCL system).
The information of both wavelengths was used for quantitative analysis of glucose and
sodium acetate even if they showed strong overlapping absorption bands typically found
in condensed phase.
The possibility of measuring at two wavelengths simultaneously was applied in paper III
for on-line monitoring of a chemical reaction. The Oxidation of sulphite to sulphate by
hydrogen peroxide was investigated. The short time constant of the system allowed
direct, real-time monitoring of sulphate formation and hydrogen peroxide depletion.
The compensation of baseline drifts via wavelength modulation is shown in paper IV.
Periodical modulation of the QCL driving parameters allows alternating measurements
on two distinct wavelengths. The gained information is used to perform a baseline
correction in order to compensate for influences on the signal of the CO2 measurement
arrising from changes in matrix composition.
Another paper included in this thesis (paper V) reviews the characteristics of QCLs and
their use in gaseous and liquid phase. Different measurement techniques are as well
discussed as their numerous fields of applications.