Mid-1 R spectroscopy in medical diagnostics using FTIR-spectroscopy and tunable quantum cascade lasers

Mid-Infrared spectroscopy has proven to be highly specific for the spectroscopic analysis of body fluids, cells and tissues. With the advent of the quantum cascade laser (QCL) in the late nineties, powerful narrow-band single wavelength IR emitters, multi-wavelength sources, or, with an external cavity (EC), tunable EC-QCLs are now available. Their power reaches to hundreds of mW and their tunability can extend over several 100 cm
⁻¹
, sufficiently broad to scan the entire IR fingerprint region within some msec. Probably their most pronounced advantage is their use in a pulsed mode, which makes them an ideal IR light source for photometric measurement of IR radiation absorbed in skin or tissues in combination with photoacoustic or photothermal detection. On this basis, IR spectroscopy can provide a highly sensitive and specific detection of body fluid constituents and lead to point-of-care methods that — at least partly — will substitute clinical laboratory analysis. The lecture presents our most recent developments in QCL applications for the measurement of skin parameters and body fluids in vitro and in vivo in comparison with FT-IR experiments. A photoacoustic detection technology optimized for the ultrasound range (around 50 kHz) with an open absorber/resonator cell is described for the detection of IR absorption in skin layers [1]. More recently, a photothermal detection method was developed that combines an Internal Reflection Element with PhotoThermal Detection (IRE-PTD) [2]. The latter allows the detection of IR absorbance using visible light and appears to be ideal for skin analysis, in particular with a depth profiling on the basis of different modulation frequencies.