Mid-infrared quantum cascade lasers for flow injection analysis

A Fabry-Perot quantum cascade laser (QCL) was used as a powerful light sour ce for mid infrared (MIR) detection in flow injection analysis. The QCL las ed at several wavelengths close to each other within a few wavenumbers (990 -1010 cm(-1)), hence fitting well to the broad absorption bands of molecule s in liquid phase. As compared with that of a state-of-the-art Fourier tran sform spectrometer, the signal-to-noise ratio could be improved by a factor of 50. Additionally, by using a QCL as the light source, optical path leng ths of more than 100 mu m could be used even in aqueous matrixes which redu ced the danger of cell clogging. In the example shown here phosphate was de termined in Diet Coke samples. The now injection system used allowed the me asurement of the sample at two pH values (5 and 13) at which the analyte wa s present as H2PO4- and PO43-, respectively. As the analytical readout the difference in IR absorption of H2PO4- and PO43- at the laser wavelengths wa s taken. The FIA-QCL measurements were corroborated by ion chromatography w hich was used as a reference method.