Quantum cascade laser modulation for correction of matrix-induced background changes in aqueous samples

A robust method for the determination of carbon dioxide in sugar containing solutions using a single distributed feedback quantum cascade laser (QCL) has been developed. By switching the driving current between two selected values, 1.6 and 1 A, emission wavelengths at 2341.4 and 2341.6 cm^-1 could be achieved. The method is based on absorbance measurements in transmission and the calculation of the absorbance differences between both wavenumbers. This allows the elimination of indirect matrix interference produced on carbon dioxide measurements with increasing sugar concentrations. A flow injection setup was employed to produce carbon dioxide standards from a series of bicarbonate solutions (0–3 g/l) by adjusting the pH with a sodium hydroxide/citric acid buffer solution to pH 3.13. Different concentrations (0–90 g/l) of sugar were also mixed on line with the analyte to study their influence on carbon dioxide measurement. As the difference in the two evaluated wavelengths is small compared to the absorption peak of CO₂, the analytical readout of the QCL modulation can be seen as a proportional parameter to the first derivative of FTIR spectra in this spectral region. PACS 42.55.Px; 42.62.Fi