A set-up for fibre-optical in-line near infrared spectroscopic monitoring of hydrocarbons in supercritical CO₂ has been developed, which can be operated under flow-through conditions. It consists of a high-pressure optical cell, which is adapted to an FT-NIR spectrometer via a fibre-optic interface. NIR spectra of squalane in supercritical CO₂ have been measured for varying concentration, temperature and pressure conditions. A partial least squares (PLS) regression model was developed with a calibration set of 36 samples spanning the 0–32 × 10^#0150;3 g cm^#0150;3 squalane concentration range and with CO₂ densities between 0.62#0150;0.86 g cm^#0150;3. Based on the corresponding absorbance data in the 4900#0150;4200 cm^#0150;1 spectral range, squalane and CO₂ were modelled with standard deviations of 0.24#0150;10^#0150;3 and 3.8 × 10^#0150;3 g cm^#0150;3, respectively. Comparable standard deviations were obtained by validating the model with independent test samples. The developed calibration models allow for the in-line quantification of both constituents under varying temperature and pressure conditions from a single spectral measurement. The results obtained encourage the use of NIR spectroscopy/PLS calibration for fast in-line analysis of continuous flow extraction processes for the deoiling of metal parts based on supercritical CO₂.

Keywords: NIR, fibre-optic spectrophotometry, squalane, supercritical CO₂, PLS regression