Effects of selected parameters on the response of the evaporative light scattering detector in supercritical fluid chromatography

Due to the renewal of the use of supercritical fluid chromatography (SFC), the coupling of SFC with evaporative light scattering detection (ELSD) needs to be revisited. Indeed, SFC and ELSD apparatus have evolved, while understanding of the nebulisation process has improved. This detector, first developed for polymer analyses can be used for a lot of compounds that do not display sufficient UV absorption. Because the response of ELSD is related to numerous parameters, such as mobile phase composition, the velocity difference between the mobile phase and the nebulisation gas, or the nebulisation temperature, many additional studies are necessary to understand its behaviour in SFC as compared to HPLC. The effects of the nature of co-solvent added to carbon dioxide (methanol, ethanol and acetonitrile), the injected volume and the flow rate on the ELSD response were studied. Certain parameters induce great changes on the nebulisation recovery (the proportion of mobile phase entering the drift tube), allowing for a dramatic improvement in peak area. Moreover, effects of the particle size in the aerosol were also observed, related, as expected, to the modifier nature, but strangely also to the injected volume. Specific and positive behaviour of the ELSD detection on the apparent peak efficiency was confirmed in SFC, for large injected volumes. Finally, a flow rate increase reduces peak area, showing that the use of high flow rates, although attractive to shorten analysis duration, is not favourable to ELSD response. Numerous parameters acting on detector response are modified by the flow rate change, such as the mobile phase velocity. Moreover, measurement of the nebulisation chamber temperature shows the strong decrease of temperature for high flow rates, due to the supercritical fluid depressurisation.