In the first part of this series of papers, we have described the efficiency variations observed in sub/supercritical fluid chromatography (SFC) for a column filled with 5μm particles, in relation to the flow rate variations, at varied temperatures, from 10 to 60°C, with 5% of methanol as modifier mixed to CO2. A cooling effect due to CO2 depressurisation along the column was reported, which decreased the efficiency at high flow rates (up to 6ml/min) for oven temperatures above 35°C. In the same way as the frictional heating observed in UHPLC, this cooling induces a radial temperature gradient which increases the C term of the efficiency curve. In this second part, we achieved a study on the outlet pressure changes (from 8 to 25MPa) on the chromatographic efficiency in term of equivalent plate number (H). Moreover, because the temperature changes related to the cooling effect also modify the fluid viscosity, i.e. the pressure drop, a specific, and simple, data treatment on inlet pressure values is applied to correlate non linear variations of the pressure drop to temperature changes along the column. This data treatment seems to describe the temperature changes along the column, without using temperature measurement. Two phenomena are observed, the previously described cooling effect due to CO2 depressurisation, but also a frictional warming effect, such as the one reported in UHPLC with sub-2μm particles. The combination of these two effects leads to varied and complex efficiency changes depending on the outlet pressure and the flow rates in SFC.
Keywords: CO(2) depressurisation; Cooling effect; Radial gradient temperature; Supercritical fluid chromatography.
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