Ultra high efficiency/low pressure supercritical fluid chromatography with superficially porous particles for triglyceride separation

• A method development in UHE-LP-SFC is described for a complex mixture (vegetable oil).
• The use of superficially porous particle (SPP) allows reduction in analysis duration.
• Coupling columns to strongly improve efficiency is favoured the low viscosity of supercritical fluids.
• C18 superficially porous particles (SPPs) display changes in selectivity.
• Derringer function allows to take into account different chromatographic parameters.

This paper reports the development of the separation of vegetable oil triglycerides (TG) in supercritical chromatography (SFC), using superficially porous particles (SPPs). The SPP, having a small diameter (2–3 μm), provide a higher theoretical plate number (N), which allows to improve separation of critical pairs of compounds. However, compared to fully porous particles of larger diameter (5 μm), the pressure drop is also increased. Fortunately, supercritical fluids have a low viscosity, which allows coupling several columns to achieve high N values, while maintaining flow rate above 1 ml/min, ensuring a ultra high efficiency (UHE) at low pressure (LP) (below 40 MPa), with regards to the one reached with liquid and sub-two micron particles (around 100 MPa). The use of two detector systems (UV and ELSD) connected in series to the UHE–LP-SFC system provides complementary responses, due to their specific detection principles. Working in a first part with three coupled Kinetex C18 columns (45 cm total length), the effect of modifier nature and percentage were studied with two reference oils, argan and rapeseed, chosen for their different and well-known TG composition. The analytical method was developed from previous studies performed with fully porous particles (FPP). Optimized conditions with three Kinetex were as follows: 17 °C, 12% of ACN/MeOH (90/10; v/v). With these conditions, and by using an increased length of Kinetex C18 column (60 cm), another additional column was selected from ten different commercial SPP C18 bonded phases, by applying a Derringer function on varied parameters: theoretical plate number (TPN), separation index (SI) for critical pairs of peaks (the peaks of compounds difficult to separate due to subtle structural differences), the analysis duration, and the total peak number. This function normalizes the values of any parameters, between 0 and 1, from the worst value to the better, allowing to take account of various parameters in the final choice. Finally, by using four Kinetex C18 plus one Accucore C18 (75 cm total column length), a high-performance separation of triglycerides was achieved, with reasonable analysis duration and isocratic conditions. These conditions can be applied to varied vegetable oils. Identification of the numerous separated peaks of rapeseed oil was achieved by using published data and chromatographic retention behaviour.