Maximizing kinetic performance in supercritical fluid chromatography using state-of-the-art instruments

Recently, there has been a renewed interest in supercritical fluid chromatography (SFC), due to the introduction of state-of-the-art instruments and dedicated columns packed with small particles. However, the achievable kinetic performance and practical possibilities of such modern SFC instruments and columns has not been described in details until now. The goal of the present contribution was to provide some information about the optimal column dimensions (i.e. length, diameter and particle size) suitable for such state-of the-art systems, with respect to extra-column band broadening and system upper pressure limit. In addition, the reliability of the kinetic plot methodology, successfully applied in RPLC, was also evaluated under SFC conditions. Taking into account the system variance, measured at ∼85 μL2, on modern SFC instruments, a column of 3 mm I.D. was ideally suited for the current technology, as the loss in efficiency remained reasonable (i.e. less than 10% decrease for k > 6). Conversely, these systems struggle with 2.1 mm I.D. columns (55% loss in N for k = 5). Regarding particle size, columns packed with 5 μm particles provided unexpectedly high minimum reduced plate height values (hmin = 3.0–3.4), while the 3.5 and 1.7 μm packing provided lower reduced plate heights hmin = 2.2–2.4 and hmin = 2.7–3.2, respectively. Considering the system upper pressure limit, it appears that columns packed with 1.7 μm particles give the lowest analysis time for efficiencies up to 40,000–60,000 plates, if the mobile phase composition is in the range of 2–19% MeOH. The 3.5 μm particles were attractive for higher efficiencies, particularly when the modifier percentage was above 20%, while 5 μm was never kinetically relevant with modern SFC instruments, due to an obvious limitation in terms of upper flow rate value. The present work also confirms that the kinetic plot methodology could be successfully applied to SFC, without the need for isopycnic measurements, as the difference in plate count between predicted and experimental values obtained by coupling several columns in series (up to 400 mm) was on average equal to 3–6% and with a maximum of 13%.