Journal of Chromatography A, 1374 (2014) 247–253 Contents lists available at ScienceDirect Journal of Chromatography A j o ur na l ho me page: www.elsevier.com/locate/chroma Exploring the speed-resolution limits of supercritical fluid chromatography at ultra-high pressures Ruben De Pauw a , Konstantin Shoykhet (Choikhet) b , Gert Desmet a , Ken Broeckhoven a,∗ a Vrije Universiteit Brussel, Department of Chemical Engineering (CHIS-IR), Pleinlaan 2, 1050 Brussels, Belgium b Agilent Technologies Europe, Hewlett-Packard-Strasse 8, 76337 Waldbronn, Germany a r t i c l e i n f o Article history: Received 29 September 2014 Received in revised form 20 November 2014 Accepted 21 November 2014 Available online 27 November 2014 Keywords: Supercritical fluid chromatography Fully porous Superficiallyporous Speed resolution Kinetic plot Ultra-high pressure a b s t r a c t The limits of supercritical fluid chromatography have been experimentally explored using inlet pressures at the limits of the current commercial instrumentation (400–600 bar), as well as pressures significantly surpassing this (up to 1050 bar). It was found that efficiencies in the range of 200,000 theoretical plates can be achieved for a void time t 0 of roughly 6 min using superficially porous particles (2.7 and 4.6 m) while remaining within the pressure limits of current commercial instrumentation and columns. If lower efficiencies are sufficient (<100, 000 plates), smaller particles (e.g. 1.8 m) provide the best trade-off between analysis time and efficiency. Apparent efficiencies of 83,000 (k ′ = 2.2) to 76,000 (k ′ = 6.6) plates could be achieved for void times around 1 min by pushing the pressure limits up to 1050 bar on a column length of 500 mm. As the optimal mobile phase velocity for these small particle columns is even higher, it is required to use narrow-bore columns (2.1 mm ID) to remain within the instrument limits of flow rate. The smaller column volume however puts a strain on the separation efficiency due to extra-column band broadening, resulting in losses up to 50% for weakly retained compounds for column lengths below 250 mm. It is also illustrated that when using sub-2 m particles, especially for separations where a significant amount of organic modifier is required, the current pressure limits of state-of-the-art instru- mentation can sometimes be insufficient. For a gradient running from 4 to 40 v% methanol on a 300 mm column at the optimal flow rate the pressure increases from 420 to 810 bar. Operating SFC-columns with a large pressure gradient induces several (undesired) side effects which have been investigated as well. It has been found that, since the viscosity increases strongly with pressure in SFC, the optimal flow rate and the minimal plate height can significantly change when the column length is changed. Whereas e.g. a 3 × 150 mm column (2.7 m particles) has an optimal flow rate of 1.5 ml/min and minimal plate height of 5.66 m, a 3 × 1050 mm column has an optimal flow rate of 1.2 ml/min and a minimal plate height of 6.25 m. Nevertheless, an increase in operating pressure drop in SFC results in a significant gain in kinetic performance. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Having similar densities as liquids but with viscosities up to 20 times lower (higher diffusion coefficients), supercritical CO 2 is expected to be the ideal (co-)solvent for fast and/or highly efficient separations without mass-transfer limitations or excessive column pressure drops. The higher diffusion coefficient results in a flatter C-term of the van Deemter-curve in supercritical fluid chromatog- raphy (SFC), allowing the use of higher flow rates with no significant performance loss.[1] To assess the limits of separation performance ∗ Corresponding author. Tel.: +32 26293781; fax: +32 26293248. E-mail address: kbroeckh@vub.ac.be (K. Broeckhoven). of a fully optimized chromatographic system (column length, par- ticle size and flow rate), it is convenient to use the Knox and Saleem (KS) limit-equation, which is given by [2] t 0 = h 2 min ·  ·  P · N 2 (1) where t 0 , h min , , , P and N are, respectively, the column hold-up time, the minimum reduced plate height, the mobile phase viscos- ity, the flow resistance, the maximal column or instrument pressure drop and the plate count. From Eq. (1), the advantage of SFC over LC is directly apparent, as the much lower viscosity of the mobile phase leads to a reduced column hold-up time to reach the same plate count. A similar observation was made for Hydrophilic Inter- action Liquid Chromatography (HILIC) separations where the high organic content of the solvent also results in lower mobile phase http://dx.doi.org/10.1016/j.chroma.2014.11.056 0021-9673/© 2014 Elsevier B.V. All rights reserved.