4 August / September 2019
The Quest for True Ultra High Performance Supercritical Fluid Chromatography: a Mini-Review
by Terry A. Berger, PhD, DIC, 9435 Downing St, Englewood, FL 34224
Some manufacturers of current supercritical fluid chromatographs (SFC’s) routinely claim that their products are ‘ultra-high performance’, similar to ultra-high performance liquid chromatographs (UPLC’s - here defined as reduced plate height ≈ 2 with sub-2 µm particles in 2.1 mm internal diameter (ID) columns). However, none of the current commercially plumbed SFC instruments are any better in terms of extra-column dispersion than instruments available in 1992, which were specifically designed for use with 4.6 mm ID, 150-250 mm long columns packed with 5 µm particles. Despite the fact that fast and ultra-fast separations are an important recent development, today, most users appear to be uninterested or unwilling to make any of the modifications necessary to approach true ultra-high performance SFC.
It is often stated that column technology has outstripped instrumentation, but this does not mean columns 3 or 2.1 mm ID are all well packed. A few groups have attempted to modify commercial SFC instruments to try to determine the quality of small diameter columns packed with sub-2 µm particles. In fact, the results from these studies suggest that few columns used in these reports were actually well packed. The results were often confusing and counter-intuitive, due to the convolution of poor packing and excessive extra-column effects. Thus, both the instruments and the columns were inadequate. This review describes the various attempts, the often confusing results, and a path forward.
Introduction
The use of 2.1 mm ID columns, packed with sub-2 µm particles, producing hmin
≈
2, defi nes ultra-high performance liquid chromatography (UHPLC). In order to achieve a minimum reduced plate height (hmin
) approaching 2, on such columns, very low extra-column dispersion, on the order of a few µL2
, or smaller, is required.
Compared to conventional HPLC’s, UHPLC’s require shorter lengths of 100 or 125 µm vs. 175 µm connector tubing, smaller detector fl ow cells, often less than 1 µL, and smaller injection volumes. In UHPLC such small particles require very high pressure pumps,
Figure 1. Two superimposed injections of caffeine on a 3x20 mm, 1.8 µm RX-Sil column at 1.75 mL/min of 7.5% methanol at 30 °C, 100 bar. The average reduced plate height was 1.65. Average plates were 7099.
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