Abstract
Increasing column/tubing aspect ratio has been shown in a feasibility study to improve column efficiency when operating in reversed phase mode. This paper contains a thorough investigation on how increases in mobile phase flow and centrifugal force field affect stationary phase retention and column efficiency (as measured by the resolution between adjacent peaks) for columns wound with rectilinear tubing of different aspect ratio. The study uses a Mini CCC instrument operating from 1500 to 2100 rpm (126–246 g) to compare three columns with the same cross-sectional area but different aspect ratio – rectangular horizontal (force field perpendicular to the flat side – aspect ratio 3.125); square (aspect ratio 1.0) and rectangular vertical (flat side parallel with force field – aspect ratio 0.32). Columns are compared by measuring stationary phase retention, resolution and normalized resolution for 3 different mobile phase flow rates 2, 4 and 8 ml/min in both normal phase and reversed phase modes. The results with rectilinear tubing are compared to conventional circular tubing with the same cross-sectional area. The results show that resolution increases with aspect ratio and that at the highest aspect ratio the highest flow rate can maintain a high efficiency only if the highest g-field of 246 g is used. When comparing the rectangular horizontal tubing which gave the best results with conventional circular tubing with the same cross-sectional area a 45% improvement was found in reversed phase mode and a 51% improvement in normal phase mode over the conventional circular cross-section tubing. In other words, a rectangular horizontally wound bobbin with half the length of tubing can achieve the same result as a circular one. These are very significant results for halving separation times analytically or enabling designers to produce new instruments of the same capacity with a much-reduced size.
Highlights
This study builds on the original feasibility study presented by Hewitson at the 9th International Counter-current Chromatography Conference (CCC2016) held in Chicago August 1–3, 2016 and recently published [1]
The variation of stationary phase volume retention (Sf) with gfield for reversed phase (RP) and normal phase (NP) for the highest flow rate of 8 mL/min is shown in Fig. 2a and b respectively
Square tubing with an aspect ratio of 1.0 gives better retention, but by the time g-field has increased to 246 g, high aspect ratio rectangular horizontal (RH) tubing is giving better retention but still not as good as the circular tubing (CIR)
Summary
This study builds on the original feasibility study presented by Hewitson at the 9th International Counter-current Chromatography Conference (CCC2016) held in Chicago August 1–3, 2016 and recently published [1]. It introduces square tubing with an aspect ratio of 1.0 instead of the circular tubing used by Hewitson – so that a full range of aspect ratio can be investigated with rectilinear tubing. Hewitson’s study was inspired by two studies that investigated potential changes in tubing geometry in a conventional J-type coil planet centrifuges. These were by Degenhardt [2] who investigated rectilinear tubing with a small aspect ratio difference and more recently by Englert [3] who showed promising results with crimped tubing
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