Over the past decade, the introduction of reliable instrumentation has led to a resurgence of the interest in supercritical fluid chromatography (SFC). Whereas theoretically several solvents could be used as a supercritical fluid, the non-toxicity and non-flammability, and the rather mild conditions to reach a supercritical state, have made sCO2 the mobile phase of choice for SFC. In order to apply the technique to a
wide range of samples over a large enough range of molecular weight and polarity, the sCO2 is mixed with a co-solvent, often MeOH, in volume fraction from 3-5% up to 30-40% in gradient elution. More recently, separations taking up the entire window from pure CO2 up to pure MeOH have been performed for the simultaneous analysis of hydrophilic and lipophilic substances [1]. Whereas a vast amount of studies have been performed to investigate the properties of pure supercritical CO2, only scarce data is available of the typically employed mixtures of CO2-MeOH in SFC. For one of the most important parameters in any separation process, i.e. the diffusion coefficient Dmol, only few compounds, often not relevant for current applications in SFC, have been investigated. Dmol is not only the underlying parameter that drives longitudinal diffusion (B-term) and improves mass-transfer (C-term) in packed bed SFC, but is only an important factor in countering peak dispersion due to packing inhomogeneities (A-term). However, before these contributions can be investigated, it is essential that accurate values for Dmol can be obtained. Whereas several methods exist to measure diffusion coefficients, the use of supercritical fluids put an extra constraint, i.e. high pressure operating, on the experimental system. Luckily, one of the most use methods, i.e. the Taylor-Aris dispersion method, can readily be implemented on modern SFC instrumentation. In this methodology, the dispersion in a well defined piece of open tubular capillary is measured and related to the theoretical expression derived by Taylor-Aris for a straight capillary to derive the diffusion coefficient. In practice, the required tubing length to obtain sufficiently accurately measurable dispersion and to avoid transient effects, is however too large (several meter) for practical implementation. When using coiled tubing, care has to be taken to avoid secondary flow effects that can affect radial dispersion and thus the apparent diffusion coefficient. Finally, the entire set-up needs to be temperature controlled and a method needs to be developed to ensure measurement of the diffusion coefficient in the desired mobile phase and not in the initial sample plug. The present contribution will present novel designed experimental procedure to measure diffusion coefficient in SFC. Dmol-values and the effects of solvent composition, temperature and pressure will be presented for a large range of neutral pharmaceutical molecules.
References:
[1] V. Desfontaine, et al. J. Chromatogr. A, 1562 (2018) 96-107
Original languageEnglish
Publication statusPublished - Jan 2020
Event16th International Symposium on Hyphenated Techniques in Chromatography and Separation Technology - Het Pand, Ghent, Belgium
Duration: 29 Jan 202031 Jan 2020
https://kuleuvencongres.be/htc16

Conference

Conference16th International Symposium on Hyphenated Techniques in Chromatography and Separation Technology
Abbreviated titleHTC-16
CountryBelgium
CityGhent
Period29/01/2031/01/20
Internet address

ID: 49174040