Modus operandi of modern SFC closely resembles with the operations of reversed-phase liquid chromatography (RPLC). In both techniques methods are started with a high percent of the principal solvent in the mobile phase, which is then gradually replaced with a co-solvent, or modifier, as a function of method time. Co-solvent percents are generally varied linearly, resulting in the solvent gradient. The main difference between RPLC and SFC is that the principal solvent is water in RPLC while in SFC it is CO<sub>2</sub>. Another difference is the compressibility of the mobile-phase. In SFC mobile-phase compressibility is non-negligible under most of the commonly used method conditions. In RPLC, on the other hand, mobile-phase is mostly incompressible.<br/>Although modern SFC widely employs solvent gradient, a clear understanding on the effect of changing composition on analyte retention is not available. Here, SFC can adopt and build upon the fundament work and methodologies developed for HPLC [1]. This adoption can be done in two different ways - (a) mitigating the compressibility effect by operating SFC instruments in average-pressure mode [2], and (b) incorporating the effect of compressibility in the fundamental equation of solvent gradient. In the former approach, because the compressibility is mitigated by the instrument itself, we should be able to employ available HPLC models to predict SFC retention times. In this presentation we will describe both approaches, describing the resultant models and presenting experimental verifications against the proposed models.

Modeling solvent gradient behavior in SFC

253rd National Meeting (2017)

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