Modeling and Scale-Up of Size-Exclusion Chromatography

Abstract

Size-exclusion chromatography (SEC) is ubiquitous in the downstream processing of a protein product in modern biotechnology. It is also known as gel filtration chromatography because its separation mechanism relies on the differences in molecular size and shape of solutes, and the separation media are often soft gels. Large molecules elute out of an SEC column first because they cannot penetrate small macropores in particles. Small molecules elute out of an SEC column later because it takes time for them to diffuse in and out of small macropores. In gradient elution of reverse phase, affinity, ion-exchange, or hydrophobic interaction LC, the feed volume can be many times of the column volume [1–3] due to the fact that the gradient operation has a concentrating effect. Because SEC separation does not rely on any binding, gradient elution using an eluitor in the mobile phase to elute out the solutes in the sample is not applicable. Thus, peak bands will invariably diffuse over time due to a lack of any concentrating effect. This means the feed loading volume can only be a small fraction of column volume, leading to the need for very large SEC columns in industrial separations [4–6]. In fact, in the separation of a small molecule from a large molecule, such as in solvent exchange or desalting SEC operations, the feed volume may be up to several percent of the column volume [7], while only a fraction of a percent is possible for the purification of different proteins because protein separation is more difficult [4]. Soft gels are less expensive and thus cost-effective in large-scale SEC [8]. One drawback of soft gels is that scale-up of SEC has to often increase column diameter, rather than column length, because a large pressure drop caused by bed height increase could compress the gels too much.