Design for Capture Chromatography

Key Insight

A batch capture process aims to adsorb a target protein selectively while separating it from unbound or weakly retained contaminants. This process typically involves at least five steps: feed load (where contaminants flow through), wash (to remove unbound/weakly retained contaminants), elution (to desorb the bound protein), clean-in-place (CIP, to remove irreversibly bound species), and equilibration (to restore column conditions). Unbound impurities typically exit rapidly, while other adsorbed contaminants may require specific elution or co-elute with the product. Elution is often performed under conditions where protein binding is minimal due to high sensitivity to mobile phase composition.

Productivity (P) in capture is calculated as the 'Amount of Protein Recovered' divided by the 'Column Volume' and 'Total Cycle Time'. The total cycle time sums the duration of loading, washing, elution, CIP, and equilibration. The amount of protein recovered is approximated by 'eta E' (fraction recovered on elution) multiplied by 'DBC 10%' (dynamic binding capacity at 10% breakthrough) and 'Vc' (column volume). Load time depends on DBC, column length, flow rate, and feed concentration. DBC generally increases with load residence time, leading to an optimum residence time that maximizes productivity. For instance, with favorable isotherms and pore diffusion control, DBC 10% relates to equilibrium binding capacity (EBC) and the number of transfer units, which depends on effective pore diffusivity and particle radius.

Wash time is estimated for removing unbound or linearly retained contaminants; achieving 99 percent removal requires specific calculations based on the error function properties. Elution time is primarily governed by isotherm parameters and mass transfer; in many cases, conditions are chosen for minimal protein binding, and the maximum allowable protein concentration in the product pool (e.g., 5 milligrams per milliliter) dictates the elution duration. CIP time is determined by cleaning agent efficacy and duration, often requiring validation studies. Equilibration time varies with stationary phase chemistry (e.g., short for strong ion exchange, HIC, affinity; longer for weak resins). Column design for capture processes also considers particle rigidity or compressibility, impacting pressure drop calculations; for rigid particles, Darcy's law applies, while compressible media require models like that of Stickel and Fotopoulos.