Affinity columns, mass transfer

The reaction kinetics approximation is mechanistically correct for systems where the reaction step at pore surfaces or other fluid-solid interfaces is controlling. This may occur in the case of chemisorption on porous catalysts and in affinity adsorbents that involve veiy slow binding steps. In these cases, the mass-transfer parameter k is replaced by a second-order reaction rate constant k. The driving force is written for a constant separation fac tor isotherm (column 4 in Table 16-12). When diffusion steps control the process, it is still possible to describe the system hy its apparent second-order kinetic behavior, since it usually provides a good approximation to a more complex exact form for single transition systems (see Fixed Bed Transitions ). 

Horstmann and Chase [35] have used the mass transfer parameters determined in stirred tank experiments to simulate the breakthrough curves of affinity chromatography experiments. Numerical methods using different computer packages were carried out to solve the differential equations of the stirred tank adsorption and to predict the performances of a packed bed chromatographic column. 

Chemical stability of carbon over the entire pH range has led to considerable interest in the development of carbon-based stationary phases for RPC. Porous graphitised carbon with sufficient hardness, well-defined and stable pore structure without micropores, which ensures sufficient retention and fast mass transfer can be prepared by a complex approach consisting of impregnation of the silica gel with a mixture of phenol and formaldehyde followed by formation of phenol-formaldehyde resin in the pores of the silica gel, then thermal carbonisation and dissolution of the silica gel by hydrofluoric acid or a hot potassium hydroxide. solution [48. The retention and selectivity behaviour of carbon phases significantly differs from that of chemically bonded pha.ses for RPC. Carbon adsorbents have greater affinity for aromatic and polar substances so that compounds can be separated that are too hydrophilic for adequate retention on a Cix column. Fixed adsorption sites make these materials more selective for the separation of geometric isomers [49]. 

One way in split-peak measurements can be performed is by injecting a small amount of analyte onto an affinity column at various flow rates. A plot of the inverse negative log of the measured free fraction is then made versus the flow rate. The slope of this graph is related to the adsorption kinetics and mass-transfer rates within the column. If the system is known to have adsorption-limited retention or if the mass-transfer rates are known, then the association rate constant for ana-... 

To investigate the effects of the different mass transfer mechanisms, breakthrough curves were generated on model monoclonal antibody affinity columns with two types of packings Sepharose 4B (Pharmacia) and controlled-pore glass (Electronucleonics, mean pore size 1273 ft) Mouse monoclonal anti-benzenearsonate IgG was produced in this laboratory by batch culture in a 15 L fermentor. The IgG was purified... 

When a mixture of two components is injected in the chromatographic column, separation occurs since each component moves according to its own adsorption affinity with the solid phase. While traveling in the column each peak broadens due to the nonequilibrium nature of chromatography as manifested, for example, by the existence of mass transfer resistance between fluid and solid phase. These states for each peak are qualitatively described by moment equations. 

---