Column chromatography
Chromatography: Substances present in a mixture are allowed to distribute themselves between two phases: stationary phase and mobile phase. Components that bind weakly to the stationary phase spent a greater proportion of time in the mobile phase and travel faster.
Column chromatography – mixture injected at one end, mobile phase passed through at constant velocity. The fastest moving components take the least time to pass through the column and are eluted first, i.e. they have the shortest retention times. Thin layer chromatography – time of travel rather than column length is fixed. Substances that are weakly absorbed by the stationary phase travel fastest and so they move furthest up the TLC plate, i.e. they have the largest Rf values. Advantages: • Many possible combinations of stationary and mobile phases. • Works with most substances that can be dissolved. • Only small differences in physical properties needed. (e.g. stereoisomers separated by using chiral stationary phase). • Work well on a small scale. Disadvantages: • Relatively expensive, both in terms of equipment and consumables (mobile phase).
Column chromatography – mixture injected at one end, mobile phase passed through at constant velocity. The fastest moving components take the least time to pass through the column and are eluted first, i.e. they have the shortest retention times. Thin layer chromatography – time of travel rather than column length is fixed. Substances that are weakly absorbed by the stationary phase travel fastest and so they move furthest up the TLC plate, i.e. they have the largest Rf values. Advantages: • Many possible combinations of stationary and mobile phases. • Works with most substances that can be dissolved. • Only small differences in physical properties needed. (e.g. stereoisomers separated by using chiral stationary phase). • Work well on a small scale. Disadvantages: • Relatively expensive, both in terms of equipment and consumables (mobile phase).
