Preparative Chromatography - Preparative Chromatography Apparatus > Page 22
This can be explained on the basis of the relative retention of the different solutes. Anisole is rapidly eluted in front of the overloaded benzyl acetate and, consequently, is only in contact with the portion of the column occupied by the benzyl acetate for a very limited time and thus, is little effected. In contrast the acetophenone is eluted much later than the benzyl acetate and thus, is only effected by the overloading of the stationary phase at the initial stages of development and this causes peak distortion and some retardation. However, as the benzyl acetate moves away from the acetophenone, it is no longer effected, and can be eluted in the normal manner and thus, is not further retarded.
The net effect of mass over load, as opposed to volume over load, is to reduce the retention of the front of all peaks, but the effect is greatest with those solutes that are eluted closed to the over loaded peak. The retention of the back of all the peaks is far less than that of the front. The reduction in retention is by far the greatest for the over loaded peak. It is also clear that in chromatography, column over load is a very effective way of increasing the throughput and by adjusting the selectivity (using temperature, selected stationary phases, or gradient elution) very large sample loads can be tolerated. This approach should always be considered first for moderate loads before contemplating large scale column design.
Preparative Chromatography Apparatus
Very large sample loads will necessitate the use of large scale chromatographic equipment. However, the conventional preparative chromatograph, although certainly more massive, is generally less complex than the analytical chromatograph. Although gradient elution has been used in preparative chromatography, it should be avoided, if possible, due to the cost of solvents and the complication involved in solvent recovery.