Topics - Retention

RetentionA chromatographic system consists of a moving phase and a stationary phase. Solutes placed in the chromatographic system distribute themselves between the two phases, but are moved through the system solely by the moving phase. Consequently, substances distributed preferentially in the moving phase move more rapidly through the system than those preferentially distributed in the stationary phase. Those that are preferentially distributed in the stationary phase are said to be retained by the stationary phase. Retention by the stationary phase is caused by the intermolecular forces (dispersive forces, polar forces and ionic forces) that exist between the solute molecules and those of the stationary phase being greater than the intermolecular forces that exist between the solute molecules and those of the mobile phase. Retention can be measured in a number of different ways. The time between the injection and the elution of the solute peak maximum is called the retention time. The volume of mobile phase passed through the column (corrected if necessary for mobile phase compressibility) between the sample injection and the elution of the peak maximum is called the retention volume. The distance between the point of injection and the peak maximum on the recorder or computer chart (or on a TLC plate) is called the retention distance. The retention of a solute in a chromatographic system is a characteristic of the solute and can be used to help identify the solute.