Topics - Retention Gap

Retention Gap The ‘split injector’ does not allow a representative sample to enter a capillary column due to the different diffusivities of the components of the sample mixture. In general low molecular weight materials with higher diffusivities are lost preferentially to the high molecular weight low diffusivity solutes. In an attempt to improve the sampling process, large bore capillary columns were introduced that allowed a syringe to enter the column and discharge all the contents into column system. However, with this procedure the sample breaks up into separate parts in the form of bubbles resulting in a form of multiple injection which produces multiple peaks and consequent loss of resolution. To avoid the production of multiple peaks the retention gap method of injection was introduced. The stationary phase is removed from the first few centimeters of the column and the sample injected into this stationary phase free column section. The column is held at a relatively low temperature. Although the sample still splits into bubbles, they all travels at the same speed down the column until they meets the stationary phase were they are all adsorbed as a narrow band. The column temperature is then programmed in the usual manner and the separation developed in the normal way. Another method of injection that avoids multiple peaks is the ‘solute focusing procedure’.