Topics - Focusing
In chromatography, the term focusing is used in two ways; either with regard to the concentration of light onto a photo cell, diode array or diffraction grating in the various detectors based on light absorption, or the concentration of an injected sample onto a limited volume of stationary phase at the start of a gas chromatographic column. Focusing of light in detectors is carried out using lenses of glass (for visible light), quartz or fused silica (for UV light) or alkyl halides and diffraction gratings for Infra Red light. There are a number of techniques used for sample focusing, a simple example would be the retention gap
injection method used with capillary columns. 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 travel at the same speed down the column until they meets the stationary phase were they are all adsorbed (focused
) into a small volume of stationary phase and form a narrow band at the start of the column. The column temperature is then programmed in the usual manner and the separation developed in the normal way.