In trace analysis small amounts of compound must be analyzed in the presence of a large amount of interfering matrix. A chromatogram can be improved by using a multiple column technique. The entire sample can be eluted from a short column to obtain a rough separation and only the fraction that contains the relevant peaks can be passed on to a longer column to do the separation. The first column is called a clean-up column or pre-column and the second column is called the analytical column.
In column switching, a relatively large volume of sample is injected onto a short analytical column. The early eluting trace component of interest passes into a second longer analytical column which has the resolving power to separate the trace component from any early eluting interferences. These components are held at the top of the second column and the column switching valve is switched to divert later components eluting from the first column directly to the detector. After passage of these compounds through the detector, the flow is again diverted through the second column and the trace components can be analyzed.