Double-column gas chromatography using packed pre-columns and glass capillary main columns.

Methods of column switching are described that make possible back-flushing, heart cutting and trapping. Conventional packed columns are employed as pre-columns for their high sample capacity, and glass capillary columns are used as the main columns for their high separation efficiency. Some typical examples of the application of double-column gas chromatography are presented: (1) solvent cutting for identification and exact quantitative determination of a phosphorus ester impurity in a sample of wheat flour (including trapping); (2) separation of trace amounts of methyl esters of fatty acids by means of back-flushing and trapping; (3) enrichment technique using trapping of trace elements in the front section of the capillary (plus solvent cutting, multiple injection and back-flushing); and (4) coupling a nitrogen-selective detector to the capillary main column to gain additional information. To illustrate the trapping effect, the separation number was determined with and without trapping.