Iterative trapping of gaseous volatile organic compounds in a capillary column

The iterative trapping method has been developed for concentrating gaseous volatile organic compounds (VOCs) prior to gas chromatographic analysis. VOCs are trapped in a 50 cm × 0.53 mm metal capillary column coated with a 7 μm thick film of polydimethylsiloxane (PDMS). Iterative trapping does not employ the two-step thermal desorption approach used by most VOC concentrating techniques. Instead, a four-step cycle involving synchronized changes in flow direction and temperature is repeated throughout the sampling process. This iterative process causes VOCs to accumulate within the capillary well past the level where a standard two-step method reaches its saturation limit. Iterative trapping is capable of sampling and desorbing C5 through C11 n-alkanes with uniform efficiency. This new technique, in its current form, is most appropriate for focusing VOCs from gas volumes on the order of 10 mL. Iterative trapping increases the focusing power of a weak sorbent like PDMS and allows narrow chromatographic peaks to be generated without the use of high desorption temperatures or a secondary focusing stage.