The fiber is left in contact with air above the sample for an appropriate period (3 to 60 minutes) depending on the nature of the sample (2). The fiber is withdrawn into its holder, removed from the vial (3) and injected through the septum of the sample system of the chromatograph into a region surrounded by a heater (4). The plunger is again depressed, and the fiber, now protruding into the heater, is rapidly heated to desorb the sample onto the GC column. The column is kept cool so the components concentrate on the front of the column. When desorption is complete (a few seconds) the column is temperature programmed to develop the separation. A chromatogram of the head-space sample taken over a sample of tobacco is shown in figure 8. 1 g of tobacco (12% moisture) contained in a 20 ml head-space vial was treated with 3.0 ml of 3M potassium chloride solution. The fiber was coated with a 100 mm film of polydimethyl siloxane, a highly dispersive (hydrophobic) adsorbent. The vial was heated to 95°C and the fiber was left in contact with the head-space for 30 min. The sample was desorbed from the fiber for one minute at 259°C. The separation was carried out on a column 30 m long, 250 mm I.D. carrying a 0.25 mm thick film of 5% phenylmethylsiloxane.
Courtesy of Supelco Inc.
Figure 8 A Chromatogram of Tobacco Head Space
The column was held isothermally at 40°C for one minute, programmed to 250°C at 6°C/min. and then held at 250°C for 2 min. A good separation of the tobacco components is achieved and the resolution permits the comparison between tobaccos from different sources. The technique is reliable, sensitive, and accurate. The concentration of sample vapor in the air, however, will depend on its distribution coefficient between the sample matrix and the air which will differ between individual components and, thus, absolute concentrations in the sample matrix can not be determined.
