Sampling Benzene Vapor in Air
This sampling procedure may be required close to an oil refinery, gasoline storage tanks, or possibly in the vicinity of a gasoline pump. The concentration range for benzene in air may extend from a toxic level of about 1 ppm or more to a relatively safe level of 1 ppb. Consequently, the amount of benzene in one cubic centimeter of air will be very small. To obtain sufficient material for analysis, the benzene must be trapped and concentrated from a significant volume of gas and the benzene regenerated in a form suitable for analysis. A diagram of the sampling apparatus that would be used for the determination of benzene in air is shown in figure 3.
Figure 3 Apparatus for Sampling Trace Amounts of Benzene in air for Subsequent GC Analysis
Traps from which the adsorbed benzene is desorbed by heating are usually packed with graphitized carbon, and are 2–4 mm I.D. and about 10 cm long (Supelco Carbotrap). The air sample is aspirated through the trap by reduced pressure applied to the outlet by a suitable pump. The sample volume taken may range from 500 ml to several liters depending on the level of the benzene. The tube can be cooled to improve the trapping efficiency but this would only be necessary for large sample volumes. However, if cooling is employed, the trap is likely to contain water that may cause difficulties in sample preparation or interference in the subsequent GC analysis. This problem may be reduced if the trap temperature is maintained no lower than 2°C. On completion, the trap is capped and stored in a thermally insulated box with some appropriate packing to protect it from mechanical shock during transport. If possible, the box should be kept refrigerated at about 2°C. On arrival, the sealed trap should be kept in a refrigerator until ready for analysis.
