If two or more are used, it is helpful if their desorption characteristics are sufficiently different so that the ratio of the amounts remaining after a period of exposure provides information about {HEAD ~ the duration} of the exposure and yields a correction factor for loss of {TAIL ~ analyte} from the sorbent as a result of reverse diffusion and desorption 
If two or more are used , it is helpful if their desorption characteristics are sufficiently different so that the ratio of the amounts remaining after a period of exposure provides information about { HEAD ~ the duration } of the exposure and yields a correction factor for loss of { TAIL ~ analyte } from the sorbent as a result of reverse diffusion and desorption
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