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 {TAIL ~ exposure} provides information about the duration of the exposure and yields a correction factor for loss of analyte from the sorbent as a result of reverse diffusion and {HEAD ~ 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 { TAIL ~ exposure } provides information about the duration of the exposure and yields a correction factor for loss of analyte from the sorbent as a result of reverse diffusion and { HEAD ~ desorption }
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