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