I don't see a problem with that. When the swab comes out of the detector it is cleaner than when it when in. That's because the swab is heated to remove all the volatiles. So, if the detector gives a negative result, that means the swab has to be clean, and it can be used again. The only time you have to worry is when the detector gives very strong positive, in that case you could have a carryover contamination of the next sample due to incomplete volatilization of the sample.