We are desorbing sorbent tubes and running GC experiments. To obtain calibration curves, we spike the sorbent tubes with a standard solution, eliminate the solvent by passing air through, and then run thermal desorption. It seems like when we run a blank tube, we get a lower background than when we run a spiked tube. The peak 207 is the major issue here. In our blank tube, the peak is 1E3 intensity in mass spectrum, however, in the spiked sample background region its intensity is about 10 times higher. What is causing this? How can we solve this issue, and if we want to subtract background, what is the right background?