Glycerin isn't quite a ring: C3H5(OH)3 but traces of it will be picked up by the ETD.

A better option under study appears to be a mass-spectrometry-based ETD which more accurately detects such traces using a charge-to-mass ratio analysis. The TSA hopes to integrate a laser-based detection system into the x-ray machine, which would improve efficiency and accuracy of scanning hand baggage for explosives.