imho Weed should be legal. I don't think anyone should smoke it but I don't think anyone should go to jail if you do (in the privacy of your own home).Mass Selective Detectors identify samples by molecular weight after they have eluted through a fused silica capillary column inside the Gas Chromatagraph, the heavier the molecular weight the longer it takes to make it through the column. Then the sample is ionized and that ion cloud is then focused through hyperbolic quadrapoles and scanned 24 times a second by ramping RF and DC through a sweep , and counted by an electron muliplier at the end. Very cool stuff. Accuracy ... If you were in a room where someone was toking and you carry high body fat , it could nail you ...6 months later. ... it will pick up LSD in the blood ! concentrations as low as 2x10 -17 grams it was most fun I ever had working , except for Lutherie.