Fig. 1

Mass spectra of tramadol (a), O-desmethylate (b), N-desmethylate (c), and N,O-didesmethylate (d). A mass-to-charge (m/z) of 264.2/58.2 was monitored for tramadol, 250.2/58.2 for O-desmethylate, 250.2/232.2 for N-desmethylate, and 236.1/218.4 for N,O-didesmethylate