m/z 44 (h) indicates the absence of the N-methyl group. In the CID mass spectra of codeine and the baine possessing a N-methyl function, the corresponding ethylidene-ammonium type ion h appears at m/z 58 (Scheme 6, Table 2). Codeine-N-oxide (21), which shows a fragmentation pattern different from that of codeine, could be identified compared with authentic sample. The N-oxide function has a remarkable influence on the fragmentation behavior of this alkaloid. Thus, compound 21 displays a base peak at m/z 162 that is shifted to m/z 168 in the [ring-13C6]-labeled derivative 21c, which comprises the isoquinoline moiety (Scheme 6, Table 2). In this case, the key ions of type h and i appear with the corresponding mass shift of 16 amu at m/z 74 and 60, respectively.