Comparison of derivatives for determination of codeine and morphine by gas chromatography/mass spectrometry

Pentafluoropropionic anhydride (PFPA) and heptafluorobutyric anhydride (HFBA) derivatives of morphine and codeine demonstrated poor spectra due to low abundances of secondary and tertiary ions. Trifluoroacetamide (MBTFA) has been a widely used derivative; however, the internal standard, nalorphine, displayed very poor stability and this resulted in split peaks by gas chromatography making MBTFA unsuitable for quantitative methods. Quantitation of codeine and morphine using bis-trimethylsilyltrifluoroacetamide (BSTFA/1%TMS) revealed a significant gradual decrease (p less than 0.05) of peak area ratio (PAR) of codeine and morphine compared to the internal standard using selected ion monitoring (SIM). The acetic anhydride derivative showed no significant differences in the peak area ratios for codeine/IS over a period of 24 hours, although the coefficient of variation (CV) was higher for the acetyl derivative than for the TMS derivative of codeine. There was a significant difference associated with the acetyl derivative of morphine at 4 h post derivatization compared to the initial injection (p less than 0.05); however, the acetyl derivative was stable for 24 hours and had a CV of less than 10% at a cutoff of 300 ng/mL.