Optimized HPLC method for tramadol and O-desmethyl tramadol determination in human plasma

The optimized method for HPLC determination of tramadol and its metabolite O-desmethyl tramadol in human plasma using sotalol as internal standard has been developed and validated by a new approach. The determination by fluorescence detection was performed on re-eluted solution, obtained after liquid–liquid extraction with ethyl acetate of the three analytes from plasma. The chromatographic separation of tramadol under a gradient elution was achieved at a temperature of 15 °C with a RP-18 column, guarded by a C18 precolumn. The mobile phase was a mixed aqueous solution containing ortho-phosphoric acid, triethylamine, acetonitrile and methanol in a complex gradient mode. The quantitative determination of tramadol was performed at different successive pairs of excitation/emission wavelengths (200/300 nm, 200/295 nm, 212/305 nm) with lower limits of quantification: LLOQ = 4.078 ng/ml for tramadol, respectively LLOQ = 3.271 ng/ml for O-desmethyl tramadol. For the LLOQ limits, were calculated the values of the coefficient of variation and difference between mean and the nominal concentration. For tramadol analyte they were CV% = 5.147% and bias% = − 7.273% in the intra-days and CV% = 4.894% and bias% = 0.836% in the between-days assay, respectively for the metabolite O-desmethyl tramadol they were CV% = 11.517% and bias% = 0.337% in the intra-days and CV% = 6.41% and bias% = 3.259% in the between-days assay.

In addition, the stabilities of the analytes were verified in different conditions. Both, tramadol and its metabolite proved to be stable in plasma for four weeks, frozen at − 20 °C, but also for 48 h at 15 °C in the re-eluted solution after liquid–liquid extraction.