Influence of structural differences of dextromethorphan and its three metabolites on their simultaneous separation using various silica columns with a simple aqueous mobile phase

The retention behaviors and efficiencies of the chromatographic separation of dextromethorphan (DM) and its metabolites (dextrorphan (DX), 3-methoxymorphinan (MM), and 3-hydroxymorphinan (HM)), on three different silica columns (Inertsil, μ-Porasil, and Lichrospher), were compared using a simple aqueous mobile phase consisting of an organic solvent (methanol or acetonitrile) and water at different volume ratios (1:9 to 9:1) containing triethylamine (TEA) and acetic acid (ACH). Results demonstrated that the retention capacities for basic compounds with the same ionization conditions of the silanol group were the largest for the Lichrospher column, followed by the μ-Porasil one, with the Inertsil column exhibiting the lowest level. Based on their physical characteristics, the larger retention capacities for DM and its three metabolites in the LiChrospher column compared to those of the Inertsil and μ-Porasil columns may have been because the LiChrospher column has the largest surface area for interaction. However, since the surface areas of the Inertsil and μ-Porasil columns are similar, the greater retention capacities for DM and its three metabolites in the μ-Porasil column can probably be attributed to the existence of a greater number of silanol groups than in the Inersil column. This also demonstrates that the interaction of tertiary amines with ionized silanol groups is greater than that of secondary amines resulting in the elution order being MM>DM and HM>DX. However, O-demethylation to expose metabolites with a phenolic hydroxy group seemed to decrease the retention capacity, thus yielding the elution order of DX>DM (tertiary amine) and HM>MM (secondary amine). This might be attributable to the negative charge repulsion between phenoxyl groups (PhO-) and ionized silanol groups (SiO-) decreasing the affinity of basic compounds.