Chromatographic retention behaviour, modelling and optimization of a UHPLC-UV separation of the regioisomers of the Novel Psychoactive Substance (NPS) methoxphenidine (MXP)

A detailed investigation into the chromatographic retention behaviour and separation of the three regioisomers of the Novel Psychoactive Substance (NPS) methoxphenidine (i.e. 2-, 3- and 4-MXP isomers) has revealed the ionization state of the analyte and stationary phase, to be the controlling factor in dictating which retention mechanism is in operation. At low pH, poor separation and retention was observed. In contrast, at intermediate pH, enhanced retention and separation of the three MXP isomers was obtained; it appeared that there was a synergistic effect between the electrostatic and hydrophobic mechanisms. At high pH, the MXP isomers were retained by hydrophobic retention. Accurate retention time predictions (<0.5%) were achievable using non-linear retention models (3 × 3). This allowed the optimization of the gradient separation of the MXP isomers using a two-dimensional gradient and temperature design space. Prediction errors for peak width and resolution were, in most cases, lower than 5%. The use of linear models (2 × 2) still afforded retention time and resolution accuracies of <2.3 and 11% respectively. A rapid and highly sensitive LC-MS friendly method (i.e. R_smin > 5 within 4 min) was predicted and verified. The developed methodology should be highly suitable for the rapid, specific and sensitive detection and control of MXP regioisomers.