Analysis of basic compounds by reversed-phase high-performance liquid chromatography using hybrid inorganic/organic phases at high pH

The retention and mass overload of five bases and a quaternary ammonium compound were studied on a bridged ethyl hybrid inorganic/organic phase (XBridge C18) over a pH range 2.7–12.0 using acetonitrile–phosphate and carbonate buffers. Some comparisons were drawn with results on a methyl hybrid (XTerra) phase. At low pH, rapid mass overload was observed with severe decreases in efficiency as sample mass was increased over the range 0.04–5 μg of solute. At intermediate pH ( pH 7 w s .0 ), generally good peak shapes for small sample mass were still obtained on the ethyl hybrid, but with somewhat increased tailing of bases compared with pH w s 2.7 . At higher pH ( pH 10 w s ), good peak shape and improved loadability were obtained for moderately strong bases, due to their occurrence mostly as neutral species. However, stronger bases gave poor efficiency, attributed to interaction of the charged basic solute with increasingly ionised column silanol groups. Results were broadly similar on the methyl hybrid at pH 10 w s . At pH 11 w s , unusual profiles of increasing followed by decreasing efficiency were obtained on the ethyl hybrid for some bases as sample mass was increased; improved results were obtained at pH 12 w s . While the column loadability increased substantially at the highest pH studied, tailing for small sample mass was still more severe than at low pH, even though all compounds were <1% ionised in the mobile phase.