Secondary interactions, an unexpected problem emerged between hydroxyl containing analytes and fused silica capillaries in anion-exchange micro-liquid chromatography

Relevant secondary interactions (hydrogen-bond type), additional to the main anion-exchange mechanism, were found when a method for As, Se and Cr speciation was developed based on microLC-inductively coupled plasma mass spectrometry (ICP-MS) coupling. In order to get the claimed analytical performance characteristics of the microbore columns, microLC systems are equipped with very narrow bore fused silica capillaries. When a mobile phase of NH4NO3 at pH 8.7 was used, a notable tailing was observed for As(III), As(V), MMA and Se(IV), species containing hydroxyl groups in its chemical structure at this pH value. However, additional interactions appeared neither when the fused silica capillaries of the capillary LC system were substituted for polyetheretherketone (PEEK) nor operating at pH below 8.5. A mechanism to explain the additional interaction observed is proposed and tested in this work. It seems that high pH values produce a partial hydrolysis of the siloxane groups of the fused silica capillaries. Under these conditions, degradation products of silica, containing ionized silanol groups, reach the column and interact with the anion-exchange resin. Then, ionized silanol groups, retained on the column, can interact with the hydroxyl moiety of the aforementioned analytes leading to severe peak tailing and broadening. Different strategies were evaluated to solve the problem. The addition of a salt containing hydroxyl groups in the mobile phase such as hydrogen phosphate, the diminution of the pH and the use of PEEK capillaries in the microHPLC system demonstrated to be suitable. Finally, two alternative microHPLC–ICP-MS separations, based on a gradient elution of NH4NO3 at pH 8.0 and NH4NO3/NH4H2PO4 at pH 8.7, were optimized and compared. Results showed better peak shapes for some species when hydrogen phosphate was added to the mobile phase.