Evaluation of a novel anion-exchange restricted-access sorbent for on-line sample clean-up prior to the determination of acidic compounds in plasma by liquid chromatography

A new kind of silica-based restricted-access material (RAM) with anionic properties has been tested in pre-columns for on-line solid-phase extraction of acidic compounds from directly injected plasma samples prior to their determination by reversed-phase liquid chromatography (LC), using the column-switching technique. The outer surface of the porous RAM particles contains hydrophilic diol groups while diethylaminoethyl (DEAE) groups are bound to the internal surface which gives the sorbent the properties of a weak anion exchanger towards low-molecular-mass compounds. Due to an appropriate pore diameter (about 6 nm), macromolecules, such as proteins, are physically excluded from the pores and flushed directly out during the sample clean-up process, while small compounds have access to the inner surface and can be retained mainly by electrostatic interactions. The retention capability of this novel packing material has been tested for some hydrophilic acidic compounds such as aspartic acid, glutamic acid, ascorbic acid and acetylcysteine as well as for some more hydrophobic drugs such as naproxen, ibuprofen and diclofenac, used as model compounds. The influence of the composition of the washing liquid on the retention of the analytes in the pre-column has been investigated. The efficiency of the sorbent to clean-up complex matrices was also tested using human plasma and urine samples. A generic washing liquid composition was then selected in order to obtain efficient and selective sample clean-up as well as a high recovery of the acidic analytes.