Using restricted-access materials and column switching in high-performance liquid chromatography for direct analysis of biologically-active compounds in complex matrices

In the bioanalytical field, sample preparation is often considered the time-limiting step. Indeed, extraction techniques (e.g., liquid–liquid extraction (LLE) and solid-phase extraction (SPE)) are commonly used off-line for biological matrices.To perform high-throughput analysis, there have been efforts to develop a faster sample-purification process. Special extraction sorbents, such as restricted-access materials (RAMs), allow direct, repetitive injection of complex biological matrices onto these supports. Coupling RAMs to column-switching high-performance liquid chromatography (HPLC) systems is a very attractive approach to biological sample preparation. This technique leads to automation, simplification and speeding up of the sample-preparation process.In this article, we review coupling of RAMs to column-switching systems and give particular attention to commercially available supports. These RAMs are used in single-column or column-switching configurations for direct analysis of compounds in various biological fluids.