In-line microextraction techniques to improve the sensitivity and selectivity of capillary electrophoresis using commercial instruments

Capillary electrophoresis (CE) handles complex samples with excellent resolution, but has poor sensitivity because of the small detection volume and suffers from destacking of high-conductivity samples; therefore, sample preparation is often required for matrix cleanup and analyte enrichment. This review discusses operationally-simple sample preparation schemes which are easily adoptable by researchers using commercial CE instruments, wherein various solid-phase microextraction (SPME) and liquid-phase microextraction (LPME) techniques are ‘in-line’ coupled (i.e., integrated) to the separation capillary without modifying the commercial CE instrument with sophisticated interfaces. In-line coupling refers to scenarios where the prepared sample is injected directly into the separation capillary. Then, sample loss is reduced and the enrichment factors can be significantly increased as the acceptor volume can be reduced to the nanoliter level. Cases involving subsequent on-line preconcentration are also discussed. In-line SPME-CE and LPME-CE are powerful tools for analyzing various samples, especially trace substances in complex biological matrices.