Conductivity detection and quantitation of isotachophoretic analytes on a planar chip with on-line coupled separation channels

A poly(methylmethacrylate) chip, provided with two separation channels in the column-coupling (CC) arrangement and on-column conductivity detection sensors and intended, mainly, to isotachophoresis (ITP) and ITP-capillary zone electrophoresis (CZE) separations was developed recently. The present work was aimed at assessing its performance relevant to the detection and quantitation of the ITP analytes. Hydrodynamic (HDF) and electroosmotic (EOF) flows of the solution in the separation compartment of the CC chip were suppressed and electrophoresis was a dominant transport process in the ITP separations with model analytes carried out in this context. When the surfaces of the detection electrodes of the conductivity sensors on the chip were appropriately cleaned qualitative indices of the test analytes [relative step heights (RSHs)], provided by a particular detection sensor, agreed within 1% (expressed via RSDs of the RSH values). Their long-term reproducibilities for one sensor, as estimated from 70 ITP runs repeated in 5 days, were 2% or less. Sensor-to-sensor and chip-to-chip fluctuations of the RSH values for the test analytes were 2.5% or less. In addition, experimentally obtained RSH values agreed well with those predicted by the calculations based on the ITP steady-state model. Reproducibilities of the migration velocities attainable on the CC chips with suppressed EOF and HDF, assessed from the migration time measurements of the ITP boundary between well-defined positions on the separation channels of the chips (140 repeated runs on three chips), ranged from 1.4 to 3.3% for the migration times in the range of 100-200 s. Within-day repeatabilities of the time-based zone lengths for the test analytes characterized 2% RSDs, while their day-to-day repeatabilities were less than 5%. Chip-to-chip reproducibilities of the zone lengths, assessed from the data obtained on three chips for 100 ITP runs, were 5-8%.