Development of an on-chip injector for microchip-based flow analyses using laminar flow

Abstract

A new on-chip injector for microchip-based flow analyses has been designed and characterized. The microchip design utilizes separate laminar flow streams of buffer and sample that are brought into parallel contact for a distance of 300 µm. The buffer flow stream is first routed through a conventional 6-port injection valve fitted with a 5 µm i.d. sample loop. When the 6-port valve is actuated from load to inject for a given time, the on-chip buffer flow stream is constricted and the sample flow stream is pressurized into the buffer flow channel. Once the valve returns to the load state the separate laminar flow streams resume. Fluorescence detection was used to characterize the injector and it was found that 50 injections of a 100 µM fluorescein sample led to an average peak height of 174.32 ± 2.05 AFU (RSD 1.18%) and average peak skew of 1.37 ± 0.06. The injector was also interfaced with amperometric detection. Injections of catechol solutions ranging in concentration from 500 nM to 100 µM resulted in a linear response (sensitivity = 2.49 pA µM⁻¹, r² = 0.998) and a limit of detection of 155 nM (S/N = 3). Compared to an off-chip injection scheme, plug dilution, band broadening, and peak asymmetry are much reduced. Finally, the injection and subsequent lysis of an erythrocyte sample was demonstrated, with an injected plug of erythrocytes being lysed 5.72 ± 0.15 s after injection into a flow stream containing sodium dodecyl sulfate (n = 10). The new injection scheme does not require complex valving mechanisms or high pressures and enables reproducible injections from a continuous sample flow stream in a manner where changes in analyte concentration can be monitored with high temporal resolution.