Fabrication of a monolithic sampling probe system for automated and continuous sample introduction in microchip‐based CE

A fabrication process for producing monolithic sampling probes on glass chips, with tip diameters of a few hundred micrometers was developed, using simple tools including a glass cutter and a bench drill. Microfluidic chips with probes fabricated by this approach were coupled to a linearly moving slotted‐vial array sample presentation system for performing continuous sample introduction in the chip‐based CE system. On‐chip horizontal tubular reservoirs containing working electrolyte and waste were used to maintain a stable hydrostatic pressure in the chip channels during prolonged working periods. The performance of the system was demonstrated in the separation of FITC‐labeled amino acids with LIF detection, by continuously introducing a train of different samples without interruption. Throughputs of 30–60/h were achieved with <1.0% carry‐over and reproducibilities in peak height of 3.6, 3.3, and 3.5% RSD for arginine, FITC, and phenylalanine, respectively ( n  = 11). Continuous analysis of a mixture of FITC‐labeled amino acids for 2 h, involving 60 analytical cycles, yielded an RSD of 7.5 and 6.8% for arginine and FITC ( n  = 60), respectively. An extremely low sample consumption of 30 nL for each analysis was obtained. Separation efficiencies in plate numbers were in the range of 0.8–2×10 5 /m. In addition to the application in sample introduction, the sample/reagent introduction system was also used to produce working electrolyte gradients during a CE separation to improve the separation efficiency. Comparing with isocratic electrophoresis separation, gradient CE demonstrated better separation efficiencies for a mixture of FITC‐labeled amino acids.