Evaluation of a microchip electrophoresis-mass spectrometry platform deploying a pressure-driven make-up flow

Integration of a pressure-driven make-up flow (MUF) into a microchip electrophoresis (MCE) platform in order to facilitate its coupling with electrospray ionization-mass spectrometric detection (ESI-MS) is described. In the glass/PDMS hybrid microchip, a MUF channel was made to intersect with the MCE separation channel at an angle of 45°. The MUF was generated by a syringe pump. Microscopic image results from simulation studies showed that the pressure-driven MUF and the potential-driven electroosmotic flow in the MCE separation channel could be run separately without interfering with each other and mixed well at the joint point by adjusting either the MUF flow rate or the potential applied for MCE separation. The MUF had several desirable functions, including making the start of electrospray easy and cleaning the nanoESI emitter continuously when not spraying. High separation efficiency was achieved with the proposed MCE-nanoESI-MS system in separating an amino acid mixture containing glutamine, serine, threonine, phenylalanine, and glutamic acid. All of them were baseline separated from each other within 3 min. Plate numbers of >10,000 (on a 2.5 cm MCE separation channel) were obtained. The analytical platform also showed a linear response for quantification of DOPA with a detection limit (S/N=3) of 0.10 μM. In addition, on-line derivatization of MCE elutes in order to enhance MS detection sensitivity was easily carried out by adding the tagging reagent into the MUF. These results indicated that the present system might have a good potential in MCE-MS applications.