Microfluidic Chip Electrophoresis/Mass Spectrometry Interface Designed to Function with Low Electroosmotic Flow for the Analysis of Proteins

摘要

Microfluidic devices have a great potential for integration of fast separations and sample preparation steps prior to MS analysis. The coupling of microfluidic device with ESI/MS provides an excellent platform for proteomic investigations. However, coupling of electrokinetically driven microfluidic systems, that use electrokinetic valving and can employ microchannel electrophoretic separations, have proven to be difficult to couple with ESI/MS. To minimize protein adsorption, coatings, which minimize surface charge, such as polyethylene glycol (PEG) are used. While such coatings minimize protein adsorption, they also minimize the bulk flow of solution, driven by electroosmotic flow (EOF), through the channel. Creating a stable electrospray from the microfluidic device requires application of the appropriate voltage at the tip and a flow rate that is compatible with the spray tip voltage. When microchannel or capillary electrophoresis is performed upstream of the electrospray, an intermediate electrode is needed to act as a current sink before the electrospray tip. Without this current sink through an intermediate electrode, the voltage drop across the separation channel is much lower than desired to achieve a separation with high efficiency as shown in Figure 1. For systems with low electroosmotic flow, a major challenge is minimizing analyte migration to this intermediate electrode that sets the ESI voltage. Additionally, it is desirable to remove the electrode from the microfluidic channel to spatially remove electrochemical reactions that can alter the sample, produce gas bubbles, and induce pH drift.