A fully integrated monolithic microchip electrospray device for mass spectrometry

A novel microfabricated nozzle has been developed for the electrospray of liquids from microfluidic devices for analysis by mass spectrometry. The electrospray device was fabricated from a monolithic silicon substrate using deep reactive ion etching and other standard semiconductor techniques to etch nozzles from the planar surface of a silicon wafer. A channel extends through the wafer from the tip of the nozzle to a reservoir etched into the opposite planar surface of the wafer. Nozzle diameters as small as 15 microm have been fabricated using this method. The microfabricated electrospray device provides a reproducible, controllable, and robust means of producing nano-electrospray of a liquid sample. The electrospray device was interfaced to an atmospheric pressure ionization time-of-flight mass spectrometer using continuous infusion of test compounds at low nanoliter-per-minute flow rates. Nozzle-to-nozzle signal intensity reproducibility using 10 nozzles was demonstrated to be 12% with single-nozzle signal stability routinely less than 4% relative standard deviation (RSD). Solvent compositions have been electrosprayed ranging from 100% organic to 100% aqueous. The signal-to-noise ratio from the infusion of a 10 nM cytochrome c solution in 100% water at 100 nL/min was 450:1. Microchip electrospray nozzles were compared with pulled capillaries for overall sensitivity and signal stability for small and large molecules. The microchip electrospray nozzles showed a 1.5-3-times increase in sensitivity compared with that from a pulled capillary, and signal stability with the microchip was 2-4% RSD compared with 4-10% with a pulled capillary. Electrospray device lifetimes achieved thus far have exceeded 8 h of continuous operation and should be sufficient for typical microfluidic applications. The total volume of the electrospray device is less than 25 pL, making it suitable for combination with microfluidic separation devices.