This zoom-in Scanning Electron Microscope image shows a five-nozzle M3 emitter, where each nozzle measures 10x12 microns. Credit: Daojing Wang, Lawrence Berkeley National Laboratory
Each emitter consists of a parallel array of silica nozzles protruding out from a hollow silicon sliver with a conduit size of 100 x 10 microns. Multiple nozzles (100 nozzles per millimeter was a typical density) were used rather than single nozzles in order to reduce the pressure and clogging problems that arise as the microfluidic channels on a chip downsize to a nanometer scale. The emitters and their nozzles were produced from a silicon wafer, with the dimension and number of nozzles systematically and precisely controlled during the fabrication process. Fabrication required the use of only a single mask and involved photolithographic patterning and various etching processes.
Said Peidong Yang, "Once integrated with a mass spectrometer, our microfabricated monolithic multinozzle emitters achieved a sensitivity and stability in peptide and protein detection comparable to commercial silica-based capillary nanoelectrospray tips. This indicates that our emitters could serve as a critical component in a fully integrated silicon/silica-based micro total analysis system for proteomics."
Added Daojing Wang, "This is also the first report of a multinozzle emitter that can be fabricated through standard microfabrication processes. In addition to having lower back pressure and higher sensitivity, multinozzle emitters also provide a means to systematically study the electrospray ionization processes because the size of each nozzle and density of nozzles on the emitters can be adjusted."
According to Wang and Yang, the fabrication and application of the microfabricated monolithic multinozzle emitters, called "M3 emitters" for short, could be commercialized immediately and should be highly competitive with current silica capillary emitters in terms of cost and mass production.