Electroosmotically driven solution mixing in borosilicate theta glass nESI emitters

The use of borosilicate theta glass capillaries as nanoelectrospray ionization emitters has recently been demonstrated as a method for mixing two solutions as they are sprayed into the mass spectrometer for analysis. All previous experiments resulted in a solution mixing timescale limited to the time the analytes spend in the Taylor cone and subsequent droplets (i.e. sub‐millisecond timescale). In an effort to extend the solution mixing timescale to the milliseconds regime, we demonstrate that solution can be moved from one channel of the theta tip to the opposite channel via electroosmosis by applying a potential difference between the two wire electrodes inserted into each channel of the theta tip. First, we establish that electroosmosis is responsible for solution movement using fluorescence microscopy to track fluorescent tracer dyes. We then demonstrate the utility of this technique in varying the extent of denaturation of holomyoglobin to apomyoglobin on the millisecond timescale just prior to analysis by mass spectrometry. Finally, we induce additional turbulence for better mixing by applying a square wave potential to one of the wire electrodes while holding the opposite wire at a constant voltage between the low and high potentials of the square wave. This experiment was found to provide nearly complete mixing after a single cycle of the square wave. The use of electroosmosis significantly expands the flexibility of theta tips for altering solutions prior to nESI without the need for off‐line sample manipulation. Copyright © 2015 John Wiley & Sons, Ltd.