Polymer Micronozzle Array for Multiple Electrosprays Produced by Templated Synthesis and Etching of Microstructured Fibers

Abstract

Microstructured fibers (MSFs) having raised polymer nozzles in each channel are custom designed, fabricated, and tested for use as multiple electrospray (MES) emitters for mass spectrometry (MS). There is strong motivation to develop electrospray emitters that operate at practical flow rates but give the much greater ionization efficiency associated with lower (nano) flow rates. This can be accomplished by splitting the flow into many lower-volume electrosprays, an approach known as MES. To couple with most modern mass spectrometers, the MES emitter must have a small diameter to allow efficient ion collection into the MS. In this work, a MSF, defined as a fiber having many empty channels running along its length, was designed to have 9 channels, 9 μm each, >100 μm apart arranged in a radial pattern, all in a fiber having a compatible diameter with both front-end LC equipment and typical MS inlets. This design seeks to promote independent electrospray from each channel while maintaining electric field homogeneity. While the MSFs themselves do not support MES, the formation of polymer nozzles protruding from each channel at the tip face enables independent electrospray from each nozzle. Microscope imaging, electrospray current measurement, and ESI-MS detection of a model analyte all confirm the MES behavior of the 9-nozzle emitter, showing significant signal enhancement relative to a single-nozzle emitter at the same total flow rate. LC/MS data from a protein digest obtained at an independent laboratory demonstrates the applicability and robustness of the emitter for real scientific challenges using modern LC/MS equipment.

A Microstructured Fiber with Defined Borosilicate Regions to Produce a Radial Micronozzle Array for Nanoelectrospray Ionization

“…The small electric current generated by the electrospray was measured for all conditions tested, with the value for a particular condition being the mean current over at least 5 minutes acquisition time and the uncertainty being the standard deviation of the mean. The dependence of spray current on total flow rate and voltage followed the expected trends as previously observed 19 . The solvent composition was varied according to a typical LC/MS gradient, from 99:1 water:methanol (by volume, with 1% acetic acid) to 50:50 water:methanol and back, as shown in Figure S4 of the Supporting Information .…”

“…The experimental setup for measuring the electrospray current and imaging the spray is similar to that used previously 19 and is shown in Figure S7 of the Supporting Information . A solution composed of deionized water (79.2% v/v), methanol (19.8% v/v) and glacial acetic acid (1% v/v), degassed and filtered, was delivered to the emitter using a nano-flow gradient pump (IDEX Health & Science LLC, Oak Harbor, WA, USA).…”

“…Polycarbonate MSFs (lacking individual nozzles) generated MES with fully aqueous samples only, due to wetting effects 18 . Alternatively, a custom silica MSF with polymeric nozzles produced MES however the emitters were fragile 19 . In this work, MSFs were designed to have channels made from a different preform material than the bulk of the fiber, such that selective etching could be used to make a nozzle at each channel exit.…”

Fabrication of a Polymer Nano Nozzle for Electrohydrodynamic Small-Molecule Solvent Inkjet Printing

“…34,36 Additionally, polymer micro-nozzles with a channel diameter of approximately 9 μm were produced by a polymerization/etching procedure. 37 Unfortunately, despite the concise of these methods, the nozzle inner diameter is merely on the micro-scale. An SU-8 nano nozzle has also been fabricated, with the nozzle channel being a nanoscale crack thermally induced by volume shrinkage; however, the size and reproducibility of the cracks are hard to control.…”

Multiplexing of Electrospray Ionization Sources Using Orthogonal Injection into an Electrodynamic Ion Funnel

“…Nonetheless, further improvement in the ion current from a single emitter is limited by the smallest droplet size that can be generated at a given flow rate and the maximum amount of charge carried by the ESI droplets, which is known as the Rayleigh limit. − Multiplexing of the ESI emitters is a promising strategy that has been employed to overcome this limitation. − One multiplexing strategy involves the development of ESI emitter arrays, which have been used both in analytical MS , and ESI-based propulsion. , In analytical MS, a circular 22-emitter array has been used to achieve more than a 2-fold improvement in the MS sensitivity . Ambient ion beam merging and focusing using specially shaped 3D-printed devices and counter-propagating beams provides an opportunity to manipulate and combine multiple ion beams before they enter the vacuum system. , Despite the significant progress in this field, multiplexing of ion beams still results in a substantial ion loss, which motivates the development of new multiplexing strategies.…”