Characterization of micromachined hollow tips for two‐dimensional nanoelectrospray mass spectrometry

Abstract

In this work an improved design of chip-based nanoelectrospray nozzles is reported. Two-dimensional matrices of out-of-plane 10 microm i.d. silicon dioxide tips with a tapered shape were manufactured using deep reactive ion etching technology. Using a peptide sample, six micromachined tips and six commercially pulled silica capillary tips were compared employing an ion trap mass spectrometer. At a flow rate of 100 nL/min, the detectability obtained was approximately the same for the two types of tips. The relative standard deviation of the signal-to-noise ratio for the peptides between six different tips was on average 22% for the micromachined tips and 45% for the pulled capillary tips. The usefulness of the micromachined tips for analysis of non-covalent protein-ligand complexes was demonstrated by the analysis of a sample of RNase A and cytidine 2'-monophosphate. In another test, analyzing a tryptic digest of 1 pmol/microL cytochrome C, 18 peptides corresponding to a 82% sequence coverage were detected. Using MS/MS, the whole sequence of an 11 amino acid cytochrome C fragment was obtained. Computer simulations were performed on the shape and magnitude of the electrical field around micromachined and pulled capillary tips. To reach the threshold electric field density at the tip apex required to initiate an electrospray, a higher electrospray voltage was needed for the chip-based tips compared with pulled capillary tips. This is due to the influence of the chip base.

Why the move to microfluidics for protein analysis?

“…These devices are now being microfabricated with technologies imported from the microelectronics industry to improve reproducibility and robustness of the system. Many technological approaches have been based on polymer [10][11][12][13] and glass or silicon [14,15] microfabrication, among them are mature devices working either with robotic stations such as the Nanomate TM system from Advion Biosciences [16 ] or stand-alone disposable devices [17]. This kind of micro-or nanospray microfabricated emitter is appearing at a time when exciting developments in mass spectrometry are starting to emerge, such as new methodologies based on the capabilities of MS for the structural elucidation of protein complexes or complex mixture analysis.…”

Analytical Performance of a Venturi-Assisted Array of Micromachined Ultrasonic Electrosprays Coupled to Ion Trap Mass Spectrometry for the Analysis of Peptides and Proteins

“…Reported miniaturized nanospray infusion devices fall into three main categories: (1) devices using fused silica capillary emitters attached to a chip 14-21 , (2) devices using a microchannel exiting the edge of a wafer 22-29 , and (3) monolithic devices using etched ESI tips [30][31][32][33][34][35][36] . Integrated miniaturized nanoESI devices comprising enrichment columns, reverse-phase separation channels and micromachined nanospray emitters have also been recently reported [37][38][39] .…”

An open design microfabricated nib-like nanoelectrospray emitter tip on a conducting silicon substrate for the application of the ionization voltage

“…Microfabrication of ESI emitter tips follows two main approaches: (1) a "needle-like" ap-proach or (2) a "channel-like" approach consisting of a sharpened interface at the outlet of a microchannel. "Needle-like" approaches were reported using silicon nitride [11], parylene [12], silicon [13][14][15], as well as polycarbonate [16] or polymethyl methacrylate (PMMA) [17]. The reported "channel-like" approaches include PMMA starshaped systems [18], polydimethyl siloxane (PDMS) devices having a triangular shape [19], devices based on a groove feature [20], machined point-like structures [21], polyimide-based triangular systems [22], and triangular parylene sheet-based devices [23].…”