Small volume and low flow‐rate electrospray lonization mass spectrometry of aqueous samples

Abstract

A new electrospray ionization source (emitter) has been developed which allows the effective utilization of very small sample volumes at much lower flow rates than previously demonstrated. A small diameter etched-tip capillary has been incorporated into a pressure-infusion electrospray ionization source. The ability to electrospray aqueous solutions without the use of an ancillary sheath flow is demonstrated with several biopolymers. High signal intensities and stable signals are observed for this source in a comparison with a standard methanolsheath source. Electrospray ionization mass spectrometry (ESI-MS)has become an important analytical technique for chemical, biochemical and biological materials.'.' The further application of ESI-MS to small-volume and difficult-to-analyze samples will require a source that utilizes sample efficiently, affords high sample sensitivity and provides a stable electrospray for extended periods of time. Several approaches have been reported for either the continuous delivery of sample or of a sample plug to an ESI source, primarily based upon pressure-driven flow. One of the most widely used approaches, initially developed at this lab~ratory,~ involves the use of a coaxial sheath liquid. The coaxialsheath-liquid ESI source design was originally developed to facilitate interfacing of capillary electrophoresis (CE) and mass spectrometry and provides considerable flexibility in solvent choice and flow rate.3.4 The use of a sheath liquid, however, results in several disadvantages. These include the unavoidable addition of ionic and neutral species through the sheath liquid, which compete in the ESI process for available charge. The maximum sensitivity obtainable is lowered, to an extent which depends on the details of mixing of the analyte and sheath-liquid streams. Past experience in our laboratory has shown that stable signals from sheath-liquid type sources can, in certain applications, be difficult to obtain for extended periods of time. Obtaining a stable electrospray with a water sheath, when non-denaturing solvent conditions are desired and/or required, is often particularly difficult to achieve. Chowdhury and Chait have described a nonsheath ESI source constructed from a highperformance liquid chromatogaphy (HPLC) syringe that functions at conventional electrospray flow rates.' The syringe needle tip was electropolished to a tapered point which enabled the stable electrospraying of proteins from aqueous solutions. Wahl and coworkers have recently reported the use of a non-sheath source incorporating an etched tip at the terminus of small-i.d. fused silica capillaries for CE-MS applications, and have achieved subfemtomole detection of protein^.^,^ The combination of low flow rates with small i.d. fusedsilica capillaries resulted in high ESI-MS sensitivity due to an increase in sample ionization efficiency.A non-sheath source for pressure-infusion ESI-MS, * Author to whom correspondence should be addressed.incorporating a small-diameter silica capillary with an etched terminus ...

A triaxial probe for on-line proteolysis coupled with hydrogen/deuterium exchange-electrospray mass spectrometry

“…Also plotted°in°Figure°6°are°the°ratios°obtained°when°the premixed solution was run through both tube 1 and tube 2 of the triaxial probe (5 L/min in each tube). The curve is indistinguishable from that obtained using the separated solvents, suggesting little (if any) effect of imperfect mixing which could have affected the solvent dielectric and therefore the charge-state distribution [39].°We°conclude°that°space°charge°and°resulting°ion sampling effects are the primary contributors to the small°differences°evident°in°Figure°5.°The°higher°cur-rents for the coaxial probe may result in higher space charge, and therefore more scattering and less efficient sampling°of°more°highly°charged°ions° [37].…”

“…Under these conditions the fibrils gave the same fragments as those observed for monomer°in°Figure°2c. Although some expected cleavages were "missed" in the°12°s°digestion°time°(evident°in°Figure°4),°the fragments produced have the potential to provide structural information about the N-terminus (1-19), C-terminus (35)(36)(37)(38)(39)(40), and part of the protected core (20-34)°[30°-32].°The°observed°cleavage°between°posi-tions 19 and 20 for the fibrils is particularly significant, since this site is believed to be involved in the ␤-sheet network°[30°-32];°dissolution°of°fibrils°evidently°pre-cedes hydrolysis, exposing sites involved in the secondary structures. HDX studies of these peptic fragments should contribute to the testing and refinement of structural°models°[30°-32].°Although°detailed°discus-sion lies outside the scope of the current study, feasibility of such HDX experiments is considered next.…”

“…To°test°for°contributions°from°space°charge°effects° [37,38]°to°the°enhancement°of°the°6ϩ°peak°in°Figure°5,°the relative intensities of the ϩ6 and ϩ5 charge state monoisotopic peaks of A␤ monomer were monitored while moving the emitter horizontally across the entrance°aperture.°Results°are°shown°in°Figure°6.°The profile for the coaxial probe was obtained while running 0.5-M A␤ monomer in 50/50/0.5 (vol/vol/vol) H 2 O/MeCN/HCOOH at 10 L/min in tube 1. The results for the triaxial probe were generated by running 1-M A␤ monomer in 100/0.5 (vol/vol) H 2 O/HCOOH at 5 L/min in tube 1 and 100/0.5 (vol/vol) MeCN/ HCOOH 100/0.5 H 2 O/HCOOH at 5 L/min in tube 2, giving the same final composition and total flow.…”

Automated orthogonal control system for electrospray ionization

“…[7]. The principal means of increasing the operable ESI flow rate was with the addition of either coaxial or cross flow sheath gas to aid in the droplet formation suitable for ion generation and mass analysis [8 -10].While much research and development effort was aimed at increasing electrospray's operable flow rate, a number of groups conducted studies at lower flow rates [11][12][13][14]. Early observations by Gale and Smith [11] showed that the flow rate could be reduced to 200 nL/min without reducing the signal-to-noise (S/N) ratio.…”

“…While much research and development effort was aimed at increasing electrospray's operable flow rate, a number of groups conducted studies at lower flow rates [11][12][13][14]. Early observations by Gale and Smith [11] showed that the flow rate could be reduced to 200 nL/min without reducing the signal-to-noise (S/N) ratio.…”