Electrospray Ionization from Nanopipette Emitters with Tip Diameters of Less than 100 nm

Abstract

Work presented here demonstrates application of nanopipettes pulled to orifice diameters of less than 100 nm as electrospray ionization emitters for mass spectrometry. Mass spectrometric analysis of a series of peptides and proteins electrosprayed from pulled-quartz capillary nanopipette emitters with internal diameters ranging from 37 to 70 nm is detailed. Overall, the use of nanopipette emitters causes a shift toward the production of ions of higher charge states and leads to a reduction in width of charge-state distribution as compared to typical nanospray conditions. Further, nanopipettes show improved S/N and the same signal precision as typical nanospray, despite the much smaller dimensions. As characterized by SEM images acquired before and after spray, nanopipettes are shown to be robust under conditions employed. Analytical calculations and numerical simulations are used to calculate the electric field at the emitter tip, which can be significant for the small diameter tips used.

What Protein Charging (and Supercharging) Reveal about the Mechanism of Electrospray Ionization

“…That is, an increasingly steep ( basicity)/charge may yield narrower CSDs for conditions that generate ions with extremely high charge densities. These considerations also rationalize observations that protein CSDs are narrower when solutions are sprayed from tips with inner diameters < 70 nm instead of 1 μ [87]. …”

“…However, in analyses of multiply charged analytes, it is seen that employing increasing analyte concentrations [85,86] or higher flow rates [34,38,85] shifts charge state distributions to lower average charge. Still odder are the observations that the larger emitter openings of standard ESI sources yield CSDs shifted to lower charge than distributions obtained from nano–ESI sources [33-35,87]. Because the CRM relates analyte charge to the final, protein–sized droplet, it predicts CSDs that would be independent of initial spray conditions, in conflict with these observations.…”

Electrothermal supercharging of proteins in native MS: effects of protein isoelectric point, buffer, and nanoESI-emitter tip size

“…tips. 37 Improved signal-to-noise ratios and higher charging with decreasing tip size have also been reported for angiotensin I 38 and insulin chain B 39 ions formed using ESI emitters with adjustable orifice sizes. 38,39 In these devices, the orifice width is varied between 1 and 10’s of microns by adjusting the position of silicon chips.…”

Characterization of Nanopipettes

“…27 Beyond electroanalysis, nanopipettes are finding novel applications as devices for electrospray massspectrometry. 28,29 Nanopipette probes, employed in different types of scanning probe microscopy (SPM) techniques, 5,11,30 are used increasingly for the study of interfacial properties across a range of materials including electrodes and living cells. 3,31,32 Examples of SPM techniques that can employ nanopipettes include scanning electrochemical microscopy (SECM), 30,33 scanning ion conductance microscopy (SICM), 31,[34][35][36][37][38] SICM-SECM 9,39 and scanning electrochemical cell microscopy (SECCM).…”