Letter pubs.acs.org/ac
Electrospray Ionization from Nanopipette Emitters with Tip Diameters of Less than 100 nm Elizabeth M. Yuill, Niya Sa, Steven J. Ray, Gary M. Hieftje, and Lane A. Baker* Department of Chemistry, Indiana University, 800 E. Kirkwood Avenue, Bloomington, Indiana 47405, United States S Supporting Information *
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. anopipettes are multifunctional fluidic tools that find important application in electrochemical and electrophysiological measurement.1 In particular, the small size of nanopipettes (from several hundred nanometers to tens of nanometers) is well matched to measurement and sampling of biological systems.30 Here, we detail the use of nanopipette emitters for nanoelectrospray ionization (nano-ESI). Electrospray ionization (ESI) is a powerful technique for soft ionization of analytes and is especially useful for MS of biological macromolecules.3 ESI produces multiply charged ions directly from small volumes of analytical solution, which is useful for MSn studies and allows for detection of high mass species in the m/z range of a typical quadrupole.4 More recently, improvements in ESI have been realized through nano-ESI approaches.5 In nano-ESI, the orifice diameter of the emitter is typically reduced to between 2 and 50 μm,6 which results in several important benefits for MS analysis.7 Nano-ESI provides increased ionization efficiency5,8,9 relative to conventional ESI as a consequence of the smaller droplets initially produced, which yields an increased surface charge-to-volume ratio that promotes more efficient ion formation. Consequently, nano-ESI has been shown to be more tolerant to buffer composition, detergents, and salt content.2,5,7,10−12 Additionally, nano-ESI operates at much lower solution flow rates (
