An atmospheric pressure ion lens that improves nebulizer assisted electrospray ion sources

Abstract

An atmospheric pressure ion lens improves the performance and ease of use of a nebulizer assisted electrospray (ion spray) ion source. The lens is comprised of an oblong-shaped stainless steel ring attached to an external high voltage power supply. The lens is located near the tip of the conductive sprayer, and is maintained at a potential less than that of the sprayer. The ion lens improves the shape of the equipotential lines in the vicinity of the sprayer tip. This lens gives approximately a 2-fold reduction in the signal RSD, a 2-fold increase in the ion signal, an increase in the number of multiply charged ions, and a much broader range of usable sprayer positions. , is a versatile technique for the formation of gas phase ions for mass spectrometry. In these early studies, the sprayer was stainless steel syringe tubing with an internal diameter of 100 m, and the solution flow rates were a few L/min or greater. Since then, ESI has been used extensively for many applications including analysis of drugs [2], proteins [3], and oligonucleotides [4], to name a few. A key concern in many of these types of analyses is maximizing the magnitude and stability of the ion signal.Electrospray ion sources can be operated over a wide range of solution flow rates, including high flow rate (Ն1 L/min), reduced flow rate (80 nL/min-1 L/ min), and very low flow rate (Ͻ80 nL/min). Wilm and Mann described the first ESI source for operation at very low liquid flow rates (Ϸ20 nL/min) [5]. Stabilization of electrospray at this flow rate required the use of tapered capillary tips, with internal diameters of 1-2 m. A thin layer of gold was deposited on the glass capillary tips for application of the electrospray potential. This technique was termed nanoelectrospray ionization. Sampling efficiency was increased, however operation with the tapered tips was more difficult than electrospray at higher flow rates.The first description of electrospray operating as a reduced flow rate ion source was by Smith and coworkers [6], in which a capillary electrophoresis system was coupled with a mass spectrometer. Shortly thereafter, Emmett et al. described a packed electrospray needle for interfacing chromatography with mass spectrometry at low flow rates (300 nL/min and above) [7]. Following these initial experiments, there have been many studies with electrospray operating in this flow regime for applications involving direct infusion [8,9], or the coupling of CE [10,11] and LC [12,13] with ESI-MS. Typically these reduced flow rate ESI sources provide improved signal stability and sampling efficiency when compared to higher flow rate ESI sources. However they require the use of tapered capillaries with a metal coating [5], incapillary electrode [14], or some other type of junction [8,15,16] for applying the electrospray potential.High flow rate electrospray ion sources are easy to operate, but typically suffer from a lower sampling efficiency and poorer signal stability when compared with reduced flow rate electrospray ion sources. This problem i...

The influence of electrospray ion source design on matrix effects

“…Modifications in ESI source design mainly involve the number and the positioning of heated gas inlets, the sprayer orientation relative to sampling orifice as well as the diameter of MS orifice or the number of transfer capillaries . Further, the ion lens system may be a starting point for improvements in ESI design as it decides on ion sampling efficiency and stability of ion signals …”

Improving mass spectrometer sensitivity using a high-pressure electrodynamic ion funnel interface

“…These efforts have resulted in significant gains in sensitivity, but design optimization efforts have generally reached the point of diminishing returns; e.g., our experience indicates that the ESI interfaces on instruments obtained from different vendors now provide similar levels of performance. This situation persists despite aggressive efforts aimed at increasing ion transmission from the ESI emitter to the capillary inlet, e.g., by means that include a Venturi device [7][8][9] and electrostatic lenses [12][13][14][15]. An electrodynamic ion funnel interface [16 -18] developed earlier in our laboratory was designed to overcome ion losses due to the small aperture of the skimmer [6] by allowing all ions that exit the inlet capillary to be efficiently captured and transmitted into the next vacuum stage.…”

Field distribution in an electrospray ionization source determined by finite element method

“…A previously described home‐built ion spray source15 was used. The sprayer tip was positioned 15 mm from the curtain plate, and the orientation of the sprayer capillary was 60° with respect to the curtain plate (Fig.…”

“…Previous work by our group14, 15 has demonstrated that an oblong‐shaped atmospheric pressure ion lens improves the performance of ESI sources at flow rates from 0.2 to 5 µL/min. Thompson and coworkers16 also reported that a hemispherically shaped electrostatic lens at atmospheric pressure was able to compress the space‐charge in nano‐ESI and increase the average current density in the plume by a factor of 3.…”