Effect of pressure on electrospray characteristics

Abstract

An experimental study of pulsating electrosprays operated at subambient pressure is reported. The pressure domain that affords stable electrospray operation appears to be limited by the vapor pressure of the liquid. The voltage driving the electrospray is shown to have a logarithmic dependence on pressure. The observed scaling amends the relationship currently used to calculate the electric field at the tip of the meniscus of an electrified liquid. © 2009 American Institute of Physics. ͓doi:10.1063/1.3258494͔ Zeleny 1 conducted the first rigorous studies of what he described as discharges from liquid points, phenomena known today as electrosprays. In his experiments, a minimal pressure difference delivered electrified liquid at the tip of a glass capillary for spraying against a planar counterelectrode. In a similar geometry, Eyring et al.2 studied the electric field E when applying a voltage V between a hyperbolic metal electrode polished to a radius of curvature r and a metallic disk situated at a distance d. In a simplified form, the electric field was expressed as a function of a constant c:This relationship is currently in use to estimate the electric field at the tip of a rounded liquid meniscus. A semispherical liquid drop is in equilibrium if the pressure drop across the liquid interface ͑p − p 0 ͒ is balanced by its surface tension ͑␥͒ : p − p 0 = ␥ / r, where r is the radius of curvature of the meniscus. Under the influence of E, r changes to compensate for the pressure produced by electrical forces p E = 0 E 2 / 2, where 0 is vacuum permittivity. Once the electric field surpasses a threshold at a critical voltage, a singularity is induced, which reshapes the meniscus into a cone and triggers the ejection of charged liquid from its apex. [4][5][6] The cone-jet electrospray regime 7 is established if the charge is generated electrochemically at the same rate as it is ejected; however, when operated close to the threshold electric field, supply disruptions lead to discontinuous regimes that have been described as burst, 8 pulsating, 9 or astable. 10One of the most important applications of electrosprays is in mass spectrometry ͑MS͒ ionization sources. 11 The small charged droplets experience cycles of solvent evaporation followed by charge reduction through Rayleigh fission events or field evaporation.12-15 The charged aerosol produced by electrosprays at atmospheric pressure must be transferred into a very low pressure environment for the MS analyses. This is generally accomplished by several stages of differential pumping. Ion funnels 16 can be employed to improve the ion transfer efficiency through intermediate chambers in the 1-30 torr range, while traditional radio frequency ͑rf͒ ion guides are broadly used in lower pressure regimes. A typical electrospray ionization ͑ESI͒-MS interface employs a heated capillary inlet enabling sampling of the electrospray while providing supplemental thermal energy to aid desolvation of the charged droplets. Most of the charged aerosol is "lost" to the front or insi...

High Pressure (>1 atm) Electrospray Ionization Mass Spectrometry

“…As shown in Supplementary Figure S3, when the operating pressure of the ion source was raised to 4 bar, both positive and negative mass spectra of various proteins (e.g., cytochrome c, lysozyme, and ubiquitin) could be obtained even from the sample solution prepared with pure deionized water. Recently, Marginean et al noticed that the electrospray condition (spray current and stability) changed with the operating pressure in a sub-atmospheric pressure environment, and the stability was limited by the vapor pressure of the liquid [16]. This pressure effect was not observed in our experiments, and the electrospray current remained nearly constant with the applied pressure once the stability of liquid flow rate was maintained.…”

Super-Atmospheric Pressure Electrospray Ion Source: Applied to Aqueous Solution

“…Some efforts had been put forward to explore the workability of ESI in the pressure range other than atmospheric pressure. For example, sub-atmospheric pressure electrospray (few 10s Torr) has been attempted by Marginean and coworkers using ESI emitter with solution flow rate of nL/min, so that the generated ions could be guided more efficiently by their focusing ion optics, namely ion funnel to the mass analyzer located in high vacuum [8,9]. On the other hand, we have recently demonstrated the super-atmospheric pressure electrospray ionization with an ion source that could be pressurized with compressed air up to 5 bars [10].…”

Super-Atmospheric Pressure Ion Sources: Application and Coupling to API Mass Spectrometer

“…1c). [17][18][19][20] e original ion transport tube for the LTQ-Velos was substituted with a custom made ion transport tube made from commercial straight cut stainless steel tube (GL Sciences, Tokyo, Japan), and it was slightly longer (∼7 cm) than the original one. To maintain the vacuum in the rst pumping stage, a Roots booster pump (PMB 003C, ULVAC, Kanagawa, Japan) was added in between LTQ and the original rotary vane pumps.…”

“…Recently, an attempt has been made to conduct the electrospray under sub-ambient pressure for improving the ion transmission. 16,17) However, under the gas pressure of 1-10 Torr, it is rather easy to induce corona or glow discharge that can disrupt the stability of electrospray severely.…”