SuperAtmospheric Pressure Electrospray Ion Source

Authors: Lee Chuin Chen Mridul Kanti Mandal Kenzo Hiraoka

Publish Date: 2011/10/12

Volume: 22, Issue: 12, Pages: 2108-2114

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Abstract

This is a followup paper of our previous report on an ion source which was operated at an operating pressure higher than the atmospheric pressure Besides having more working gas for desolvation the reduction of mean free path of electrons in a higher pressure environment increases the threshold voltage for gaseous breakdown thus enabling a stable electrospray for the sample solution with high surface tension without the occurrence of electric discharge In our previous work the ion source was not coupled directly to the mass spectrometer and significant amount of ions were lost before entering the vacuum of the mass spectrometer In this paper we report the new design of our second prototype in which by using a modified ion transport capillary the pressurized ESI ion source was coupled directly to the first pumping stage of the mass spectrometer without additional modification on the vacuum pumping system Demonstrations of the new ion source on the sensitive detection of native proteins from aqueous solution in both positive and negative ion modes are presentedStandard electrospray ion source is operated under an atmospheric pressure ambient 1 2 3 4 Working at atmospheric pressure is not only convenient to the user the bath gas also provide sufficient thermal energy to the charged droplets for the vaporization of solvent without freezing them This feature allows the electrospray to work with wide range of solvents including those commonly used in the liquid chromatography while its vacuum counterpart electrohydrodynamic EHD spray 5 6 can only handle limited types of solvent Generation of ions in the atmospheric pressure rather than in vacuum also helps to keep the internal and translational energies of the ions at a level corresponding to the bath gas temperature 3 Nowadays the mass spectrometer which is designed to work with ESI and other atmospheric pressure ionization API source is usually referred as API mass spectrometer 7Some efforts had been put forward to explore the workability of ESI in the pressure range other than atmospheric pressure For example subatmospheric 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 superatmospheric pressure electrospray ionization with an ion source that could be pressurized with compressed air up to 5 bars 10 Raising the operating pressure of the ESI ion source offers some interesting features that can supplement the conventional ESI or nanoESI ion source especially in the case of handling labile compound in aqueous solution Aqueous solution has so far been handled by pneumaticassisted ESI 11 12 heated ESI 13 ESI with electron scavenging sheath gas 14 and more popularly by nanoESI 15 16 17 The approach used in our ion source is somewhat different from these existing methodsDue to its high surface tension water is known to have higher ESI onset voltage compared with organic solvent mixtures eg 50 vol/vol methanol/water solution Thus even with pneumatic assistance it is rather difficult to achieve stable and efficient electrospray ionization for aqueous solution because the threshold voltage for the gaseous breakdown could fall below or about the same with the onset voltage of electrospray Once the gaseous breakdown occurs it leads to a corona or arc discharge that seriously affects the electric field near the ESI emitter and degrades the performance of electrospray Theoretical estimations for the onset voltage of electrospray are given by Taylor 18 Smith 19 and more recently by Wilm and Mann 15 The ESI onset voltage depends primarily on the surface tension of the solution and the relative permittivity of the ambient medium If the ambient medium is air or gas and for a solution which is relatively incompressible eg water the electrospray onset voltage is almost unaffected by the ambient gas pressure up to several bars 10 The threshold voltage for inducing the gaseous breakdown however will increase with the rise of gas pressure due to the reduction of mean free path for electrons This phenomenon is usually referred as Paschen law 20 Thus besides using CO2 or SF6 as sheath gas 14 21 it is also possible to quench the gaseous breakdown completely by pressurizing the ESI ion source with air or N2 to an appropriate levelBy operating the ion source at air pressure higher than 4 bars we have previously verified that asides from maintaining stable steady conejet mode electric field around the ESI emitter could be increased up to the formation of multijet mode nomenclature due to Cloupeau and PrunetFoch 22 for both positive and negative ion modes without the occurrence of corona discharge Besides solving the gaseous breakdown problem there are other advantages by working under a high pressure condition Since more bath gas is used for the desolvation process the electrospray ionization is expected to be even softer compared to that operated at atmospheric pressure The gas pressure of the ion source can also be adjusted to tune the pressure in the first pumping stage of the mass spectrometer which is known to influence the collision induced dissociation and the softness of ion desolvation in vacuum 23 24 25 When the heating of the sample solution becomes necessary eg to study the thermally induced denaturation of protein DNA etc a high pressure condition can even allow the solution to be heated beyond its boiling point to realize a superheated electrospray ionizationIn this paper we report a new design of our second prototype of the high pressure ESI source In our previous prototype ions produced from the ESI were not coupled directly to the vacuum stage of the mass spectrometer but were transported out from the high pressure vessel to an atmospheric pressure side via a metallic capillary The inner diameter id of the capillary was 08 mm and the gas flow rate was ~17 L/min The typical gas sampling rate for API mass spectrometers with differential pumping and ~05 mm inlet orifice is about 1 L/min This means that most of the ion rich gas from our previous ion source could not be fed directly to the mass spectrometer due to the limitation of gas load allowed for maintaining the operational vacuum level In the present work endeavor has been made to transport the electrospray ions and fine charged droplets directly from the high pressure vessel to the vacuum stage of the mass spectrometersThe functionality of the present ion source has been tested using two commercial mass spectrometers a linear ion trap LTQVelos Thermo Fisher Scientific San Jose CA USA and a bench top Orbitrap Exactive Thermo Fisher Scientific Bremen Germany Working pressures in the highest vacuum stages that house the mass analyzer were 10−5 mbar for LTQVelos and 10−10 mbar for Exactive

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