Radiant energy – Ion generation – Field ionization type
Reexamination Certificate
2001-07-19
2004-08-31
Lee, John R. (Department: 2881)
Radiant energy
Ion generation
Field ionization type
C250S281000, C250S288000, C250S287000, C250S493100, C250S492100, C250S492300, C436S172000, C436S177000, C436S120000
Reexamination Certificate
active
06784439
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to electrostatic spray devices, and more particularly to an improved electrospray ion source assembly.
BACKGROUND OF THE INVENTION
The electrospray (ES) process generally includes flowing a sample liquid into an electrospray ion source comprising a small tube or capillary which is maintained at a high voltage, in absolute value terms, with respect to a nearby surface. Conventional ES systems for mass spectrometry apply high voltage (relative to a ground reference) to the emitter electrode while holding the counter electrode at a lower, near ground reference voltage. For the positive ion mode of operation, the voltage on the emitter is high positive, while for negative ion mode the emitter voltage is high negative.
However, the emitter electrode can be held at (or near) the ground voltage. In this alternate configuration, the counter electrode is held at high negative voltage for positive ion mode and at high positive potential for negative mode. The voltage drop is the same between the electrodes and the electron flow in the circuit is the same in both the conventional and alternate bias configurations.
The liquid introduced into the tube or capillary is dispersed and emitted as fine electrically charged droplets (plume) by the applied electrical field generated between the tube or capillary which is held at high voltage, referred to as the working electrode, and the nearby surface. The nearby (e.g. 1 cm) surface is commonly referred to as the counter electrode.
The ionization mechanism generally involves the desorption at atmospheric pressure of ions from the fine electrically charged particles. The ions created by the electrospray process can then be used for a variety of applications, such as mass analyzed in a mass spectrometer.
The electrospray ion source operates electrolytically in a fashion analogous to a two-electrode controlled current (CCE) flow cell, effectively forming an electrochemical cell in a series circuit. A metal capillary or other conductive contact (usually stainless steel) placed at or near the point from which the charged ES droplet plume is generated (the ES emitter) is the working electrode in the system. The analytically significant reactions (in terms of ES-mass spectrometry (MS)) generally occur at this electrode.
The rate of charged droplet production by the electrospray source defines the average current (droplet generation rate times average charge per droplet) that flows in the cell (i.e., the ES current, i
ES
). This rate is determined by several interactive variable parameters including the magnitude of the electric field applied between the working and counter electrodes, the solution flow rate, the solution viscosity and electrical conductivity. When used as an ion source for mass spectrometry, the counter electrode of the circuit is generally the atmospheric sampling aperture plate or inlet capillary, the various lens elements and detector of the mass spectrometer.
In a typical ES-MS process, a solution containing analytes of interest is pumped through the ES emitter which is held at high voltage, resulting in a charged solvent droplet spray or plume. The droplets drift towards the counter electrode under the influence of the electric field. As the droplets travel, gas-phase ions are liberated from the droplets. This process produces a quasi-continuous steady-state current with the charged droplets and ions constituting the current and completing the series circuit.
To sustain the buildup of an excess net charge on the surface of the liquid exiting the emitter, heterogeneous (electrode-solution) electron transfer reactions (i.e., electrochemical reactions) must occur at the conductive contact to the solution at the spray end of the ES device. Accordingly, oxidation reactions in positive ion mode (positive high voltage potentials) and reduction reactions in negative ion mode (negative high voltage potentials) will dominate at the ES emitter electrode. Electron transfer reactions also must occur at the counter electrode. Charge can flow in no other way than through these electrode circuit junctions. Thus, electrochemical reactions are inherent to the basic operation of the electrostatic sprayer used in ES applications, such as ES-MS.
The electrolysis reactions that take place in the ES emitter can influence the gas-phase ions formed and ultimately analyzed by the mass spectrometer, because they may change the composition of the solution from the composition that initially enters the ion source. These changes include, but are not limited to, analyte electrolysis resulting in ionization of neutral analytes or modification in the mass or charge of the original analyte present in solution, changes in solution pH through electrolytic H
+
or OH
−
production/elimination, and the introduction/elimination of specific species to/from solution (e.g., introduction of Fe
2+
ions from corrosion of a stainless steel emitter).
Other than direct electrolysis of a particular species, redox chemistry or other chemistry can take place via homogenous solution reactions with a species that may be created at the working electrode. Homogeneous solution reactions are also used in controlled-current coulometry.
Applied to electrospray, a homogeneous solution reaction can occur though creating a species at the working electrode, and then diffusing the created species into solution and interacting it with another species causing an effect. This is a homogenous solution reaction, whereas reaction at the working electrode is heterogenous process. Homogeneous solution reactions provide the ability to greatly increase reaction efficiency because not all the analyte needs to get to the working electrode surface to react.
Sufficient time must generally be provided for the homogenous reaction to take place before the material is sprayed. Time between electrochemical reaction and spraying can be provided by an upstream working electrode contact. The electrochemical creation of reactants for the homogenous solution reaction can also buffer the potential to a given level, provided the species reacting is in high enough concentration or the reaction is not diffusion limited. A particular advantage of this approach is the ability to generate unstable reactants (e.g., the oxidant bromine) in situ.
Determining the extent and nature of these solution compositional changes is a complex problem. Because the magnitude of i
ES
is known to be only weakly dependent on solvent flow rate, the extent of any solution compositional change that the electrolytic reactions can impose necessarily increases as flow rate decreases. The interfacial potential distribution of the working electrode ultimately determines what reactions in the system are possible as well as the rates at which they may occur.
However, in an ES ion source, the interfacial potential is not fixed, but rather adjusts to a given level depending upon a number of interactive variables to provide the required current to the circuit. The variables that are expected to materially affect the interfacial electrode potential include, but are not limited to, the magnitude of i
ES
, the redox character and concentrations of all species in the system, the solution flow rate, the electrode material and geometry. Control over the electrochemical operation of the ES ion source is essential both to avoid possible analytical pitfalls it can cause (e.g. changes to the sample to be analyzed) and to fully exploit the phenomenon for new fundamental and analytical applications which are available through use of ES-MS.
Currently available electrospray emitter designs have not considered structures which can permit improved control of the electrochemistry of the electrochemical cell which can be used for analytical benefit. For example, current electrospray emitter designs do not perform efficient mass transport to the working electrode surface.
SUMMARY OF INVENTION
An electrospray device includes a high voltage electrode chamber having an inlet for receiving a fluid to be i
Akerman & Senterfitt
Lee John R.
U-T Battelle, LLC
Vanore David A.
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