Patent Application
20070063138
1. Field of the Invention
The present invention relates to devices and methods for separation of ions from a neutral carrier fluid. More specifically, the invention relates to transfer of ions in a first carrier gas to a second carrier gas.
2. Description of the Related Art
In an application of Field Asymmetric Ion Mobility Spectroscopy (FAIMS), a sample gas is partially ionized and the ions in the ionized gas are separated according to each ion's mobility by application of an asymmetric electric field. In many situations, the neutral molecules or atoms in the carrier gas can reduce the ability of the FAIMS to fractionate the ions and can also contribute to a noise component in the detector electrode. Therefore, there remains a need for devices and methods that can transfer the ions in a first carrier gas to a second carrier gas.
An ion gate is disposed between a first volume occupied by a first carrier gas and ions of the first carrier gas and a second volume occupied by a second carrier gas. The ion gate includes at least one channel connecting the first volume to the second volume, a first electrode disposed on an inlet surface of the ion gate facing the first volume, and a second electrode disposed on an outlet surface of the ion gate facing the second volume. Ions are transported from the first volume to the second volume through the channel under an electric field produced by the first and second electrodes.
One embodiment of the present invention is directed to a device comprising: a first carrier gas occupying a first volume, the first carrier gas including ions; a second carrier gas occupying a second volume; an ion gate disposed between the first and second volumes, the ion gate including at least one channel allowing ions in the first volume to enter the second volume, a first electrode at a first electric potential disposed on an inlet surface of the ion gate, a second electrode at a second electric potential disposed on an outlet surface of the ion gate, the first and second electric potential providing an electric driving force to transport ions in the first volume to the second volume through the at least one channel. In an aspect of the present invention, the at least one channel is characterized by a channel length that is less than 1 mm. Preferably, the channel length is less than 500 microns, and most preferably the channel length is less than 300 microns. In an aspect of the present invention, the at least one channel is characterized by a channel cross-sectional area that is between 10,000 μm2 and 1 μm2. Preferably, between 2,500 μm2 and 10 m2, and most preferably between 1,000 μm2 and 10 μm2.
The invention will be described by reference to the preferred and alternative embodiments thereof in conjunction with the drawings in which:
In a preferred embodiment, the first volume contains a first carrier fluid and ionized molecules of the first carrier fluid. The second volume contains a second carrier fluid that is preferably different from the first fluid. The fluid may be a liquid or a gas depending on the application of the ion gate. For example, the first and second carrier fluids may be gaseous when the ion gate is used in an ion mobility spectrometer. Alternatively, the first and second carrier fluids may be liquid when the ion gate is used in electrophoresis.
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In a preferred embodiment, the ion gate is made of a high resistivity material such as, for example, silicon, quartz, silica, or Pyrex. Channels 135 may be manufactured using known MEMS processing methods such as, for example, Deep Reactive Ion Etching (DRIE) or laser drilling. The channel length, or the distance between the first and second volumes, is less than 1 mm, preferably less than 500 microns, and most preferably less than 300 microns. The cross-sectional area of each channel is between 1 μm2 and 10,000 μm2, preferably between 10 μm2 and 2,500 μm2, and most preferably between 10 μm2 and 1,000 μm2. The number of channels may be selected such that the total cross-sectional area of the channels is between 0.01 and 5 cm2 and preferably between 0.1 and 1 cm2.
In some embodiments, the channels may have a rectangular cross-section such as, for example, a slot where the width of the channel is very much smaller than the height of the channel. Other configurations may include a serpentine slot. The width of the slot may be between 1 μm and 100 μm, preferably between 5 μm and 60 μm, and most preferably between 10 μm and 40 μm. The height of the slot may between 10 and 10,000 times the slot width and preferably between 100 and 1,000 times the slot width.
In some embodiments, the second volume may be at a higher pressure relative to the pressure in the first volume. The pressure difference between the first and second volume creates a pressure head across the ion gate that induces a flow from the second volume to the first volume. It is believed that the high fluidic impedance of the ion gate reduces the transport of the second carrier gas into the first volume while still allowing ions in the first volume to be driven by the electrodes into the second volume. The reduction in transport is relative to a single convex channel with a cross section equal to the cumulative cross-sectional areas of the one or more channels in the ion gate.
Having thus described at least illustrative embodiments of the invention, various modifications and improvements will readily occur to those skilled in the art and are intended to be within the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.
