Atmospheric Pressure FocusingAtmospheric Pressure Focusing
Chem-Space Associates, Inc., Pittsburgh, PA
University of South Florida, St. Petersburg, FL
Copyright © 2004 by Chem-Space Associates, Inc. ALL RIGHTS RESERVED
Background
The generation of ions at atmospheric pressure is accomplished by a variety of means; including electrospray, atmospheric pressure chemical ionization (APCI), atmospheric pressure MALDI, and photoionization. A general characteristic of all these ion sources is the need for a large volume in order to form ions or charged particles/droplets. Needle/capillary sources such as electrospray and APCI, disperse ions radially from the axis in high electric fields and gases emanating from tubes and nebulizers. Even desorption techniques, such as atmospheric pressure-MALDI, will disperse ions radially in a solid angle from a surface.
This radial dispersion can be a large as 5-10 cm in diameter!!
The motion of ions at atmospheric pressure is influenced by electric, viscous, and thermal forces (see below). In low electric fields, the motion is dominated by random motion (thermal) and gas flow (viscous). In contrast to this, the motion of ions in high electric fields is dominated by electric field and gas stream lines. With electric fields driving the motion of the ions in the direction of the field and viscous forces imparting an acceleration (forward force), a deceleration (backward force), or radial movement (orthogonal force) on the motion of the ion. The total motion in high electric fields can then be described as the addition of the force vectors of both electric fields and gas stream lines.
Methods
One solution to collecting and focusing this dispersive plasma is to place a high-transmission element (HTE), between the plasma and the sampling aperture or capillary. Three types of HTE's are considered:
Surface Laminated Surface Toroidal Surface
Various HTE's were sandwiched between an electrospray nebulizer and a collection surface. A dilute of blue dye (0.01%, TPM 5EO/5PO, Milliken, Corp.) was electrosprayed. All points, from the aerosol chamber to the collection area were observed for the deposition of dye. In addition, ion current was measured at all surfaces and inside a vacuum system. Utilizing the program SIMION, ion trajectories, depositon, and loses were predicted.
Copyright © 2004 by Chem-Space Associates, Inc. ALL RIGHTS RESERVED
Motion of Ions Due to Viscous Forces
at Atmospheric Pressure
Motion of Ions in Electric Fields at Atmospheric Pressure Focusing
Copyright © 2004 by Chem-Space Associates, Inc. ALL RIGHTS RESERVED
High Transmission Surface
High Transmission Surface
High Transmission Surface
Laminated Surface
Laminated Surface
Laminated Surface
Toroidal Laminated Surface
Toroidal Laminated Surface
Toroidal Laminated Surface
Atmospheric Pressure Focusing
Conclusions
To focus and collect ions at atmospheric pressure one needs to:
Control or sculpt all the electric fields, from the regions of ion generation to ion focusing; and
Control the viscous forces, both concurrent and counter-current, from ion generation to sampling through sampling apertures.
Put simply:
Ions Motion in High Electric Fields at Atmospheric Pressure is the Sum of
Electric & Viscous Forces
The SIMION ion optics simulation program can be used to predicted and verify the motion of ions at atmospheric pressure incorporating the both electric and viscous force components.
By matching high electric and low electric field sources with an optimized high-transmission surface and gas flow one can collect, focus, and deliver gas-phase ions from dispersive atmospheric sources onto small cross- sectional areas—2-3 mm!
Having the ability of forming these small cross-sectional beams one must match the dimensional of the ion beam with small openings for transfer into vacuum systems. Transferring this ion beam into the vacuum system is addressed at our companion poster, Poster #100.
Acknowledgements
The authors would like to thank the National Institute of Health for financial support through the SBIR Program (grant # R43 RR143396-1) and David Fries and his staff at the College of Marine Sciences (St. Petersburg, FL) for the use of their labs and manufacturing facilities.
References
Single Surface High-Transmission Element: Sheehan, E.W., Willoughby, R.C., "Apparatus and method for focusing ions and charged particles at atmospheric pressure", US Patent 6,744041 (June 1, 2004).
Laminated High-Transmission Element: Sheehan, E.W., Willoughby, R.C., US Patent Application No. 10/449,167.
Toroidal High-Transmission Element: Sheehan, E.W., Willoughby, R.C., US Patent Pending.
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