Efficient Ionizer for an Array of Mass Spectrometers

NASA's Jet Propulsion Laboratory, Pasadena, California

An electron-beam ionizer has been designed to deliver ions to the entrance apertures of nine miniature quadrupole mass spectrometers in an array. A similar electron-beam ionizer could also be designed for an array of more or fewer mass spectrometers. Principal issues that had to be addressed in formulating the design were (1) generation of a collimated, suitably dimensioned electron beam that passes near all the entrance apertures so that ions are generated for each aperture at a sufficient rate; (2) application of suitable extraction potentials to nearby, suitably configured electrodes so that a large flux of ions can be extracted from the electron-beam region wherein the analyte is ionized; and (3) choosing a configuration and potentials for a system of electrostatic lenses (that is, assemblies of electrodes and apertures) to transport and focus the extracted ions into beams of desired diameters and kinetic energies that impinge on the entrance apertures of the mass spectrometers at the desired angles.

The Electron-Beam Ionizer is integrated with electrodes for extraction of ions and focusing the ions into beams that impinge on the entrance apertures of quadrupole mass spectrometers.

The design process began with the initial choice of an ionizer geometry compatible with the input-beam requirements and other aspects of the design of the miniature mass-spectrometer array, and with regard to the locations of such fixtures as grounded screws, nuts, and posts. A commercial software package was then used to compute the performance of the ionizer and iterate upon the design. The computational modeling involved taking account of phenomena that are difficult to visualize, including electron space charge, the effect of the space charge on ion trajectories, and effects of electrostatic fringing fields.

The final design reached after iteration provides for a space-charge-limited electron beam that passes near all the spectrometer entrance apertures, optimal extraction of ions via penetration of the electric fields of the electrostatic lenses into the electron-beam ionization region, and optimal focusing of the extracted ions by the electrostatic lenses, which are arrayed in a compact system.

The figure presents a cross section of the ionizer in a plane that contains two of the spectrometer entrance apertures. Electrons are emitted from the cathode, which is a straight wire perpendicular to the page. The two shim plates help to collimate the emitted electrons into a ribbon beam. The beam passes through a collision volume (the ionization region) and makes its exit to the left. Within the collision volume, the electron beam knocks electrons off analyte molecules, which thereby become positive ions.

Two electric fields act together to extract ions from the collision volume into the electrostatic lenses above the collision volume. One of these electric fields is that generated by the repeller plate at the bottom. The other field is the one that is generated by the lowermost electrostatic-lens electrode and that penetrates the collision volume via the electrostatic-lens entrance apertures. The electrostatic lenses focus the extracted ions into beams that impinge on the mass-spectrometer entrance apertures with the desired characteristics as described above. The focusing action occurs over a range of nascent ion trajectories and energies in the collision volume.

This work was done by Ara Chutjian, Murray Darrach, and Otto Orient of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to: Technology Reporting Office, JPL, Mail Stop 122-116, 4800 Oak Grove Drive, Pasadena, CA 91109. (818) 354-2240 Refer to NPO-20252, volume and number of this NASA Tech Briefs issue, and the page number.