TIME-OF-FLIGHT MASS SPECTROMETER OPERATIVE AT ELEVATED ION SOURCE PRESSURES

• US 3,553,452 A
• Filed: 02/17/1969
• Issued: 01/05/1971
• Est. Priority Date: 02/17/1969
• Status: Expired due to Term

• Current Assignee: Carrol D. Miller, Fred P. Abramson, Jean H. Futrell, Thomas O. Tiernan
• Original Assignee: Carrol D. Miller, Fred P. Abramson, Jean H. Futrell, Thomas O. Tiernan
• Inventors: Tiernan, Thomas O., Miller, Carrol D., Futrell, Jean H., Abramson, Fred P.
• Application Number:
  US04/799,849
• Time in Patent Office: 687 Days
• US Class Current: 250/287
• CPC Class Codes: H01J 49/24 Vacuum systems, e.g. maintaining desired pressures
• CPC Class Codes: H01J 49/403 characterised by the acceleration optics and/or the extraction fields

Abstract

A time-of-flight mass spectrometer for the investigation of ionmolecule reactions at ion source pressures up to about 1 Torr. The ion source is operated with a continuous electron beam and a constant repeller voltage of the minimum value necessary to extract ions from the source. A shaped repeller and low electron trap potential are used to provide an uniform field in the source, resulting in a minimum kinetic energy range for the extracted ions. Focusing takes place in the low-pressure region outside the source bounded by the first and second grids between which the focusing pulse is applied. A concurrent blocking pulse is applied to the first grid to prevent ions entering the focusing region during the focusing operation.

Claims

1. Apparatus situated in the evacuated housing of a time-offlight mass spectrometer for generating, focusing and accelerating ions for introduction into the drift tube of said spectrometer, said apparatus comprising:

an ion source structure providing an enclosed ionization chamber having an electrically insulated porous repeller plate and an exit plate containing a small exit aperture as parallel opposite walls, means providing a continuous electron beam passing centrally through said chamber, means for introducing a gas under relatively high pressure into said chamber through said repeller plate, and means for applying a constant small positive potential to said repeller plate for expelling ions through said exit aperture;

ion focusing means comprising two spaced grids located outside said source structure in the evacuated space of said spectrometer housing and situated parallel to said exit plate and opposite said exit aperture for receiving ions expelled through the aperture into the space between said grids, and means for periodically and concurrently applying a positive blocking pulse to the grid nearer the exit aperture and a larger negative focusing pulse to the other grid; and

ion accelerating means comprising a third grid parallel to and spaced from the grid receiving said focusing pulse, and means for maintaining said third grid at a constant high negative potential.

2. Apparatus as claimed in claim 1 in which means are provided for heating said ion source structure to a temperature high enough to prevent the formation of a surface charge on the walls of said chamber.

3. A time-of-flight mass spectrometer having, in a gas tight housing under constant evacuation, a unipotential drift tube, an ion detector located at the exit end of the drift tube, and apparatus for generating, focusing, and accelerating ions for introduction into said drift tube, said apparatus comprising:

an ion source structure situated opposite and spaced from the entrance to said drift tube and providing an enclosed ionization chamber centered on the extended axis of said drift tube and bounded on two opposite sides by an electrically insulated porous repeller plate and an exit plate both normal to said extended axis, a said exit plate beiNg nearer the drift tube entrance than said repeller plate and having a small exit aperture concentric with said extended axis, means providing a continuous electron beam passing centrally through said chamber and normal to the extended axis of the drift tube, means for introducing a gas for analysis under relatively high pressure into said chamber through said porous repeller plate, and means for applying a small constant positive direct potential to said repeller plate for expelling ions from said chamber through said exit aperture;

first, second, and third grids normal to the extended axis of the drift tube and situated between said exit plate and the entrance to said drift tube, said first grid being close to said exit aperture, said third grid being at the entrance to said drift tube, and said second grid being intermediate the first and third grids;

means for periodically applying a negative focus pulse to said second grid;

means synchronized with said focus pulse for applying concurrently therewith a positive pulse to said first grid for blocking the exit of ions from said chamber during said focus pulse;

means for continuously maintaining said third grid at a high negative potential; and

means for maintaining said drift tube at the potential of said third grid.

4. Apparatus as claimed in claim 3 in which said repeller plate is provided with an extension into said chamber, said extension being in the form of an open ended box only slightly smaller than said chamber and extending to a point near said electron beam for making the electric field in said chamber produced by the potential of said repeller plate more uniform particularly in the region between the beam and the exit aperture.

5. Apparatus as claimed in claim 4 in which said electron beam is established between a cathode and a plate acting as an electron trap both situated outside and on opposite sides of said chamber with said beam passing through apertures in opposite walls of said chamber, and in which said plate is maintained at a low positive potential relative to the walls of said chamber to reduce the field in said chamber due to said plate.