Mass spectrometer

Details Mass spectrometer Mass spectrometer

1990-07-19

1992-05-26

Anderson, Bruce C.

Radiant energy

Ionic separation or analysis

Ion beam pulsing means with detector synchronizing means

250281, B01D 5944, H01J 4900

Patent

active

051171074

DESCRIPTION:

BRIEF SUMMARY
The present invention relates generally to the field of Mass Spectrometry, and in particular the invention provides an improved Time-of-Flight (TOF) Mass Spectrometer.
Time-of-Flight mass spectrometers have generally employed one of three different means of ion formation:
In prior art TOF mass spectrometers the first of these methods, when employed, suffers from an inherently limited mass resolution caused by the formation of ions both over a significant region of space and having a spread of thermal energies. Each of these factors affect the time taken for ions to travel the length of the flight tube of the spectrometer and therefore affect the resolution of the instrument.
The remaining two methods, each work with solid samples and succeed in creating ions in a much better defined spatial plane. However, each of these methods still suffers from the problem of ion energy spread. The energy spread problem is usually compensated for by the use of a Reflectron (B. A. Mamyrin and D. V. Schmikk, Sov. Phys. JETP, 49, 762 [1979]) thereby obtaining better mass resolution. However this is of no avail for routine "gaseous" samples such as those derived from a Gas Chromatograph.
The present invention consists in a time-of-flight mass spectrometer comprising a source of ions, beam forming means to produce a substantially parallel beam of the ions generated by said source, an ion accelerator arranged to accelerate the ions of said beam in a direction orthogonal to the direction of the beam and means to measure the times of arrival of said ions at a target located at a predetermined distance from the accelerator, the accelerator comprising at least two parallel planar electrodes disposed about the path of said beam to define a first-stage acceleration chamber, at least one of the electrodes being a grid, and a pulsed voltage source connected between said electrodes such that when no voltage is applied the electrodes define a field free region and when a voltage pulse is applied between the electrodes an electric field will be generated and ions located between said electrodes will be accelerated orthogonally to the direction of the beam.
In a preferred embodiment of the invention, ions leaving the accelerator pass into a second-stage accelerator region having an electric field equal to that in the first-stage of the accelerator when the push-out pulse is applied to the parallel electrodes and then into a main accelerator region (third-stage) having a strong potential gradient to accelerate the ions toward the target.
Embodiments of the present invention may employ any of the prior art ion sources, including electron impact ionisation, chemical ionisation and fast atom bombardment sources.
An embodiment of the invention will now be described with reference to the accompanying drawings in which:
FIG. 1 schematically illustrates a TOF mass spectrometer according to the present invention; and
FIG. 2 schematically illustrates the orthogonal accelerator the stage-two region and the main accelerator of the embodiment of FIG. 1 in greater detail.
Referring to FIG. 1 the illustrated TOF Mass Spectrometer which is contained within a continuously pumped high vacuum housing, has an electron impact ionisation source 10 into which a gaseous sample is admitted. The source includes a heated cathode 12 to emit an electron beam through a sample chamber 13 to an anode (electron trap) 11 such that collisions between electrons and atoms of the sample gas within the chamber produce positive ions which are then repelled by a positively biased repeller 14, such that some of the ions will pass out through an aperture 15 as an ion leakage. While the cathode 12 and electron trap 11 are schematically illustrated as being above and below the source housing 10 they are in reality above and below the plane of the page. The source chamber is held at a positive voltage, e.g. +50 V. The differential pumping baffle 16 in which is placed the source slit 17 is held at a negative potential (e.g. -250 V), and the leaked ions are accelerated towards the baff

REFERENCES:
patent: 2938116 (1960-05-01), Benson et al.
patent: 3576992 (1971-05-01), Moorman et al.
patent: 3986111 (1976-10-01), Sellers

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