Double-focussing mass spectrometer

Claim 1

A double-focussing mass spectrometer having a large entrance aperture to accept ions emitted by an ion source into a solid angle encompassing an axis, the said ions having various ratios of charge to mass; said mass spectrometer comprising an entrance diaphragm having an annular opening through which a diverging beam of said ions enters, said beam having a central ray in each radial section in a plane passing through said axis; an analyzing system to select ions of a predetermined ratio of charge to mass; an exit diaphragm having an annular opening through which said selected ions pass to ion detecting means, wherein said annular openings of said entrance and exit diaphragms are essentially coaxial to said axis and said analyzing system includes, along the paths of said ions between said entrance and exit diaphragms in the order named: a. energy analyzer means for direction focussing ions of equal energy to respective loci corresponding to their respective energies, said energy analyzer means being essentially rotationally symmetrical relative to said axis, further having a predetermined first energy dispersion coefficient, and deflecting the beam of ions entering through said opening of said entrance diaphragm in such a way towards the said axis, that the said central rays of the deflected beam of ions run essentially parallel to said axis; b. energy diaphragm means, having an annular opening and arranged in the plane of said loci to limit the energy range of the ions transmitted through said annular opening, which opening is essentially coaxial relative to said axis; c. electrical annular lens means for collimating the ions of equal energies diverging from said loci, said lens being essentially coaxial to said axis and having a predetermined second energy dispersion coefficient; and d. momentum analyzer means for focussing ions with an equal charge-to-mass ratio, which have passed through said opening of said energy diaphragm means to respective further loci lying in a plane which comprises said exit diaphragm, said momentum analyzer being essentially symmetrically to said axis and having a predetermined third energy dispersion coefficient; said annular lens having a focal length selected such that the energy dispersion resulting from said first energy dispersion coefficient of said energy analyzer means is opposite and equal to the energy dispersion resulting from the combination of said second and third energy dispersions coefficients of said annular lens and said momentum analyzer means.