US 4,731,533 AGranted

Method and apparatus for dissociating ions by electron impact

AssigneeVestec Corporation

InventorMarvin L. Vestal

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Abstract

Improved methods and apparatus suitable for use in mass spectrometry are provided for dissociating ions by achieving electronic excitation of the ions using electrons. In MS-MS applications, ion dissociation is induced by electron impact on an ion beam after the beam has exited a first mass analyzer, and before the beam enters a second analyzer. An efficient method for dissociating ions is achieved by providing a low velocity ion beam traveling substantially along the axis of an RF excited multipole field, and intersecting sheet electron beams traveling transverse to the axis of the field. When used in the analysis of high molecular weight, nonvolatile molecules, the teachings of the present invention yield the extensive and reproducible fragmentation characteristics generally associated with electron ionization of lower mass, volatile molecules. The methods and apparatus of the present invention may therefore be used in conjunction with mass spectrometry to qualitatively and quantitatively analyze a significantly greater quantity of samples. The techniques of the present invention are also well suited for dissociating high-mass, even-electron ions produced by thermospray and other soft ionization techniques.

A mass spectrometer for the analysis of samples, including ionizing means for ionizing sample molecules of widely varying molecular weights to produce ions derived from the sample, fragmentation means for producing ionized fragments or products of the produced ions, mass analyzer means for scanning the ionized fragments or products and selecting ionized fragments or products of a particular weight over a wide range of atomic mass units, and detector means at the output of the mass analyzer means for receiving the selected ionized fragments or products and for producing a representation therefrom of a mass spectrum for identifying the sample or components thereof, the fragmentation means comprising:

a first plurality of electrically conducting rods for forming a multipole electrodynamic field having a central axis and for transmitting a substantially cylindrical-shaped elongate beam of the produced ions from the ionizing means to the mass analyzer means along substantially the central axis;
a second plurality of electrically conducting rods each spaced radially outwardly from the first plurality of electrically conducting rods with respect to the central axis for passing electrons from the second plurality of electrically conducting rods toward the central axis;
and
shielding means spaced radially between the second plurality of electrically conducting rods and the first plurality of electrically conducting rods for prohibiting electrons passing from the second plurality of electrically conducting rods toward the central axis from engaging the first plurality of electrically conducting rods and transmitting radially inward past the first plurality of electrically conducting rods elongate sheets of electrons for intersecting the elongate beam of produced ions so as to excite at least some of the produced ions to electronic states above their dissociation limits and producing the ionized fragments or products.

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Abstract

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