Ion trap mass spectrometer method and apparatus for improved sensitivity
AssigneeVarian Associates, Inc.
InventorsMingda Wang, Edward G. Marquette
Life of this patent
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Abstract
An ion trap mass spectrometer system providing superposition of an AC dipole and/or monopole field on the quadrupole field to provide one preferential ejection direction.
Claims25
In a method for improving the sensitivity of a quadrupole ion trap (QIT) spectrometer, said QIT having a ring electrode, a pair of end caps, an RF trapping voltage source having an RF trapping frequency W.sub.0 and an amplitude V, and means for changing the trapping RF amplitude V as a function of time, including the steps of:
- (a) applying said RF trapping voltage V to said ring electrode at RF frequency W.sub.0 ;
- (b) providing ions of a sample in said QIT;
- (c) modifying the field within said QIT so that said field is not a pure quadrupole field;
- (d) scanning the amplitude of the trapping voltage;
- (e) detecting ions being ejected from said QIT;
- (f) creating a mass spectrum of said sample by correlating the instantaneous amplitude of said trapping voltage with the number of ions detected,
- the improved method comprising,
- wherein said step (c) of modifying the field within said QIT so that said field is not a pure quadrupole field includes superimposing an AC field on said quadrupole field which is a lower order than a quadrupole field.
In a QIT having a shaped ring electrode substantially enclosing a volume except for top and bottom openings, a pair of end cap electrodes enclosing the top and bottom of said volume, means to develop a quadrupole trapping field potential for retaining ions in said volume by applying voltages to said ring electrode and to said end cap electrodes, said voltage being applied to said ring electrode including a fixed RF frequency .omega..sub.0, an ion detector external to said volume and adjacent one of said end caps which end cap is perforated for passing ions from said volume to said ion detector, and QIT parameter scanning apparatus to provide an indication of a scanned parameter versus the number of ions detected by said detector,
- the improvement comprising,
- means for modifying the potential field within said volume so that said field in said volume is not a pure quadrupole, said means including means to electrically superimpose a lower order AC field on said quadrupole field, whereby said lower order field is less than a third order.
Dependencies are inferred from the claim text.