Noise reduction in negative-ion quadrupole mass spectrometry

#1. Field of the Invention

#The present invention relates to mass spectrometry. In particular, the present invention relates to noise reduction in negativeion quadrupole mass spectrometry.

#2. Discussion of Background

#In the drawings,

#FIG. 1 is a schematic view of a quadrupole mass spectrometer system;

#FIG. 2 is a cross-sectional view of a quadrupole mass spectrometer system through plane A of FIG. 1;

#Referring now to FIG. 3A, there is shown a schematic view of an apparatus according to a preferred embodiment of the present invention. Quadrupole mass spectrometer 80 includes ion source 82, quadrupole mass filter 84 (similar to quadrupole 14 of QMS system 10 above) and ion collector/recorder system 86. Ion source 82 produces ionized particles from a sample by some convenient means. Interposed between quadrupole 84 and ion collector 86 is a magnetic field, indicated generally at 90.

#When QMS 80 is operated in negative ion mode, ion source 82 produces primarily negatively-charged particles, including electrons as well as ions. These particles are formed into incident beam 92, accelerated by an electric field (not shown), and passed through quadrupole 84, emerging as beam 94. Beam 94 may include additional electrons and ions, generated by collisions between particles, collisions with the rods of quadrupole 84, and so forth. As noted above, the electrons exiting quadrupole 84 are largely uncollimated and have a random energy distribution. Beam 94 enters magnetic field 90, where electrons 96 are deflected away from the path of beam 94 to electron collector 98. Magnetic field 90 is transverse to the path of beam 94, with field strength sufficient to deflect electrons 96 from beam 94, but weak enough that the heavier negative ions 100 are undeflected. Magnetic field 90 is preferably in a plane perpendicular to that of the figure as shown. Negative ions 100 pass to ion collector 86 where they are detected and recorded as a mass spectrum.

#The radius of curvature of a charged particle moving with velocity v in a magnetic field H is