Journal
EUROPEAN JOURNAL OF MASS SPECTROMETRY
Volume 22, Issue 3, Pages 115-122Publisher
IM PUBLICATIONS
DOI: 10.1255/ejms.1416
Keywords
quadrupole mass filter; dipolar excitation; transmission contour; instability band; resonance frequency; frequency spectrum of ion oscillations; resolution power
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Dipole resonant excitation of ions creates instability bands which follow iso-beta lines where beta is the characteristic exponent (stability parameter). Instability bands are exited most effectively on the fundamental frequency w = beta Omega/2. Here w is the angle resonance frequency of the dipolar voltage applied to x or y pair of rods of the analyzer, and Omega is the angle frequency of the main drive voltage. Our goal is to study the mass peak shape in the third stability region with dipolar resonance excitation of the instability band with respect to the resonance frequency w and the dipolar potential amplitude. Numerical integration of the ion motion equations with a given ion source emittance is used to investigate peak shapes and ion transmission. We show that it is possible to vary the resolution power at any part of the third stability region. A change of the dipolar potential phase leads to a periodical variation of the resolution with period p. The most effective dipolar excitation in the y direction is along by, near the stability boundary. The mass peak shape is calculated also for a quadrupole with round rods. The best peak shape (small tails and high resolution) takes place for the rod set with r/r(0) = 1.130. Dipolar excitation increases the transmission by approximately 5-10% at a given resolution.
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