Nonlinear field effects in quadrupole mass filters

The performance of a quadrupole mass filter (QMF) generally degrades when u sing electrodes of circular cross section in place of mathematical ideal hy perbolic electrodes. The circular cross section of electrodes produces nonl inear resonances resulting in distortion and peak splitting in mass spectra . In addition, resonances reduce the actual working cross section, resultin g in limited ion yield. In this article we study nonlinear resonances and i ntensities of resonance lines passing through the tip of the stability diag ram of the QMF. We have found that balancing of multipole terms, rather tha n eliminating individual multipole terms, improves the sensitivity of the Q MF considerably. The theory for assessing intensities of nonlinear resonanc es is presented in detail along with rescaling laws to adjust current QMF p arameter settings. A general formula is presented from which the location a nd intensity of nonlinear can be derived, which then may be used for the de sign of special purpose QMFs. (C) 1999 American Institute of Physics. [S003 4-6748(99)01609-3].