Quadrupole mass filter operation with auxiliary quadrupolar excitation: theory and experiment

Greatly improved peak shapes are obtained by applying auxiliary quadrupolar excitation to the first stability region of a resolving quadrupole mass fi lter. This phenomenon is investigated theoretically and experimentally, and is conceptualized as a splitting of the Mathieu stability diagram into mul tiple islands, yielding multiple operating points available for ion filteri ng. The modified stability diagram is calculated exactly using the solution s to Hill's equation, and yields excellent agreement with experiment. All e xperimental measurements are made using an inductively coupled plasma mass spectrometer. Theoretical mapping of the stability diagram is confirmed usi ng the Ar-80(2)+ ion. Peak shape enhancements are demonstrated by measuring m/z = 39 in the presence of an abundance of Ar-40(+), where an operating p oint is found that gives > 1000X increase in abundance sensitivity and also yields only a weak broadening with increasing translational ion energy up to 20 eV. Good separation of high concentrations of Ga-69(+) and Ba-137(2+) is demonstrated. (C) 2001 Elsevier Science B.V.