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 filter. This phenomenon is investigated theoretically and experimentally, and is conceptualized as a splitting of the Mathieu stability diagram into multiple islands, yielding multiple operating points available for ion filtering. The modified stability diagram is calculated exactly using the solutions to Hill's equation, and yields excellent agreement with experiment. All experimental measurements are made using an inductively coupled plasma mass spectrometer. Theoretical mapping of the stability diagram is confirmed using 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 point 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.