High-resolution mass spectrometry with a quadrupole operated in the fourth stability region

The properties of a quadrupole mass filter operated in the fourth stability region with Mathieu parameters (a,q) = (2 x 10(-3), 21.3) have been investigated experimentally and theoretically. A resolution at half-height (R1/2) of 13,900 has been obtained for 40 eV 39K+ ions formed by thermal ionization. With ion energies of 750 eV or more, a resolution at half-height of up to 5000 can be obtained. Extrapolation of resolution data obtained with ions of 40 eV-4800 eV energy shows that unit resolution should be possible for ions with energies of approximately 10 keV. For low-energy ions (e.g., 75 eV), the transmission of the quadrupole operated in the fourth region is many orders of magnitude less than that of a quadrupole operated in the first or second stability regions. Acceptance calculations show this is a result of the combination of a much lower acceptance and strongly defocusing fringing fields. However, for higher ion energies, the transmission with operation in the fourth region is found to be comparable to that of a quadrupole operated at the same resolution in the second region. The implications for inductively coupled plasma mass spectrometry are briefly discussed.