High-resolution mass spectrometry with a quadrupole operated in the fourthstability 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 investi gated experimentally and theoretically. A resolution at half-height (R-1/2) of 13 900 has been obtained for 40 eV K-39(+) ions formed by thermal ioniz ation, 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 similar to 10 keV, For low-energy ions (e.g., 75 eV), the transmission of the quadrupole operated in the fourth region is ma ny 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 hing ing fields. However, for higher ion energies, the transmission with operati on in the fourth region is found to be comparable to that of a quadrupole o perated at the same resolution in the second region. The implications for i nductively coupled plasma mass spectrometry are briefly discussed.