Electron ionization in superimposed magnetic and radio frequency quadrupolar electric fields.

An improved design of a novel electron ionization source for orthogonal acceleration time-of-flight mass spectrometry is described, based on the superimposition of an axial magnetic field with cylindrical symmetry around a radio frequency-only quadrupole. A tubular permanent magnet was designed to generate the required strong magnetic field and field profile. An axial electric field along the ion guide for efficient ion extraction was introduced using segmented quadrupole rods. Details of the source design and the effects of various operating parameters are described. The source produces high-quality mass spectra with regard to fragmentation, relative abundances, and isotopic ratios. Preliminary results have shown excellent sensitivity, with limits of detection in the subfemtogram range (octafluoronaphthalene, full spectrum acquisition) in gas chromatography/mass spectrometry operation.