Superimposition of a magnetic field around an ion guide for electron ionization time-of-flight mass spectrometry.

A new electron ionization source was developed for orthogonal acceleration time-of-flight mass spectrometry (TOFMS) based on the superimposition of a magnetic field around a radio frequency-only (rf-only) ion guide. The cylindrically symmetric magnetic field compresses the electron beam from the electron source into a long narrow volume along the ion guide axis. The magnetic field also helps to maintain a narrow energy distribution of electrons that penetrate the full length of the ion guide despite the influence of the radial rf field. Ionization occurs inside the ion guide with improved efficiency resulting from efficient use of electrons, prolonged interaction time, and nontraditionally large ionization volume. At the same time, the rf field effectively focuses ions radially and confines them to the axis of the ion guide by collisional focusing, leading to high ion transmission efficiency. Furthermore, the source can also be operated in a trap-and-pulse mode to improve the ion sampling duty cycle of orthogonal acceleration TOFMS. To validate the design concept of this new ion source, a simple prototype using a single set of cylindrical rods was constructed and retrofitted to an orthogonal acceleration TOFMS. A significant increase in ion signal intensity was observed by operating the source in a pulsed ion extraction mode. Low detection limits (for example, 12 fg for toluene) were determined at 12.5 spectra s(-1) in the full spectrum mode.