ABSTRACT
Historically, electron ionization (EI, electron impact, electron bombardment) has been the most commonly used method to ionize relatively small (<1.2 kDa) molecules which can be vaporized without thermal decomposition (often after derivatization). A typical EI source (Figure 2.1) is a small box (
∼
1 mL volume, stainless steel, maintained at
∼
200°C to prevent sample condensation) with an inlet tube for the introduction of gaseous samples or vapors from liquid or solid samples, and another inlet for a direct probe tip (Section 2.4.2). Electrons (20-300
µ
A current), produced by thermionic emission from a rhenium or tungsten filament heated to 2000°C, enter the source through a small slit and are accelerated by a potential drop between the filament and an electron trap at the opposite side of the box in such a way that the electrons attain 70 eV energy in the middle of the ion source where they encounter the analyte molecules. A small permanent magnet (50-300 gauss) forces the electrons to move in a narrow helical path (collimation). This leads not only to a better-regulated electron beam but also provides a smaller volume within which the ionization takes place. The end results are a reduction in the initial energy spread of the ions and an increased ionization cross section. The source slit is usually grounded and all other source potentials are positive with respect to it. The ion repeller electrode is usually at a positive potential with respect to the source to aid in pushing the positive ions in the box toward the exit slit. The ion beam current (10
to 10
A) is drawn out of the ionization chamber perpendicular to the electron beam, and a set of extracting, focusing, and accelerating lenses form and shape the exiting ion beam prior to its entering the mass analyzer. Sample vapor pressure in the ion source must be in the 10
to 10
torr range. The ion source must be differentially pumped to keep the immediate region of ion formation almost vacuum tight to optimize the residence time of the analytes (maximize sensitivity), while the rest of the source should be exposed to fast pumping.
