Pa to avoid collisions of ions with other molecules. To keep ion source pressure low and constant, the amount of sample introduced must be small. Inlet systems are designed to satisfy these requirements. EI is a fairly energetic process which tends to fragment sensitive compounds severely, often to the extent that the molecular ion is not observed in the mass spectrum. Sample molecules are introduced perpendicular to the electron beam and ionized.
Ionization efficiencies are typically low, on the order of one per thousand molecules.
Although an accelerating voltage of 70 eV is standard, lower voltages may produce more molecular ions and fewer fragment ions, thus providing molecular weight information. The energy required to ionize a neutral organic molecule is typically under 20 eV and the excessive electron energy tends to produce further fragmentation of the molecular ion to smaller charged or neutral species. A quadrupole mass analyzer will be used for mass separation in this experiment. The fast scan capability of the quadrupole is advantageous if compounds elute in narrow bands from chromatographic columns. Quadrupoles are, however, insensitive to metastable ions.