Multiply-Charged Ions in FI-MS

Multiply charged ions of minor abundance are frequently observed in FI and FD mass spectra. Their increased abundance as compared to El spectra can be rationalized by either of the following two-step processes i) Post-ionization of gaseous M ions can occur due to the probability for an M ion to suffer a second or even third ionization while drifting away from the emitter surface. [69,70] Especially ions generated in locations not in line-of-sight to the counter electrode pass numerous whiskers on their first 10-100 pm of flight  

Note For the recognition of multiply charged ions, it is important to keep in mind that the m/z scale is compressed by a factor equal to the charge state z (Chap. 3.5 and 4.2.2). 

Although electrospray ionization and matrix-assisted laser desorption ionization allow to transfer much larger ions into the gas phase, it is FD that can be regarded the softest ionization method in mass spectrometry. [27,74] This is mainly because the ionization process itself puts no excess energy into the incipient ions. Problems normally arise above 3000 u molecular weight when significant heating of the emitter causes thermal decomposition of the sample. 

Example The extraordinary stable trityl ion, PhsC, m/z 243, tends to dominate mass spectra (Chap. 6.6.2). Thus, neither the El spectrum of chlorotriphenyl-methane nor that of its impurity triphenylmethanol show molecular ions (Fig. 8.11). An isobutane PICI spectrum also shows the trityl ion almost exclusively, although some hint is obtained from the Ph2COH ion, m/z 183, that cannot be explained as a fragment of a chlorotriphenylmethane ion. Only FD reveals the presence of the alcohol by its molecular ion at m/z 260 while that of the chloride is detected at m/z 278. Both molecular ions undergo some OH or Cl loss, respectively, to yield the Ph3C fragment ion of minor intensity. 

In field ionization (as an experimental configuration) field ionization (the process) is the major pathway of ion generation. In field desorption from activated emitters, the analyte may also undergo field ionization. Presuming that the molecules are deposited in layers on the shanks of the whiskers or between them, this requires that i) analytes of low polarity are polarized by action of the electric field, ii) become mobile upon heating, and iii) finally reach the locations of ionizing electric 

The most commonly formed types of ions in FI are summarized in Table 8.1. 

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