Field-free region

Figure Bl.7.4. Schematic diagram of a reverse geometry (BE) magnetic sector mass spectrometer ion source (1) focusing lens (2) magnetic sector (3) field-free region (4) beam resolving slits (5) electrostatic sector (6) electron multiplier detector (7). Second field-free region components collision cells (8) and beam deflection electrodes (9).

Figure Bl.7.5. (a) MIKE spectrum of the iinimoleciilar decomposition of 1-butene ions (m/z 56). This spectrum was obtained in the second field-free region of a reverse geometry magnetic sector mass...

Metastable Peaks. If the mass spectrometer has a field-free region between the exit of the ion source and the entrance to the mass analyzer, metastable peaks m may appear as a weak, diffuse (often humped-shape) peak, usually at a nonintegral mass. The one-step decomposition process takes the general form ... 

Certain regions of a mass spectrometer have no electric or magnetic fields to affect an ion trajectory (field-free regions). Figure 32.3 illustrates three such regions in a conventional double-focusing instrument. 

Ion trajectory through a conventional (EB) sector instrument, showing three field-free regions in relation to the sectors, the source, and the ion detector. 

In a normal quadrupole instrument, the field-free regions are very short. Ions formed in region t will be transmitted by the quadrupole as normal ions. In region 2 there is no differentiation between metastable and normal ions. 

In a sector instrument, which acts as a combined mass/velocity filter, this difference in forward velocity is used to effect a separation of normal and metastable mj" ions (see Chapter 24, Ion Optics of Magnetic/Electric-Sector Mass Spectrometers ). However, as discussed above, the velocity difference is of no consequence to the quadmpole instrument, which acts only as a mass filter, so the normal and metastable mj ions formed in the first field-free region (Figure 33.1) are not differentiated. 

Metastable Ions Decomposing in the First Field-Free Region... 

In an EW- of a B/E-linked scan using an electric/magnetic-sector instrument, a precursor ion is selected. In this case it is m, which might be a molecular ion but equally could be any fragment ion. All product ions (mj, m3, m4) from decomposition of m, in the first field-free region between the ion source and the ion collector are found, thereby giving connections mpm, m -m3, m -m4. 

Metastable ion. An ion that is sufficiently excited to dissociate into a particular daughter ion and neutral species during the flight from the ion source to the detector. The dissociation is most readily observed when it takes place in one of the field-free regions in a mass spectrometer. 

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