Ion optic

This chapter should be read in conjunction with Chapter 3, Electron Ionization. In electron ionization (El), a high vacuum (low pressure), typically 10 mbar, is maintained in the ion source so that any molecular ions (M +) formed initially from the interaction of an electron beam and molecules (M) do not collide with any other molecules before being expelled from the ion source into the mass spectrometer analyzer (see Chapters 24 through 27, which deal with ion optics). 

For either the in-line or hybrid analyzers, the ions injected into the TOF section must all begin their flight down the TOF tube at the same instant if arrival times of ions at a detector are to be used to measure m/z values (see Chapter 26, TOF Ion Optics ). For the hybrid TOF instruments, the ion detector is usually a microchannel plate ion counter (see Chapter 30, Comparison of Multipoint Collectors (Detectors) of Ions Arrays and MicroChannel Plates ). 

The Q in Q/TOF stands for quadrupole (see Chapter 25, Quadrupole Ion Optics ). A Q/TOF instrument — normally used with an electrospray ion inlet — measures mass spectra directly to obtain molecular or quasi-molecular mass information, or it can be switched rapidly to MS/MS mode to examine structural features of ions. The analyzer layout is presented in Figure 20.2. 

A brief description of this hybrid system appears in Chapter 20. For further information on the quadrupole or TOF instruments, see Chapters 25 and 26, Quadrupole Ion Optics and Time-of-Flight Ion Optics. ... 

Ion Optics of Magnetic/Electric-Sector Mass Spectrometers... 

Double-focusing forward geometry ion optics is a combination in which the ESA is placed before the magnet, as shown in Figure 24.5. 

Through the use of sequential electric (electrostatic) and magnetic fields (sectors) and various correcting lenses, the ion beam leaving the ion source can be adjusted so that it arrives at the collector in focus and with a rectangular cross-section aligned with the collector slits. For the use of crossed electromagnetic fields. Chapter 25 ( Quadrupole Ion Optics ) should be consulted. 

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. 

The system of electric and magnetic fields or lenses is called the ion optics of the mass spectrometer. Electric lenses correct aberrations in the shape of the ion beam. 

After the ion source, the ion optics split the ion beam into its component m/z values (compare splitting white light into a spectrum of colors). 

The ion optics of a magnetic-sector mass spectrometer cause the ion beam leaving the ion source to arrive at a collector after being separated into individual m/z values and focused. 

By adjusting magnetic and electric fields in the ion optics (see Chapters 33 and 34 on linked scanning), metastable ions can be investigated. 

Thermospray interface. Provides liquid chromatographic effluent continuously through a heated capillary vaporizer tube to the mass spectrometer. Solvent molecules evaporate away from the partially vaporized liquid, and analyte ions are transmitted to the mass spectrometer s ion optics. The ionization technique must be specified, e.g., preexisting ions, salt buffer, filament, or electrical discharge. 

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