Electric sector

2 Double-Focusing Mass Spectrometers 4.6.2.1 Electric Sector 

Substituting mv for 2zV [Relation (4.6) linking kinetic energy to the acceleration voltage of the ions] yields  

Relation (4.11) shows that, in the case of the electrostatic sector, the radius of the ion trajectory is independent of its mass and charge it depends only on the V/E ratio. The value of r is constant because the position of the collector slot is fixed the scanning of E allows the selection of ions according to their kinetic energy. 

Alternatively, the instrument can operate in the voltage scanning mode, that is, the magnetic field strength is kept constant and the electric field is varied. The electric sector 

Finally, the instrument can be operated in the peak-matching mode, which provides optimum mass resolving power and mass accuracy. Here the magnetic field strength is kept constant and the electric sector and acceleration voltages are scanned over a relatively small m/z range. This mode of operation is suitable when two ions that are very close in mass need to be separated or when the elemental composition of a molecule is to be determined at high resolution. 

A mass accuracy better than 1 ppm is routinely obtained by the better instruments when they are operated in a peak matching mode. An interlaboratory study of mass accuracy of different instruments and operating modes can be found in Reference 231. 

The sensitivity depends on the mode of operation. In the full scan mode the sensitivity is limited, while in the selected ion monitoring mode the sensitivity is high. The desired resolution also affects sensitivity (see above). 

The speed of the analyzer highly depends on the mode of operation. The scan speed affects both the resolution and the mass accuracy, so if high quality data is needed, a 

---

After ions have been formed by El, they are examined for mass and abundance by the analyzer part of the mass spectrometer, which can incorporate magnetic sectors, electric sectors, qua-drupoles, time-of-flight tubes, and so on. The region in which the ions are first formed is called... 

In general terms, the main function of the magnetic/electric-sector section of the hybrid is to be able to resolve m/z values differing by only a few parts per million. Such accuracy allows highly accurate measurement of m/z values and therefore affords excellent elemental compositions of ions if these are molecular ions, the resulting compositions are in fact molecular formulae, which is the usual MS mode. Apart from accurate mass measurement, full mass spectra can also be obtained. The high-resolution separation of ions also allows ions having only small mass differences to be carefully selected for MS/MS studies. 

The AutoSpec -TOF hybrid mass spectrometer combines the advantages of a magnetic/electric-sector instrument with those of time-of-flight to give a versatile instrument capable of MS or MS/MS at high or low resolution. 

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

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. 

Alternatively, the electric focusing potential E can be changed, but this method needs another ion collector sited at the electric-sector focus, and it must be a collector that can be raised out of the ion beam when the mass of the ion being examined is required. This arrangement is not convenient. A better solution is obtained by linked scanning of the E/V voltages (see later discussion). 

V = accelerating voltage at the ion source E = electric-sector voltage B = magnetic-field strength. 

Commercial mass analyzers are based almost entirely on quadrupoles, magnetic sectors (with or without an added electric sector for high-resolution work), and time-of-flight (TOE) configurations or a combination of these. There are also ion traps and ion cyclotron resonance instruments. These are discussed as single use and combined (hybrid) use. 

An added consideration is that the TOF instruments are easily and quickly calibrated. As the mass range increases again (m/z 5,000-50,000), magnetic-sector instruments (with added electric sector) and ion cyclotron resonance instruments are very effective, but their prices tend to match the increases in resolving powers. At the top end of these ranges, masses of several million have been analyzed by using Fourier-transform ion cyclotron resonance (FTICR) instruments, but such measurements tend to be isolated rather than targets that can be achieved in everyday use. 

Magnetic/electric-sector instruments are used to manipulate ion beams by making use of the deflection of charged species (ions) in magnetic or electric fields. 

This focusing action gives an ion beam, in which the m/z values can be measured so accurately that the resolution of a magnetic/electric-sector instrument (separation of ions of different m/z values) is measured as a few parts per million, compared to the more modest few parts per thousand in, say, a quadmpole or ion-trap instrument. 

A magnetic-sector/TOP hybrid has two means of measuring m/z values, one very accurately in a conventional magnetic/electric-sector sense, and the other somewhat less accurately in a time-of-flight sense. 

The accelerating voltage V for ions leaving the source and the electric-sector voltage E for energy focusing are the two electric ones. The field B is the magnetic one. 

LC can be combined with all kinds of mass spectrometers, but for practical reasons only quadrapolar, magnetic/electric-sector, and TOP instruments are in wide use. A variety of interfaces are used, including thermospray, plasmaspray, electrospray, dynamic fast-atom bombardment (FAB), particle beam, and moving belt. 

A simple mass spectrometer of low resolution (many quadrupoles, magnetic sectors, time-of-flight) cannot easily be used for accurate mass measurement and, usually, a double-focusing magnetic/electric-sector or Fourier-transform ion cyclotron resonance instrument is needed. 

---