The Magnetic Sector Analyser

The field strength is scanned by an electromagnet, and the dispersion of adjacent masses (i.e. the resolution) decreases with increasing ion mass. The high secondary ion extraction voltage employed results in efficient transmission of secondary ions from the sample surface to the detector, although it is difficult to analyse samples with surfaces that are fractured or rough. 

Magnetic sectors have medium transmission (10-50 percent), large mass range (m/z 10000, where m is the mass of the ion and z the charge) and excellent mass resolution (m/Am 104). This high mass resolution makes these analysers appropriate for SIMS analysis in semiconductor research. 

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Figure 2.244 Exponential mass calibration of the magnetic sector analyser.

The analyser will always be preceded by some form of collection optics, and followed by an ion detector (usually a channel electron multiplier which converts ions into electron showers). There are three types of analyser for use in SIMS spectrometers, the magnetic sector instrument, the quadrupole analyser and time-of -flight (TOF) systems. 

Different mass analysers can be combined with the electrospray ionization source to effect analysis. These include magnetic sector analysers, quadrupole filter (Q), quadrupole ion trap (QIT), time of flight (TOF), and more recently the Fourrier transform ion cyclotron resonance (FTICR) mass analysers. Tandem mass spectrometry can also be effected by combining one or more mass analysers in tandem, as in a triple quadrupole or a QTOF. The first analyzer is usually used as a mass filter to select parent ions that can be fragmented and analyzed by subsequent analysers. 

Mass analysis is simply a method of separating ions of different mass-to-charge ratio (m/z). However, since the ions of interest are almost exclusively singly charged, then m /z is equivalent to mass for practical purposes. There are two types of mass analyser commonly employed for ICP-MS, namely the quadrupole and the magnetic sector. 

Currently used magnetic analyser mass spectrometers constitute a logical evolution of the previously described instrument. They provide very accurate m/z values, but are limited for high mass analyses (problems produced in the magnetic sector). They also include an electrostatic sector E placed after ion acceleration and before the magnetic field, B (Fig. 16.3). 

Instruments with combined magnetic and electric analysers can be assembled according to either of two configurations. The electric sector is located either in front of the magnetic sector, which is the most frequent case, or behind it. The magnetic sector is labelled B and the electric sector is labelledE. The first configuration is called EB (or also Nier-Johnson ). 

Consider an ion that fragments between the source and the first analyser. The fragment always has the velocity of the precursor. If the precursor is focused in the magnetic sector for a field Bp such that... 

Double focusing magnetic sector analysers use two successive separations, first electrostatic, which separates species on the basis of charge, and then magnetic analysis for mass separation the combined separation is thus made on the basis of the m/z ratio. 

Schnitzer and Anbar s experiments are rather similar to those of Baumann, Heinicke, Kaiser and Bethge Both employed plasma ion sources that employed by Schnitzer and Anbar was a hollow cathode duoplasmatron. Also, both used einzel lenses and momentum/charge analysis. But the different lifetimes of the doubly-charged negative ions studied dictated somewhat different analyses. Baumann, et al. used an electric deflection analysis after the magnetic sector, as already seen, whereas Schnitzer and Anbar employed a Wien velocity filter and einzel lens voltage variation prior to the magnetic sector. 

Figure 16.5 Mass spectrometer with magnetic analyser, (a) Nier-Johnson assembly (b) View of the directional focusing by the magnetic sector (the entrance and exit planes are oblique with respect to the angle of incidence of the beam, ensuring focusing) (c) Mattauch-Herzog assembly (d) Arrangement of a double focusing spectrometer (e.g. R = 40 cm and R = 60 cm).

For decades, the most widely used mass spectrometers employed magnetic sector analysers for sorting ions by mass, or more correctly, mass-to-charge ratio, commonly referred to as m/z. Magnetic sectors use a magnetic field to deflect the trajectory of... 

Magnetic sector analysers are considered to be medium-resolution instruments with f <2000. They are also an important component in the high-resolution double-focusing spectrometers discussed next. 

Figure 12.1 Schematic diagram of a two-sector mass spectrometer. The sample is ionised in the source and mass analysed in the magnetic sector. Energy focusing is carried out in the...

High energy CID, if the first mass analyser is a magnetic sector analyser. The primary ion has then an energy of several keV. 

This home-built instrument [6] consists of two magnetic sector analysers with a collision cell located in the region between the two analysers. The first stage consists of a Varian MAT CH7 analyser (90 , 21.4 cm radius), and the second stage is a home built 90 , 10.2 cm radius analyser. The precursor ion beam is monitored by the off-line electron multiplier detector No. 1. A voltage of 30 V applied to the deflection... 

In the classical mass-spectroscopic detection, the entirety of the gas molecules and atoms of to be analysed atmosphere is first ionised and accelerated. It is done under high vacuum conditions. The ions are separated according to their mass/charge ratio in the magnetic sector field and subsequently detected. 

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