Magnetic sector mass analyzer with electron ionization

Fig. 10.3. Diagram of a magnetic sector mass analyzer with electron ionization.

The combination of electrospray ionization with magnetic sector mass spectrometers proved to be less technically challenging than the interfacing of MALDI. Electrospray ionization produces ions continuously, and thus it combines easily to scanning mass analyzers such as the magnetic sector. Thus, no modifications of the mass analyzer or detector are required for electrospray ionization. However, the ions produced by electrospray must be accelerated to kilo-electron-volt energies, and this requires some innovation with the design of the ion source. One solution is to float the entire electrospray source and... 

In both electron post-ionization techniques mass analysis is performed by means of a quadrupole mass analyzer (Sect. 3.1.2.2), and pulse counting by means of a dynode multiplier. In contrast with a magnetic sector field, a quadrupole enables swift switching between mass settings, thus enabling continuous data acquisition for many elements even at high sputter rates within thin layers. 

TOF mass analyzers are based on bombardment by a pulse of electrons or photons to periodically produce positive ions. The pulses have frequencies between 10 and 50 kFIz. The generated ions are then accelerated by an electric sector (voltages from 103 to 104 V) at the same frequency as the ionizing bombardment but with a certain gap. The accelerated ions pass to a 1 m long analyzer rod, which is not subjected to an electrical or magnetic field. As all the ions have the same kinetic energy, their velocities along the analyzer rod must be inversely proportional to the m/z ratio. In this way, those ions with lower m/z ratios reach the detector first. The times to reach the detector (the TOF) are between 1 and 30 ps. 

There are several types of ionization sources [MALDI, ESI, FAB (fast atom bombardment), PD (Cf-252 plasma desorption), El (electron ionization), Cl (chemical ionization) etc.], different types of mass analyzers [combinations of magnetic and electric sectors, quadrupolar filters (Q) and ion traps (IT), time-of-flight (TOF) and FT-ICR] and different detectors, each with its own advantages and drawbacks. We describe herein only the systems that presently have widespread use for the study of biomolecules ESI coupled to a quadrupole (or triple quadrupole, QqQ) mass analyzer or an ion trap, the MALDI source with the linear or reflectron TOF analyzer, and the FT-ICR system which can be equipped with both ESI and MALDI sources. 

Installing additional accessories in front of the ion source can render analytes amenable to ionization and subsequent mass spectrometric analysis. On-line sample treatment is especially important when analyzing liquid-phase, complex, and/or concentrated samples. For example a thermal vaporizer was used to enable analysis of liquid samples by a process mass spectrometer designed for gas analysis [196], This system has been successfully implemented in the monitoring of an esterification reaction [197]. The obtained data were in a good agreement with those recorded by in-line mid-infrared spectrometry. The setup incorporated a magnetic sector analyzer with two detectors an electron multiplier detector... 

The first commercially available multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) setup was the Plasma 54, introduced in 1992 by VG Elemental. This mass spectrometer incorporated the detector platform from the Sector 54 thermal ionization mass spectrometer and included an electrostatic analyzer before the entrance to the magnetic sector. This instmment featured seven Faraday cups and a Daly detector. The Daly detector [7] incorporates an A1 knob maintained at +25 kV together with a scintillator screen and photomultiplier (Figure 3.2). Incoming ions are accelerated to the A1 knob and large numbers of secondary electrons are produced as result of the impact of these ions on the aluminum surface. These electrons are then accelerated towards the scintillator. 

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