Mass spectrometer, detectors ionization sources

Figure 15.1. The three basic components of a mass spectrometer an ionization source, a mass analyzer, and an ion detector.

Various mass spectrometer configurations have been used for the detection of explosives, such as ion traps, quadrupoles and time-of flight mass analyzers and combinations as MS/MS systems. The ionization method is usually APCI with corona discharge [24, 25]. An example is given in Figure 20, which shows the schematic diagram of an explosive mass spectrometer detector [25]. It is based on an ion trap mass analyzer, an APCI source with corona discharge and a counter-flow introduction (CFI) system. The direction of the sample gas flow introduced into the ion source is opposite to that of the ion flow produced by the ion source. 

A personnel screening portal (Figure 21) was developed using a MS/MS mass spectrometer detector [27]. The MS detector consisted of ion trap and time-of-f-light mass (IT-TOF) analyzers with a discharge ionization source (Figure 22). MS/ MS product ions of the various explosives were used for identification. 

As shown in Figure 15.1, there are three main components of every mass spectrometer. The ion source is used to produce gas-phase ions by capture or loss of electrons or protons. In the mass analyzer, the ions are separated according to their mlz ratios ions of a particular mlz value reach the detector, and a current signal is produced. This section describes the soft ionization sources, mass analyzers, and detectors that are used in experiments involving biological macromolecules. 

Figure 20-51, p. 239, shows overall transfer line connection between the GLC and the electron ionization (El) and chemical ionization (Cl) ion source of the mass spectrometer detector. 

We describe below the rotating-beam-source-photofragmentation apparatus [70] of the Wilson design used in our laboratory (see Fig. 1). The apparatus can be divided into three main components an excimer excitation laser, a photodissociation chamber in which a rotatable supersonic molecular beam intersects the laser beam, and a linearly movable, ultrahigh vacuum-electron ionization mass spectrometer detector. 

Inductively coupled plasma-mass spectrometry is a very rapid technique for the determination of long-lived radionuclides. This technique is based on the ionization of elements in the plasma source. Typically, radiofrequency and argon are used to reach plasma excitation temperatures ranging from 4900 to 7000 K [18,19]. The ions produced are introduced through an interface into a vacuum chamber and are analyzed by a quadru-pole mass spectrometer. Other attempts are being made to use faster mass-spectrometer detectors, such as time-of-flight mass spectrometers, but methods are still not available. 

A major advantage of the TOF mass spectrometer is its fast response time and its applicability to ionization methods that produce ions in pulses. As discussed earlier, because all ions follow the same path, all ions need to leave the ion source at the same time if there is to be no overlap between m/z values at the detector. In turn, if ions are produced continuously as in a typical electron ionization source, then samples of these ions must be utihzed in pulses by switching the ion extraction field on and off very quickly (Figure 26.4). 

There are two common occasions when rapid measurement is preferable. The first is with ionization sources using laser desorption or radionuclides. A pulse of ions is produced in a very short interval of time, often of the order of a few nanoseconds. If the mass spectrometer takes 1 sec to attempt to scan the range of ions produced, then clearly there will be no ions left by the time the scan has completed more than a few nanoseconds (ion traps excluded). If a point ion detector were to be used for this type of pulsed ionization, then after the beginning of the scan no more ions would reach the collector because there would not be any left The array collector overcomes this difficulty by detecting the ions produced all at the same instant. 

A multipoint ion collector (also called the detector) consists of a large number of miniature electron multiplier elements assembled, or constructed, side by side over a plane. A multipoint collector can be an array, which detects a dispersed beam of ions simultaneously over a range of m/z values and is frequently used with a sector-type mass spectrometer. Alternatively, a microchannel plate collector detects all ions of one m/z value. When combined with a TOP analyzer, the microchannel plate affords an almost instantaneous mass spectrum. Because of their construction and operation, microchannel plate detectors are cheaper to fit and maintain. Multipoint detectors are particularly useful for situations in which ionization occurs within a very short space of time, as with some ionization sources, or in which only trace quantities of any substance are available. For such fleeting availability of ions, only multipoint collectors can measure a whole spectrum or part of a spectrum satisfactorily in the short time available. 

More than 20 different kinds of commercial mass spectrometers are available depending on the intended application, but all have three basic parts an ionization source in which sample molecules are given an electrical charge, a tnass analyzer in which ions are separated by their mass-to-charge ratio, and a detector in which the separated ions are observed and counted. 

The maximum column temperatures used in GC/MS are usually 25-50° lower than those used in capillary GC with a flame ionization detector. Higher temperatures can be used in GC/MS but there will be more column bleed, which will require more frequent cleaning of the ion source of the mass spectrometer. 

Morristown, NJ) for the ion source. No carrier gas separator was used. For determination of nitrosamines and TBDMS derivatives of hydroxy-nitrosamines, columns and operating conditions were identical to those for GC-TEA analyses For most work, the He flow rate was 15 cc/min and the column effluent was split 1 1 between a flame ionization detector and the mass spectrometer. The stainless steel splitter, solvent vent valve (Carle Instruments, Fullerton, CA), and associated plumbing were... 

Figure 2.3. A. Mass spectrometer consisting of an ionization source, a mass analyzer and an ion detector. The mass analyzer shown is a time-of -flight (TOF) mass spectrometer. Mass-to-charge (m/z) ratios are determined hy measuring the amount of time it takes an ion to reach the detector. B. Tandem mass spectrometer consisting of an ion source, a first mass analyzer, a collision cell, a second mass analyzer and a detector. The first mass analyzer is used to choose a particular peptide ion to send to the collision cell where the peptide is fragmented. The mass of the spectrum of fragments is determined in the second mass analyzer and is diagnostic of the amino acid sequence of the peptide. Figure adapted from Yates III (2000).

Mass spectrometers measure the mass-to-charge ratio (m/z) of ions. They consist of an ionization source that converts molecules into gas-phase ions and a mass analyzer coupled to an ion detector to determine the m/z ratio of the ion (Yates III, 2000). A mass analyzer uses a physical property such as time-of-flight (TOF) to separate ions of a particular m/z value that then strike the detector (Fig. 2.3). The magnitude of the current that is produced at the detector as a function of time is used to determine the m/z value of the ion. While mass spectrometers have been used for many years for chemistry applications, it was the development of reproducible techniques to create ions of large molecules that made the method appropriate for proteomics. 

All mass spectrometers have different stages of pumping in order to maintain the analyzer and detector regions under high vacuum, i.e. 10 7 10 8 torr or higher. Depending upon the ionization technique, the inlet system and the ion source must be/... 

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