Combination spectrometers

Figure 1-21. The principle of the novel miniaturised Raman/LIBS combined spectrometer for the...

In essence, a guided-ion beam is a double mass spectrometer. Figure A3.5.9 shows a schematic diagram of a griided-ion beam apparatus [104]. Ions are created and extracted from an ion source. Many types of source have been used and the choice depends upon the application. Combining a flow tube such as that described in this chapter has proven to be versatile and it ensures the ions are thennalized [105]. After extraction, the ions are mass selected. Many types of mass spectrometer can be used a Wien ExB filter is shown. The ions are then injected into an octopole ion trap. The octopole consists of eight parallel rods arranged on a circle. An RF... 

A connnon approach has been to measure the equilibrium constant, K, for these reactions as a fiinction of temperature with the use of a variable temperature high pressure ion source (see section (Bl.7.2)1. The ion concentrations are approximated by their abundance in the mass spectrum, while the neutral concentrations are known from the sample mlet pressure. A van t Hoff plot of In K versus /T should yield a straight Ime with slope equal to the reaction enthalpy (figure B1.7.11). Combining the PA with a value for basicityG at one temperature yields a value for A.S for the half-reaction involving addition of a proton to a species. While quadnipoles have been tire instruments of choice for many of these studies, other mass spectrometers can act as suitable detectors [19, 20]. 

Although GGMS is the most widely used ana lytical method that combines a chromatographic sep aration with the identification power of mass spectrometry it is not the only one Chemists have coupled mass spectrometers to most of the mstru ments that are used to separate mixtures Perhaps the ultimate is mass spectrometry/mass spectrome try (MS/MS) m which one mass spectrometer gener ates and separates the molecular ions of the components of a mixture and a second mass spec trometer examines their fragmentation patterns ... 

Exact Mass Differences. If the exact mass of the parent or fragment ions are ascertained with a high-resolution mass spectrometer, this relationship is often useful for combinations of C, H, N, and O (Table 1.15b) ... 

The thermospray inlet/ion source does not produce a good percentage yield of ions from the original sample, even with added salts (Figure 11.2). Often the original sample is present in very tiny amounts in the solution going into the thermospray, and the poor ion yield makes the thermo-spray/mass spectrometer a relatively insensitive combination when compared with the sensitivity attainable by even quite a modest mass spectrometer alone. Various attempts have been made to increase the ion yield. One popular method is described here. 

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. 

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. 

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. 

Mixtures passed through special columns (chromatography) in the gas phase (GC) or liquid phase (LC) can be separated into their individual components and analyzed qualitatively and/or quantitatively. Both GC and LC analyzers can be directly coupled to mass spectrometers, a powerful combination that can simultaneously separate and identify components of mixtures. 

In a GC/MS combination, passage of the separated components (A, B, C, D) successively into the mass spectrometer yields their individual spectra. 

By connecting a gas chromatograph to a suitable mass spectrometer and including a data system, the combined method of GC/MS can be used routinely to separate complex mixtures into theii individual components, identify the components, and estimate their amounts. The technique is widely used. 

For some kinds of analyses, it is convenient to have two combined mass spectrometers. This combination naturally increases costs therefore purchase of such hybrid instruments tends to require much... 

Tandem mass spectrometers most commonly used for MS/MS smdies include the following analyzer combinations, although many others are possible ... 

Computers, often combined with transputers, are used for three main functions when connected to a mass spectrometer. The foremost requirements involve the acquisition and preprocessing of basic data and the control of the instrument s scanning operations. Additional software programs are available to manipulate the preprocessed data in a wide variety of ways depending on what is required, e.g., a mass spectrum or a total ion chromatogram. 

Dynamic FAB is an interface between a liquid chromatograph and a mass spectrometer and is, at the same time, an ion source. As an inlet/ion source, this technique fulfils a similar function to plasmaspray and electrospray, both of which are combined inlet/ion sources. 

TOF instmments can be operated as stand-alone mass spectrometers (in-line with an ion source), but they frequently are used in combination with other techniques to give hybrid instmments. In such... 

Hybrid time-of-flight (TOF) mass spectrometers make use of a TOF analyzer placed at right angles to a main ion beam. Ions are deflected from this beam by a pulsed electric fleld at right angles to the ion beam direction. The deflected ions travel down the TOF tube for analysis. Hybrid TOF mass spectrometers have many advantages arising from the combination of two techniques, neither of which alone would be as useful. 

A detector is needed to sense when the separated substances are emerging from the end of the column. A mass spectrometer (MS) makes a very good, sensitive detector and can be coupled to either GC or LC to give the combined techniques of GC/MS or LC/MS, respectively. 

Gas chromatography/ma.ss spectrometry (GC/MS) is an analytical technique combining the advantages of a GC instrument with those of a mass spectrometer. 

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. 

Different combinations of stable xenon isotopes have been sealed into each of the fuel elements in fission reactors as tags so that should one of the elements later develop a leak, it could be identified by analyzing the xenon isotope pattern in the reactor s cover gas (4). Historically, the sensitive helium mass spectrometer devices for leak detection were developed as a cmcial part of building the gas-diffusion plant for uranium isotope separation at Oak Ridge, Tennessee (129), and heHum leak detection equipment is stiU an essential tool ia auclear technology (see Diffusion separation methods). 

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