Mass spectrometer, types instruments

In Dynamic Secondary Ion Ma s Spectrometry (SIMS), a focused ion beam is used to sputter material from a specific location on a solid surface in the form of neutral and ionized atoms and molecules. The ions are then accelerated into a mass spectrometer and separated according to their mass-to-charge ratios. Several kinds of mass spectrometers and instrument configurations are used, depending upon the type of materials analyzed and the desired results. 

In essence, any of the mass spectrometer types described in the GC/MS section can be used in combination with LC/MS. Double focusing mass spectrometers provide high resolution, but are very expensive. The most popular choices in LC/MS are quadrupole and ion trap mass spectrometers. TOP mass spectrometers are quickly gaining popularity in LC/MS. Recent advances in the technology make it possible to measure exact masses of the ions, which allows the unequivocal determination of their elemental composition. Thus, results comparable to those produced by high-resolution double focusing instruments can be obtained at a fraction of the cost. 

Tbe reader new to mass spectrometry is advised to consult an appropriate introductory text [2-9]. A few mass spectrometric terms will be explained here by way of background and to outline the principles of choosing a flame ionization detector, ions are produced in the mass spectrometric detector, but the mass spectrometer is able to analyze these ions further according to their molecular weights or rather, mass-to-charge ratios (m/z, see below) to provide a mass spectrum. Different principles are employed to achieve this in a variety of types of mass spectrometer. The instruments most commonly used in GC—MS are known as magnetic sector, quadrupole and ion trap mass spectrometers. Their differences are not further described here. Bench-top systems are of the quadrupole or ion trap type. 

TOP mass spectrometers of the reflectron type are particularly suitable because they compensate, via different path lengths in an electrostatic reflector, for the time spreading caused by the energy distribution of the sputtered particles. In contrast to quadrupole or magnetic sector mass spectrometers, TOP instruments enable parallel detection of all species in a bunch of postionized sputtered particles. Repetition... 

Before discussing the basic operating principles of those mass spectrometer types used in ICP-MS instrumentation, some important characteristics need to be introduced mass resolution, abundance sensitivity, mass range, and scanning speed [8]. 

Combination of two different mass spectrometer types in so-called hybrid instruments such as linear ion trap/orbitrap (LTQ-Orbitrap) has shown excellent detection and identification capabilities for low molecular weight compounds in various... 

The modern ionization techniques of MALDI and ESI have enabled most types of carbohydrate to be amenable to analysis by mass spectrometry and progress in the field shows no sign of slowing. New instrumentation such as the Orbitrap mass spectrometer and instruments that are capable of ion-mobility separations provide the opportunity for new method development and could possibly provide answers to questions that are not currently answerable by existing techniques. 

Note 2—The instrument conditions and calibration matrix inverses described in this method are based on the use of a 180 magnetic-deflection type mass spectrometer (CEC Model 21-103). It is not known if the calibration matrix inverses, included in this method, are suitable for use on other mass spectrometer types. 

Another instrument used in physical chemistry research that employs quadnipole mass filters is the guided ion beam mass spectrometer [31]. A schematic diagram of an example of this type of instrument is shown in figure B 1.7.13. A... 

In the other types of mass spectrometer discussed in this chapter, ions are detected by having them hit a detector such as an electron multiplier. In early ICR instruments, the same approach was taken, but FT-ICR uses a very different teclmique. If an RF potential is applied to the excitation plates of the trapping cell (figure B 1.7.18(b)) equal to the cyclotron frequency of a particular ion m/z ratio, resonant excitation of the ion trajectories takes place (without changing the cyclotron frequency). The result is ion trajectories of higher... 

The term Q/TOF is used to describe a type of hybrid mass spectrometer system in which a quadrupole analyzer (Q) is used in conjunction with a time-of-flight analyzer (TOP). The use of two analyzers together (hybridized) provides distinct advantages that cannot be achieved by either analyzer individually. In the Q/TOF, the quadrupole is used in one of two modes to select the ions to be examined, and the TOF analyzer measures the actual mass spectrum. Hexapole assemblies are also used to help collimate the ion beams. The hybrid orthogonal Q/TOF instrument is illustrated in Figure 23.1. 

Other types of mass spectrometer may use point, array, or both types of collector. The time-of-flight (TOF) instrument uses a special multichannel plate collector an ion trap can record ion arrivals either sequentially in time or all at once a Fourier-transform ion cyclotron resonance (FTICR) instrument can record ion arrivals in either time or frequency domains which are interconvertible (by the Fourier-transform technique). 

The choice of a mass spectrometer to fulfill any particular task must take into account the nature of the substances to be examined, the degree of separation required for mixtures, the types of ion source and inlet systems, and the types of mass analyzer. Once these individual requirements have been defined, it is much easier to discriminate among the numerous commercial instruments that are available. Once suitable mass spectrometers have been identified, it is then often a case of balancing capital and running costs, reUability, ea.se of routine use, after-sales service, and manufacturer reputation. 

Each type of mass spectrometer has its associated advantages and disadvantages. Quadrupole-based systems offer a fairly simple ion optics design that provides a certain degree of flexibility with respect to instrument configuration. For example, quadrupole mass filters are often found in hybrid systems, that is, coupled with another surface analytical method, such as electron spectroscopy for chemical analysis or scanning Auger spectroscopy. 

The various SNMS instruments using electron impact postionization differ both in the way that the sample surface is sputtered for analysis and in the way the ionizing electrons are generated (Figure 2). In all instruments, an ionizer of the electron-gun or electron-gas types is inserted between the sample surface and the mass spectrometer. In the case of an electron-gun ionizer, the sputtered neutrals are bombarded by electrons from a heated filament that have been accelerated to 80—... 

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