Types of Mass Spectrometers

Ideally, the kinetic energy of an ion expelled from the source is zeV, where ze is the charge of the ion and V is the voltage on the backplate. 

More than 104 ion mobility spectrometers are deployed at airport security checkpoints to detect explosives, and perhaps 105 hand-portable devices are used by military and civil defense personnel. Although functionally similar to mass spectrometers, mobility spectrometers are operated in air at ambient pressure and ion mobility spectrometry is not a form of mass spectrometry. Ion mobility spectrometry does not measure molecular mass and provides no structural information. However, it is so widely used that we introduce it here. 

Electrophoresis, which is discussed in Chapter 26, is the migration of ions in solution under the influence of an electric field. Ion mobility spectrometry is gas-phase electrophoresis, which separates ions according to their size-to-charge ratio. Unlike mass spectrometry, ion mobility spectrometry is capable of separating isomers. 

Dry air doped with a chemical ionization reagent (such as Cl2 for anions and acetone or NH3 for cations) sweeps the vapor through a tube containing 10 millicuries of 63Ni. Reagent gas ionized by (3-emission from 63Ni reacts with analyte to generate analyte ions. 

Fenn received part of the Nobel Prize in chemistry in 200223 for electrospray ionization. 

The oldest type of mass analyzers used, going back to the early 1920s, are magnetic mass analyzers. They consist of a curved flight tube located between the poles of an electromagnet, so that the field is perpendicular to the flight direction of the ions. 

The ions first pass through an electrical field and then enter a magnetic field where mass selection takes place. The electrical field is used to extract the ions by a force  

The ions then enter a magnetic field B and start to follow a curved trajectory on which they are held by the centrifugal force so that  

By sweeping the field B it is possible to scan the mass spectrum (m/z) at an exit slit. As ions with different masses are deflected towards different locations in the focal plane simultaneous detection of ions with different masses is thus possible with these spectrometers, provided that multichannel detection is applied. 

Advanced types of mass spectrometers such as ion traps, ion cyclotron resonance and specifically time-of-flight mass spectrometers have also been considered for use in plasma mass spectrometry. 

In commercial instrumentation fairly low-cost quadrupole mass spectrometers and also expensive double-focussing sector field mass spectrometers are usually used (for a survey of mass spectrometers for analytical use, see Ref. [71]) and today new types of mass spectrometers such as time-of-flight mass spectrometers are being find utilized in plasma atomic spectrometry. 

When bringing the magnetic sector and the electrostatic sector together, a double- 

This spectrometer is based on the detection of ions produced from a single temporally well-defined event with high time resolution. The principle is well known and has long been applied in secondary ion mass spectrometry [72] and in laser micro mass spectrometry [73]. 

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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... 

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... 

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). 

Other types of mass spectrometer can use point, array, or both types of ion detection. Ion trap mass spectrometers can detect ions sequentially or simultaneously and in some cases, as with ion cyclotron resonance (ICR), may not use a formal electron multiplier type of ion collector at all the ions can be detected by their different electric field frequencies in flight. 

All commercially available SIMS systems have in common some type of computer automation, an ion source, a high-vacuum environment, and some type of mass spectrometer. While the specifics may vary from system to system, the basic requirements are the same. The hardware feature that tends to distii uish the various systems is the type of mass spectrometer used. These fall into three basic catego-... 

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. 

To minimize surface damage, static SIMS mass spectrometers should be as efficient as possible for detecting the total yield of secondary ions from a surface. Also, to be able to separate elemental from molecular species, and molecular species from each other, the mass resolution usually given as the mass m divided by the separable mass Am, should be very high. With this in mind, two types of mass spectrometer have been used - in early work mainly quadrupole mass filters and, more recently, time-of-flight mass spectrometers. 

To appreciate the types of analytical information that may be obtained from each of the different types of mass spectrometer likely to be encountered when carrying out LC-MS. 

M-Scan Ltd. (Ascot, U.K..) specialize in retrofitting all types of mass spectrometers. 

Schematic representation of one type of mass spectrometer. An electron beam fragments gas atoms or molecules into positively charged ions. The ions are accelerated and then deflected by a magnet. Each fragment follows a trajectory that depends on its mass.

Mass spectrometers, workhorse instmments described in Chapter 2, require a vacuum to function. A mass spectrometer generates a beam of ions that is sorted according to specifications of the particular instrument. Usually, the sorting depends on differences in speed, trajectory, and mass. For instance, one type of mass spectrometer measures how long it takes ions to travel from one end of a tube to another. Residual gas must be removed from the tube to eliminate collisions between gas molecules and the ions that are being analyzed. As the diagram shows, collisions with unwanted gas molecules deflect the ions from their paths and change the expected mass spectral pattern. 

The FTMS instrument operates in a very different fashion from most other types of mass spectrometers. With FTMS, the principal functions of ionisation, mass analysis and ion detection occur in the same space... 

Description of several types of mass spectrometers used as detectors for GC. 

The ion trapping capability of the FT-ICR is much greater than the other types of mass spectrometer and ions may be trapped for several minutes under ideal conditions. 

Usually, concentration is measured as a pressure and may differ widely according to the type of mass spectrometer used. The triple quadrupole mass spectrometer may operate with pressures up to 1 x 10 1 Pa in the reaction region. At the other extreme, ion cyclotron resonance mass spectrometers operate poorly at pressures >1 X 10 4 Pa. A pressure of 1 x 10 4 Pa may be regarded as fairly high pressure for FT-ICR measurements. Converting the pressure into a more normal value of concentration means that reactions are carried out at concentrations < 10 9M (often several orders of magnitude < 10 0 M). 

Other types of mass spectrometers are available, but those mentioned earlier are the most commonly used in analyses of soil and soil extracts. 

In some cases, more than one mass spectrometer may be used to carry out the analysis of a sample. Thus, the components from the first MS analysis may be passed to another MS for further analysis. In this type of analysis, a fragment from the first MS analysis is further broken down and the resulting new fragments are analyzed. This allows for analysis of complex samples. Because there are several types of sample ionization (e.g., El, Cl, and electrospray) and different types of mass spectrometers (e.g., quadrupole, time of flight [TOF], and magnetic sector) there are several different ways an MS-MS analysis can be carried out. 

Precise measurement of isotopic ratios requires a special type of mass spectrometer. There are important differences between the IRMS and a regular organic mass spectrometer. 

There are many types of mass spectrometer, each having special design features, some offering very sophisticated modes of analysis and these details will not be described here. Rather, the fundamental principles and instrumental aspects associated with the technique in general are covered. More detailed information is available in specialist books or instrument manufacturers publications. 

The two ionization techniques can be used with all types of mass spectrometers. Here, only those that are the most commonly used in proteomics will be described. Because mass spectrometers use electric and magnetic fields to separate ions, they can only measure mass divided by charge values. In the examples used this is assumed implicitly. In most cases the charge state of an ion can be determined from the mass spectrum. 

Table 2.4. Typical ion flight times in different types of mass spectrometers...

In order to successfully interpret a mass spectrum, we have to know about the isotopic masses and their relation to the atomic weights of the elements, about isotopic abundances and the isotopic patterns resulting therefrom and finally, about high-resolution and accurate mass measurements. These issues are closely related to each other, offer a wealth of analytical information, and are valid for any type of mass spectrometer and any ionization method employed. (The kinetic aspect of isotopic substitution are discussed in Chap. 2.9.)... 

The two main types of mass spectrometers used for analysis and detection of explosives are the quadrupole and the ion trap. These two types of mass analyzers are relatively small, when compared with magnetic sector instruments. They can be miniaturized to make mobile detectors weighing less than 15 kg. 

Most methods of metabolite identification are done with online LC-MS. As mentioned earlier there is no ideal mass spectrometer for this type of work and the sample has to be reanalyzed several times on different types of mass spectrometer. The consequence is that metabolic investigation is often time-consuming. A concept has been described by Staack et al. [82] (Fig. 1.40) where, during the LC-MS run, fractions are collected onto a 96-well plate. 

The detection of a test gas using mass spectrometers is far and away the most sensitive leak detection method and the one most widely used in industry. The MS leak detectors developed for this purpose make possible quantitative measurement of leak rates in a range extending aaoss many powers of ten (see Section 5.2) whereby the lower limit = 10 mbar l/s, thus making it possible to demonstrate the inherent gas permeability of solids where helium is used as the test gas. It is actually possible in principle to detect all gases using mass spectrometry. Of all the available options, the use of helium as a tracer gas has proved to be especially practical. The detection of helium using the mass spectrometer is absolutely ( ) unequivocal. Helium is chemically inert, non-explosive, non-toxic, is present in normal air in a concentration of only 5 ppm and is quite economical. Two types of mass spectrometer are used in commercially available MSLD s ... 

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