Mass-spectrometer

To have a common platform to build on, we need to define mass spectrometry and several closely related issues, most of them being generalized or refined in later chapters. Then, we may gather the pieces of the puzzle to get a rough estimate of what needs to be known in order to understand the subject. Finally, it is indicated to agree on some conventions for naming and writing. [25-27] 

Obviously, almost any technique to achieve the goals of ionization, separation and detection of ions in the gas phase can be applied - and actually has been applied -in mass spectrometry. This leads to a simple basic setup having all mass spectrometers in common. A mass spectrometer consists of an ion source, a mass 

For molecules of very high molecular weights, the differences between the different masses can become notable. Let us consider two examples. 

In conclusion, the monoisotopic mass is used when it is possible experimentally to distinguish the isotopes, whereas the average mass is used when the isotopes are not distinguishable. The use of nominal mass is not recommended and should only be used for low-mass compounds containing only the elements C, H, N, O and S to avoid to making mistakes. 

Collision and reaction cell techniques have been used for many years in the study of organic and biological mass spectrometry, but only in the last few years in ICP-MS. The development of collision and reaction cells extended the capability of the technique by allowing the selective attenuation or removal of problematic spectral interferences. Today a variety of collision/reaction cells using various gasses (H2, He, CH4, NH3...) are available, virtually able to eliminate the problems associated with polyatomic interferences for most elements in food and beverage matrices. However, the simultaneous multi-element capability and maximum productivity of ICP-MS is partially reduced by the different CRC tuning conditions required to eliminate a specific interference in a specific matrix. 

The detectors used in ICP-MS are not significantly different from the photomultiplier tube used in AA and ICP-OES. The ions passing through the mass filter strike the active surface of the detector, known as dinode, and generate a measurable electronic signal. 

Obviously, almost any technique to achieve the goals of ionization, separation and detection of ions in the gas phase can be applied - and actually has been applied -in mass spectrometry. Fortunately, there is a simple basic scheme that all mass spectrometers follow. A mass spectrometer consists of an ion source, a mass analyzer, and a detector which are operated under high vacuum conditions. A closer look at the front end of such a device might separate the steps of sample introduction, evaporation, and successive ionization or desorption/ionization, respectively, but it is not always trivial to identify each of these steps as clearly separated from each other. If the manufacturing date of the instrument is relatively recent, it will have a data system which collects and processes data from the detector. Since the 1990s, mass spectrometers are fully equipped and controlled by data systems (Fig. 1.3). 

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The chromatogram can finally be used as the series of bands or zones of components or the components can be eluted successively and then detected by various means (e.g. thermal conductivity, flame ionization, electron capture detectors, or the bands can be examined chemically). If the detection is non-destructive, preparative scale chromatography can separate measurable and useful quantities of components. The final detection stage can be coupled to a mass spectrometer (GCMS) and to a computer for final identification. 

In a mass spectrometer, the molecules, in the gaseous state, are ionized and fragmented. The fragments are detected as a function of their mass-to-charge ratio, m/e. The graphical representation of the ion intensity as a function of m/e makes up the mass spectrogram as illustrated In Figure 3.1. 

All have molecular weights of 226 to the nearest integer (C = 12, H = 1, S = 32), but the exact molecular weights differ slightly. A resolution of 2500 is necessary to separate molecules 1, 2 and 3 but 75,000 is required to separate molecule 4 from molecule 3 which explains why high resolution mass spectrometers are sdiiglit. 

Gas chromatography is not an identification method the components must be identified after their separation by capillary column. This is done by coupling to the column a mass spectrometer by which the components can be identified with the aid of spectra libraries. However tbe analysis takes a long time (a gasoline contains aboutTwo hundred components) so it is not practical to repeat it regularly. Furthermore, analysts have developed te hpiques for identifying... 

Ions are also used to initiate secondary ion mass spectrometry (SIMS) [ ], as described in section BI.25.3. In SIMS, the ions sputtered from the surface are measured with a mass spectrometer. SIMS provides an accurate measure of the surface composition with extremely good sensitivity. SIMS can be collected in the static mode in which the surface is only minimally disrupted, or in the dynamic mode in which material is removed so that the composition can be detemiined as a fiinction of depth below the surface. SIMS has also been used along with a shadow and blocking cone analysis as a probe of surface structure [70]. 

The most widely used type of trap for the study of ion-molecule reactivity is the ion-cyclotron-resonance (ICR) [99] mass spectrometer and its successor, the Fourier-transfomi mass spectrometer (FTMS) [100, 101]. Figure A3.5.8 shows the cubic trapping cell used in many FTMS instmments [101]. Ions are created in or injected into a cubic cell in a vacuum of 10 Pa or lower. A magnetic field, B, confines the motion in the x-y... 

Figure A3.5.8. Schematic diagram of the cell used in a Fourier transfomr mass spectrometer.

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... 

The thenuodynamic quantities are derived from equilibrium measurements as a fiinction of temperature. The measurements are frequently made in a high-pressure mass spectrometer [107]. The pertinent equation is In... 

The mass spectrometer tends to be a passive instrument in these applications, used to record mass spectra. In chemical physics and physical chemistry, however, the mass spectrometer takes on a dynamic function as a... 

The chapter is divided into sections, one for each general class of mass spectrometer magnetic sector, quadnipole, time-of-flight and ion cyclotron resonance. The experiments perfonned by each are quite often unique and so have been discussed separately under each heading. 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

A third method for generating ions in mass spectrometers that has been used extensively in physical chemistry is chemical ionization (Cl) [2]. Chemical ionization can involve the transfer of an electron (charge transfer), proton (or otlier positively charged ion) or hydride anion (or other anion). 

A schematic diagram of a reverse geometry mass spectrometer is shown in figure Bl.7.4. 

Figure Bl.7.4. Schematic diagram of a reverse geometry (BE) magnetic sector mass spectrometer ion source (1) focusing lens (2) magnetic sector (3) field-free region (4) beam resolving slits (5) electrostatic sector (6) electron multiplier detector (7). Second field-free region components collision cells (8) and beam deflection electrodes (9).

Magnetic sectors can be used on their own, or in conjunction with energy analysers to fomi a tandem mass spectrometer. The unique features of the reverse geometry instrument are presented from this point. 

A single magnetic sector can be used as a mass filter for other apparatus. However, much more infonnation of the simple mass spectrum of a species can be obtained using the tandem mass spectrometer. 

In the FFR of the sector mass spectrometer, the unimolecular decomposition fragments, and B, of tire mass selected metastable ion AB will, by the conservation of energy and momentum, have lower translational kinetic energy, T, than their precursor ... 

Another approach to mass analysis is based on stable ion trajectories in quadnipole fields. The two most prominent members of this family of mass spectrometers are the quadnipole mass filter and the quadnipole ion trap. Quadnipole mass filters are one of the most connnon mass spectrometers, being extensively used as detectors in analytical instnunents, especially gas clnomatographs. The quadnipole ion trap (which also goes by the name quadnipole ion store, QUISTOR , Paul trap, or just ion trap) is fairly new to the physical chemistry laboratory. Its early development was due to its use as an inexpensive alternative to tandem magnetic sector and quadnipole filter instnunents for analytical analysis. It has, however, staned to be used more in die chemical physics and physical chemistry domains, and so it will be described in some detail in this section. 

The principles of operation of quadnipole mass spectrometers were first described in the late 1950s by Wolfgang Paul who shared the 1989 Nobel Prize in Physics for this development. The equations governing the motion of an ion in a quadnipole field are quite complex and it is not the scope of the present article to provide the reader with a complete treatment. Rather, the basic principles of operation will be described, the reader being referred to several excellent sources for more complete infonnation [13, H and 15]. 

Aside from the smgle mass filter, the most connnon configuration for quadnipole mass spectrometers is the triple-quadnipole instrument. This is the simplest tandem mass spectrometer using quadnipole mass filters. Typically, the... 

It is possible to detemiine the equilibrium constant, K, for the bimolecular reaction involving gas-phase ions and neutral molecules in the ion source of a mass spectrometer [18]. These measurements have generally focused on tln-ee properties, proton affinity (or gas-phase basicity) [19, 20], gas-phase acidity [H] and solvation enthalpies (and free energies) [22, 23] ... 

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]. 

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... 

Figure Bl.7.13. A schematic diagram of an ion-guide mass spectrometer. (Ervin K M and Annentrout P B 1985 Translational energy dependence of Ar + XY —> ArX + Y from thennal to 30 eV c.m. J. Chem. Phys. 83 166-89. Copyright American Institute of Physics Publishing. Reproduced with pemiission.)...

Figure Bl.7.14. Schematic cross-sectional diagram of a quadnipole ion trap mass spectrometer. The distance between the two endcap electrodes is 2zq, while the radius of the ring electrode is (reproduced with pennission of Professor R March, Trent University, Peterborough, ON, Canada).

Figure Bl.7.16. Mass spectra obtained with a Finnigan GCQ quadnipole ion trap mass spectrometer, (a)...

Probably the simplest mass spectrometer is the time-of-fiight (TOP) instrument [36]. Aside from magnetic deflection instruments, these were among the first mass spectrometers developed. The mass range is theoretically infinite, though in practice there are upper limits that are governed by electronics and ion source considerations. In chemical physics and physical chemistry, TOP instniments often are operated at lower resolving power than analytical instniments. Because of their simplicity, they have been used in many spectroscopic apparatus as detectors for electrons and ions. Many of these teclmiques are included as chapters unto themselves in this book, and they will only be briefly described here. 

Figure Bl.7.17. (a) Schematic diagram of a single acceleration zone time-of-flight mass spectrometer, (b) Schematic diagram showing the time focusing of ions with different initial velocities (and hence initial kinetic energies) onto the detector by the use of a reflecting ion mirror, (c) Wiley-McLaren type two stage acceleration zone time-of-flight mass spectrometer.

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