Analyzer ion separation

Type of analyzer Ion separation principle Comments Accuracy/ ppm m/z range resolution... 

Similarly to the magnetic analyzer, ion separation is based on the circular motion of a charged species in a magnetic field in ICR instruments as well. The difference is that in the ICR, ions undergo several full cycles in the ICR cell. Other differences, such as the application of electrostatic trapping fields, are also crucial and the ion motion is, in fact, much more complex than implied by the simplified discussion below. For the technically inclined reader, we recommend the book by Marshall and Verdun [42]. If only ions with the same mtz ratios are present, such as indicated in Fig. 20, ion motion can be related to a regular sine function, the frequency of which (co) is inversely proportional to the mtz ratio ... 

All mass spectrometers analyze ions for their mass-to-charge ratios (m/z values) by separating the individual m/z values and then recording the numbers (abundance) of ions at each m/z value to give a mass spectrum. Quadrupoles allow ions of different m/z values to pass sequentially e.g., ions at m/z 100, 101, 102 will pass one after the other through the quadrupole assembly so that first m/z 100 is passed, then 101, then 102 (or vice versa), and so on. Therefore, the ion collector (or detector) at the end of the quadrupole assembly needs only to cover one point or focus for a whole spectrum to be scanned over a period of time (Figure 28.1a). This type of point detector records ion arrivals in a time domain, not a spatial one. 

All mass spectrometers analyze ions for their mass-to-charge ratios (m/z values) and simultaneously for the abundances of ions at any given m/z value. By separating the ions according to m/z and measuring the ion abundances, a mass spectrum is obtained. 

It is worth noting that some of these methods are both an inlet system to the mass spectrometer and an ion source at the same time and are not used with conventional ion sources. Thus, with electrospray, the process of removing the liquid phase from the column eluant also produces ions of any emerging mixture components, and these are passed straight to the mass spectrometer analyzer no separate ion source is needed. The particle beam method is different in that the liquid phase is removed, and any residual mixture components are passed into a conventional ion source (often electron ionization). 

Momentum spectrum. A spectrum obtained when a beam of ions is separated according to the momentum-to-charge (m/z) ratios of the ionic species present. A magnetic-sector analyzer achieves separation of the various ionic species in this way. If the ion beam is homogeneous in translational energy, as is the case with sector instruments, separation according to the m/z ratios is also achieved. 

Rapid scanning mass spectrometers providing unit resolution are routinely used as chroaatographic detectors. Ion separation is accomplished using either a magnetic sector, quadrupole filter or ion trap device. Ions can also be separated by time-of-flight or ion cyclotron resonance mass analyzers but these devices are not widely used with chromatograidiic inlets and will not be discussed here [20]. 

The determination of the activity size distribution of the ultrafine ions is of particular interest due to their influence on the movement and deposition of Po-218. These ultrafine ions are the result of radiolysis and their rate of formation is a function of radon concentration, the energy associated with the recoil path of Po-218, and the presence of H O vapor and trace gases such as S02 a joint series of experiments utilizing a mobility analyzer, the separate single screen method, and the stacked screen method were conducted to examine the activity size distribution of the ultrafine mode. 

The ions thus produced are accelerated and driven from the source to the analyzer that separates them according to their m/z ratios. Finally, the detector reveals ions. 

Once the ions are formed in the ion source, they are accelerated towards the mass analyzer where separation according to their m/z ratio occurs. Before describing the main analyzers, it is better to consider one feature common to all of them the resolution. 

Based on the main methods of ion separation, analyzers can be classified into two main groups those based on the separation of ions in space or the separation of ions in time. The former separate ions while they are travelling over a distance (some metres in the case of sector instruments, centimetres in the quadupole) (Table 2.4). In contrast, when separation occurs in time, ions are confined in a small region of space and their separation occurs by varying some parameters (Table 2.4). 

The situation is different for analysis in time (Figure 2.14b). In this case, the three main processes, i.e. ion separation, CID and analysis of product ions, occur inside the same analyzer just changing the forces acting on the ions over time. Hence, in an ion trap, it is possible to isolate the ions of interest, by application of a suitable RF and voltages to its electrodes, to apply a supplementary voltage for CID with the helium present inside it, and to... 

Focal plane detectors are used primarily to detect ions separated in space by, for example, magnetic sector analyzers (see Section 2.2.2). The objective of an ideal focal plane detector is to simultaneously record the location of every ion in the spectrum. In many ways the photoplate (see Section 2.3.1) is the original focal plane detector, but it has today been more or less replaced with designs that rely on EM detectors (see Section 2.3.3). A common arrangement is to allow the spatially disperse ion beams simultaneously to impinge on an MCP (see Section 2.3.3.2). The secondary electrons generated by the ion impacts then strike a one- or two-dimensional array of metal strips and the current from the individual electrodes is recorded. A tutorial on the fundamentals of focal plane detectors is found in Reference 283. Reference 284 provides a relatively recent review of MS detector-array technology. 

These are not the only types of tandem mass spectrometers. There are numerous configurations of instruments that are based on the type of ion separation and many new terms associated with these instrument types. For example, there are instruments known as ion traps. The ion trap is a device that can measure mass, fragment a selected mass (as could be done in a collision cell) and then measure the mass of the fragment. The product ion produced by this all in one device is the same product ion that would be produced in a tandem quadrupole instrument. However, there is only one mass analyzer that functions as both the collision cell and mass measuring device. These types of instruments are sometimes referred to as tandem mass spectrometers, but are not abbreviated as MS/MS. The MS/MS analysis is done by separating the analysis in time (tandem in time) rather than two devices separated in space. A more generic term is best suited. This term is MS , where the n represents... 

This fundamental equation explains that the velocity of heavier ions (iq of ions with mass m,) is lower than of lighter ions (v2 of ions with mass m2, with m, > m2). Equation (10) is used directly in time resolved measurements, for example in time-of-flight mass spectrometers (ToF-MS). The charged ions of the extracted and accelerated ion beam are separated by their mass-to-charge ratio, m/z, in the mass analyzer. Mass-separated ion beams are subsequently recorded by an ion detection system either as a function of time or simultaneously. Mass spectrometers are utilized for the determination of absolute masses of isotopes, atomic weights, relative abundance of isotopes and for quite different applications in survey, trace, ultratrace and surface analysis as discussed in Chapters 8 and 9. 

Dynamic ion separation systems are based on another physical principle and use the different flight time of ions with different masses and different velocity (e.g., in ToF mass analyzers). In addition, in dynamic ion separation systems there is a time dependent variation of one or more system parameters, e.g., changing of electrical or/and magnetic field strengths, which means the ion motion during the measurement procedure is crucial for the mass spectrometric analysis. 

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