Mass spectrometers common features

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-... 

An ion source, as the name suggests, is the part of the mass spectrometer where the gas phase ions are generated. All of the various ion generation techniques that are available to us have one feature in common they both vaporize and ionize our analyte molecules. In most cases, the vaporization stage occurs first, but with some techniques the ionization stage occurs first. The most commonly employed techniques are discussed in Section 5.2. 

A feature common to the pyrazine, quinoxaline and phenazine ring systems is their remarkable stability in the mass spectrometer and in all cases with the parent heterocycles the molecular ion is the base peak. In the case of pyrazine, two major fragments are observed at m/e 53 and 26, and these fragments are consistent with the fragmentation pattern shown in Scheme 1. 

In El mass spectrometers, 1-phenyl-substituted 1,2,4-thiadiazine 1-oxides can undergo migration of the phenyl group from the sulfur atom to nitrogen, a common feature in the mass spectra of aryl alkyl sulfoximes (77JOC952). 

While the actual relative abundances are often dependent on the type of mass spectrometer on which the spectrum is recorded because of the time window that is used to sample the ions, it is important to emphasize the significant contribution of the EH+ fragment and of the neat metal cation in the recorded spectra. In fact, the presence of the bare metal cation and of EH+ among the fragment ions is a common feature in the mass spectra of organogermanes and organostannanes. 

All mass spectrometers share common features. (See Figure 1.2) Some sort of chromatography usually accomplishes introduction of the sample into the mass spectrometer, although many instruments also allow for direct insertion of the sample into the ionization chamber. All mass spectrometers have methods for ionizing the sample and for separating the ions on the basis of mlz. These methods are discussed in detail below. Once separated, the ions must be detected and quantified. A typical ion collector consists of collimating slits that direct only one set of ions at a time into the collector, where they are detected and amplified by an electron multiplier. The method of ion detection is dependent to some extent on the method of ion separation. 

Both Q and SF mass spectrometers are scanning (sequential) analyzers and multiisotope analysis can be achieved at the expense of the measurement sensitivity and precision. The sequential measurement of m/z at different points within a time-dependent concentration proble of a transient signal can result in peak distortions and quantisation errors commonly referred to as spectral skew. The alternative is TOF-MS which features the ability to produce a complete atomic mass spectrum in less than 50 xs and thus allows very brief transient signals to be recorded with high bdelity. This is especially useful in the on-line isotope ratio determination. However, a 10-fold loss in sensitivity of a TOF-ICP-MS instrument in comparison with the latest Q instruments often creates an obstacle for the wider application of TOF-ICP-MS as a detector in the CE of metallobiomolecules in biological samples. 

MALDI sources do not continuously generate ions, but work in a pulsed manner. This feature matches perfectly the pulsed operation of time-of-flight (TOP) analyzers (see below). Consequently, MALDI-TOF mass spectrometers are quite common. 

A common 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). 

Figure 20-24 is a block diagram of the computerized control and dala-acquisition system of a triple quadrupole mass spectrometer. This figure shows two features encountered in any modern instrument. I hc first is a computer that serves as the main instrument controller. The operator communicates via a keyboard with the spectrometer by selening operating parameters and conditions via easy-to-use interactive software. The computer also controls the programs responsible for data manipulations and output. The second rcature common to almost all instruments is a set of microprocessors (often as many as six) that are responsible for specific aspects of instrument control and the transmission of information between the computer and spectrometer. 

Experimental variables do, however appear to be critical in obtaining reproducible Cl spectra with ion traps, this is a feature common to all mass spectrometer instruments. But improper selection of RF voltages, time intervals, or operating pressures can yield unexpected results, and could be at the origin of some criticisms concerning this mode of ionization in ion trap mass spectrometry. Obviously, and as a matter of fact, under methane Cl conditions, ion traps present characteristic properties such as a smaller m/z ratio for the CH5 /C2H ions, and a reduced yield of adduct ions [M-f29], [M-f41]. The lack of effective stabilizing conditions for the removal of internal energy has been proposed to account for the absence of such adducts [6]. 

The ion mobility spectrum has many forms that share one common feature The ion current intensity is measured as a function of an ion s mobility in a gas. As with other types of spectrometry, the ion mobility spectrum is obtained by correlating a change in a spectrometer s parameter with a physical property of the ions. In light spectrometry, the number of photons is recorded as a function of photon energy in mass spectrometry, the number of ions is recorded as a function of mass, and in ion mobility spectrometry (MS), the number of ions is recorded as a function of an ion s collision cross section, which is related to its mobility. The type of IMS depends on the instrumental parameter that is scanned to produce the intensity versus mobility spectrum. To understand the many types of mobility spectra, we must first consider the relation among mobility, electric field, and pressure. 

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