Linked-scanning

Linked Scanning and Metastable Ions in Quadrupole Mass Spectrometry... 

Linked Scanning with Triple Quadrupole Analyzers... 

Linked scanning techniques by which fragmentation reactions can be examined are particularly easy to apply with QqQ instmments. The ease with which RF and DC voltages can be changed rapidly means that the scanning can be done very quickly. Three common and popular types of linked scan are briefly described here and serve to illustrate its principles. 

An example of linked scanning on a triple quadrupole instrument. A normal ion spectrum of all the ions in the ion source is obtained with no collision gas in Q2 all ions scanned by Q1 are simultaneously scanned by Q3 to give a total mass spectrum (a). With a collision gas in Q2 and with Q1 set to pass only m+ ions in this example, fragment ions (f, fj ) are produced and detected by Q3 to give the spectrum (b). This CID spectrum indicates that both f, and fj are formed directly from m+. 

The importance of linked scanning of metastable ions or of ions formed by induced decomposition is discussed in this chapter and in Chapter 34. Briefly, linked scanning provides information on which ions give which others in a normal mass spectrum. With this sort of information, it becomes possible to examine a complex mixture of substances without prior separation of its components. It is possible to look highly specifically for trace components in mixtures under circumstances in which other techniques could not succeed. Finally, it is possible to gain information on the molecular structures of unknown compounds, as in peptide and protein sequencing (see Chapter 40). 

The study of metastable ions concerns substances that have been ionized by electrons and have undergone fragmentation. The stable molecular ions that are formed by soft ionization methods (chemical ionization. Cl field ionization, FI) need a boost of extra energy to make them fragment, but in such cases other methods of investigation than linked scanning are generally used. 

Scanning for metastable ions requires adjustment of electric and magnetic fields which can be adjusted individually or in conjunction with each other. If two fields are automatically adjusted at the same time, it is known as linked scanning. It is important to remember that metastable ions are not the product ions m2 but are the ions undergoing fragmentation (precursor ions mj). 

Alternatively, the electric focusing potential E can be changed, but this method needs another ion collector sited at the electric-sector focus, and it must be a collector that can be raised out of the ion beam when the mass of the ion being examined is required. This arrangement is not convenient. A better solution is obtained by linked scanning of the E/V voltages (see later discussion). 

There are a variety of possible linked scanning methods, but only those in more frequent use are discussed here. They differ from the linked scanning methods used in triple quadrupole instruments and ion traps in that two of the three fields (V, E, and B) are scanned simultaneously and automatically under computer control. The most common methods are listed in Table 34.1, which also defines the type of scanning with regard to precursor and product ions. 

Some Methods of Linked Scanning in the First Field-Free Region... 

In an EW- of a B/E-linked scan using an electric/magnetic-sector instrument, a precursor ion is selected. In this case it is m, which might be a molecular ion but equally could be any fragment ion. All product ions (mj, m3, m4) from decomposition of m, in the first field-free region between the ion source and the ion collector are found, thereby giving connections mpm, m -m3, m -m4. 

In a B /E-linked scan, a product ion (fragment ion) is selected. In this case it is m4, which can be any fragment ion but not a molecular ion (connects with itself). All precursor ions (m, mj, m3), which decompose to give the product ion m4, are found, giving connections m -m4, m2-m4, m3-m4. Any one of the precursor ions could be a molecular ion. 

Linked Scanning, Ion Traps, and Hybrid Mass Spectrometers... 

By adjusting magnetic and electric fields in the ion optics (see Chapters 33 and 34 on linked scanning), metastable ions can be investigated. 

By linking the way in which the first and third quadrupoles are scanned, specific metastable fragments can be detected, which is one form of linked scanning. 

Again, as for metastable ions, linked scanning of the first and third quadrupoles reveals important information on fragmentation processes, viz., which normal ions fragment to give which product ions. 

Linked scanning provides important information about molecular structure and the complexities of mixtures, and it facilitates the detection of trace components of mixtures. 

Linked scanning is particularly easy with a triple quadrupole instrument. 

Triple quadrupole instruments can be used to detect metastable ions or can be used for linked scanning to obtain information about molecular structure. 

Each of the fields V, E, and B can be varied by itself to examine metastable ions and their connections, but with modern automated techniques it is better to use linked scanning. 

By automating the linked scanning under computer control, a complete mass spectrum can be scanned for metastable ions in just a few seconds. 

Automated linked scanning of metastable ions is valuable for deducing a whole or partial molecular structure of an unknown substance. 

Linked scan. A scan, in an instrument with two or more analyzers, in which two or more of the analyzer fields are scanned simultaneously to preserve a predetermined relationship between parameters that characterize these fields. Often these parameters are the field strengths, but they can be the frequencies in the case of analyzers that use alternating fields. 

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