Linked-Scanning Techniques

This section is concerned with just one aspect of the relatively new development that has been termed in various ways such as mass separation/mass spectrometry or mass spectrometry/mass spectrometry (both offering the same acronyms MS/MS), two-dimensional mass spectrometry, tandem mass spectrometry, etc.. The acronym MS/MS is the most appropriate and is certainly the most widely used. The definitions and the nomenclature relevant to these techniques will be introduced as they will arise in our mathematical development of the scan laws. 

MS/MS techniques can be performed on double-focusing sector instruments as well as on quadrupole or hybrid instruments. For the purpose of this discussion will concentrate on a condensed survey of double focusing sector instruments. References that provide substantial accounts on quadrupole and hybrid instruments used for MS/MS can be found in the bibliography at the end of this chapter. 

The magnetic sector creates a dispersion of ions according to their momentum-to-charge ratios, while the radial electric sector disperses them according to their ratios of kinetic energy-to-charge. In a one-dimensional model these results yield the following set of equations  

Equation (30) Dissociative process Equation (31) Conservation of mass 

Equation (32) Conservation of the linear momentum. Holds only if the dissociative process takes place in a field free region. 

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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. 

The oxygen transfer from an ortho nitro group to a carbon-carbon triple bond has been studied by high-resolution mass spectrometry, linked scan techniques and chemical substitution81. Oxygen transfers to both acetylenic carbons were detected as parallel fragmentation pathways81. 

Boyd, R.K. Linked-Scan Techniques for MS/MS Using Tandem-in-Space Instra-ments. Mass Spectrom. Rev. 1994, 13, 359-410. 

D. B. Kassel, B. D. Musselman, and J. A. Smith, Primary structure dtermination of peptides and enzymatically digested proteins using capillary Uquid chroamtogra-phy/mass spectrometry and rapid linked-scan techniques, A a/. Chem. 63 (1991), 1091-1097. 

AC1470) with linked-scan techniques to demonstrate the stereochemical effects regulating the fragmentation pattern in trichothecene mycotoxins (84TH2 87UP4). These data were correlated to those obtained by X-ray diffraction (84MI3). 

The controversy around the two possible origins of the BH ion (from direct pyrolysis or from the nucleoside-like ion) necessitated studies using linked scan techniques and metastable-ion mapping." From those results it is clear that the BH ions, as well as methylfuran particles, are formed directly by the pyrolitic process. 

Figure 26. Various linked scan techniques for determination of neutral loss, precursor and daughter ions.

These examples show the applications of MIKE method. In the same way, linked scan techniques may be particularly valuable. These methods are rapid, thus allowing the accumulation of iterative results, since there is a satisfying reproducibility. 

The process described above imphes a chemical reaction between the substance of interest, M, and an electron, e% followed by further non-reversible dissociation reactions. The latter is true for all modes of ionisation, as shown in the following sections. Hence, it must be noted that mass spectrometry is a destructive method of analysis. Fortunately, the technique is very sensitive and only a minute amount of material is required to perform an analysis (often of the order of few picograms or less). Furthermore, as shown later in the section dealing with linked-scanning techniques, a single sample introduction step is often sufficient to allow for the performance of several experiments. 

Boyd, R. K. 1994. Linked-scan techniques for MS/MS using tandem-in-space instruments. Mass Spectrom. Rev. 13 359-410. 

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