Mass spectrometry common fragments

Tandem mass spectrometry, commonly abbreviated to MS/MS or MS, is a method which usually applies a dissociation process to analyse ions of selected tn/z values. Because of this, a minimum of two mass analysers are required for this technique. The most common application of MS/MS is isolation of a parent ion, nip, of interest with the first analyser, which then either spontaneously dissociates or is activated to yield fragment ions, OT/, and neutral fragments, m , detected by the second analyser. 

The behaviour under electron impact of IV- and C-trimethylsilylpyrazoles (mono-, di-and tri-substituted) has been studied by Birkofer et al. (740MS 8)347). Loss of a methyl radical followed by loss of HCN is the most common fragmentation feature of these compounds. When more than one trimethylsilyl group is present, a neutral fragment CaHgSi is expelled. Mass spectrometry of pyrazolium salts has been studied by Larsen etal. (8i OMS377, 830MS52). 

In most cases, ion activation in the reaction region or fragmentation zone is applied to increase the internal energy of the ions transmitted from the ion source. The most common means of ion activation in tandem mass spectrometry is collision-induced dissociation. CID uses gas-phase collisions between the ion and neutral target gas (such as helium, nitrogen or argon) to cause internal excitation of the ion and subsequent dissociation... 

The first part of this book is dedicated to a discussion of mass spectrometry (MS) instrumentation. We start with a list of basic definitions and explanations (Chapter 1). Chapter 2 is devoted to the mass spectrometer and its building blocks. In this chapter we describe in relative detail the most common ion sources, mass analyzers, and detectors. Some of the techniques are not extensively used today, but they are often cited in the MS literature, and are important contributions to the history of MS instrumentation. In Chapter 3 we describe both different fragmentation methods and several typical tandem MS analyzer configurations. Chapter 4 is somewhat of an outsider. Separation methods is certainly too vast a topic to do full justice in less than twenty pages. However, some separation methods are used in such close alliance with MS that the two techniques are always referred to as one combined analytical tool, for example, GC-MS and LC-MS. In effect, it is almost impossible to study the MS literature without coming across at least one separation method. Our main goal with Chapter 4 is, therefore, to facilitate an introduction to the MS literature for the reader by providing a short summary of the basic principles of some of the most common separation methods that have been used in conjunction with mass spectrometry. 

Note Although the discussion of common fragmentation pathways of organic ions is embedded here in the context of EI mass spectrometry, their occurrence is not restricted to this technique. The reactions of isolated gaseous ions do not directly depend on the ionization method, but are almost exclusively governed by intrinsic properties of the respective ion and by its internal energy (Chap. 2). 

With a focus on trace forensic detection of explosives, especially for use in counterterrorism and to counter narcotics investigations, Fetterolf et al. [75] evaluated the use of ion mobility-mass spectrometry for explosives determinations. In this, explosives residues were collected on a membrane filter by a special attachment on a household vacuum cleaner. Although subsequent thermal desorption and analysis required only 5 s, fimits of detection for most common explosives were as low as 200 pg. The persistence of explosives on hands and transfer to other surfaces were also examined as were post-blast residues of NG on fragments of improvised explosive devices constructed with double-based smokeless powder. Finally, postblast residue from C-4, Semtex, and other explosives was found by IMS analyses on items of forensic and evidentiary value. These few out of many examples demonstrate that mobihty spectrometers are well suited tools for laboratory and on-site investigations, before and after the use of explosives. 

There are various methods to conduct a fragment screening campaign. The most commonly utilized methods include various NMR techniques, mass spectrometry, SPR, and biochemical screens. X-ray crystallization is a preferred method since it provides a binding conformation, but can only be used when the target protein is well behaved. Various calorimetry techniques have also been used for fragment screening, but these have been less commonly utilized. The merits of each method have been discussed in the literature (26) and will not be outlined here. 

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