Principles tandem

Typically, in order for motion to occur, several muscle sets must work together to perform even the simplest movements. The bicep is a two-muscle set the tricep is a three-muscle set. Each set works in tandem. Within each muscle group, muscle fibers obey the aU. or none principle, ie, aU. muscle fibers contract or none contract. Therefore, if the muscle fibers of a muscle group are stimulated enough by nerve impulses to contract, they contract to the maximum. 

Confirmation of suspected residue findings relies on the various chromatographic principles of cleanup and determination (GPC, NP-LC, GC), and is further supported by re-analysis of the final extract(s) on a GC stationary phase of different polarity, providing modified selectivity, or by the use of GC with specific mass spectrometric detection [GC/MS or gas chromatography/tandem mass spectrometry (GC/MS/MS)]. 

Principles and Characteristics Analytical multistage mass spectrometry (MSn) relies on the ability to activate and dissociate ions generated in the ion source in order to identify or obtain structural information about an unknown compound and to analyse mixtures by exploiting two or more mass-separating steps. A basic instrument for the currently most used form, tandem mass spectrometry (MS/MS), consists of a combination of two mass analysers with a reaction region between them. While a variety of instrument set-ups can be used in MS/MS, there is a single basic concept involved the measurement of the m/z of ions before and after a reaction in the mass spectrometer the reaction involves a change in mass and can be represented as ... 

Principles and Characteristics Particle-induced X-ray emission spectrometry (PIXE) is a high-energy ion beam analysis technique, which is often considered as a complement to XRF. PIXE analysis is typically carried out with a proton beam (proton-induced X-ray emission) and requires nuclear physics facilities such as a Van der Graaff accelerator, or otherwise a small electrostatic particle accelerator. As the highest sensitivity is obtained at rather low proton energies (2-4 MeV), recently, small and relatively inexpensive tandem accelerators have been developed for PIXE applications, which are commercially available. Compact cyclotrons are also often used. 

As mentioned above, not only general principles but also concrete examples of the use of the tandem strategy by Prof. Denmark were considered in two monographs (427, 428). These data should be supplemented by the synthesis of azafenestranes, which has been recently performed by Prof. Denmark and his group (429, 430) (Scheme 3.173). 

A very attractive feature of radical chemistry is the generation of a novel radical after cyclization or any other radical translocation. This feature allows the inclusion of a second carbon—carbon bond-forming event and can, in principle, be extended even further. The resulting tandem reactions [38] can be extremely useful for the construction of complex molecules. Impressive early results have been reported by Stork in applications directed towards the synthesis of prostaglandins [39]. Our catalytic conditions also allow the realization of tandem reactions. An example including a mechanistic proposal is shown in Scheme 12.20. 

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. 

A tandem-in-space mass spectrometer consists of an ion source, a precursor ion activation device, and at least two nontrapping mass analyzers. The first mass analyzer is used to select precursor ions within a narrow m/z range. Isolated precursor ions are allowed to enter the ion activation device, for example, a gas-filled collision cell, where they dissociate. Created fragments continue on to the second mass analyzer for analysis. The second mass analyzer can either acquire a full mass fragment spectrum or be set to monitor a selected, narrow, m/z range. In principle the second mass analyzer could be followed by more ion activation devices and mass analyzers for MSn experiments. However, due to rapidly decreasing transmission and increasing experimental... 

In tandem MS, two or more stages of mass analysis are combined in one experiment. [79,80] Each stage provides an added dimension in terms of isolation, selectivity, or structural information to the analysis. Therefore, a tandem MS stage is equivalent to a chromatographic separation, provided the separation of isomers is not required. While chromatography distinguishes substances by their retention time, tandem MS isolates them by mass. [2,3,25] The principles of tandem MS have been discussed and some applications for stmcture elucidation and quantitation have already been shown (Table 12.1). However, the aspect of increased selectivity has not been addressed so far. [81]... 

From an exhaustive retrosynthetic analysis and from the experimental work performed by Curran [29] [32], it was clear that the synthesis of modhephene required an elaborate strategy. In the first place, the tandem radical cyclisation should be conducted individually rather than just in one step since it allows more flexibility. In the second place, Curran s observation that the precursor of modhephene (54) could be the olefmic exocyclic derivative 55 allows the application of a series of heuristic principles already familiar to us, which greatly simplifies the retrosynthetic analysis and leads to diquinane and, finally, through a second radical retroannulation to the very simple cyclopentanone derivative (Scheme 7.24). 

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