One-dimensional techniques

Using suitable combinations of APT or DEPT spectra, it is possible to perform spectral editing whereby subspectra containing signals of only one multiplicity appear. For DEPT spectra, the editing uses spectra with final 

2 Solvent suppression. If the use of a protonated solvent is essential, a number of techniques are available to substantially reduce the solvent signal. The principal use of these techniques has been to eliminate the H2O signal in aqueous solutions of biopolymers, as reviewed in [37]. Applications to synthetic polymers are uncommon, but it is worth mentioning them briefly as part of the NMR armoury. 

The simplest technique is pre-saturation in which the solvent is subjected to CW irradiation for a short time at a level sufficient to saturate the solvent without affecting other solute peaks. The saturating field is removed immediately before the observation pulse so that the solvent peak has no opportunity to relax. Several solvent peaks can be saturated if rapid frequency switching can be performed during the saturation period. If proton exchange between solvent and solute occurs, for example exchange of OH and NH with H2O, the solvent saturation can be transferred to the solute. However, this is rarely a problem in synthetic polymers, and because of its ease of application the pre-saturation method is the preferred method. 

A wide range of other methods for solvent suppression has been developed which may collectively be classed as tailored excitation. These rely on the application of appropriate combinations of pulses to excite protons lying outside a narrow band of frequencies while leaving those within that band (i.e. the solvent) undisturbed. Examples of these are the Redfield pulse [38], the jump-return technique [39] and binomial sequences [40]. 

The sine-bell method multiplies the FID by half a sine cycle using the function sin (nt/T) where T is the acquisition time of the FID. There are no adjustable parameters, although a variant technique, a shifted sine-bell using the function [43] 

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Initial shock-wave overpressure can be determined from a one-dimensional technique. It consists of using conservation equations for discontinuities through the shock and isentropic flow equations through the rarefaction waves, then matching pressure and flow velocity at the contact surface. This procedure is outlined in Liepmatm and Roshko (1967) for the case of a bursting membrane contained in a shock tube. From this analysis, the initial overpressure at the shock front can be calculated with Eq. (6.3.22). This pressure is not only coupled to the pressure in the sphere, but is also related to the speed of sound and the ratio of specific heats. 

Single linear developments are mostly employed in the vertical mode. The apph-cabihty of the horizontal mode is discussed in Chapter 6. For circular and anticircular developments, the movement of the mobile phase is two-dimensional however, from the standpoint of sample separation it is a one-dimensional technique. Circular developments result in higher hRp values compared to linear ones imder the same conditions, and compoimds are better resolved in the lower-AR range. The same effect is noticed on plates with a layer thickness gradient (see Section 5.2.1). On the other hand, using antieircular developments, compounds are bettCT resolved in the upper-M range. 

Certainly two-dimensional techniques have far greater peak capacity than onedimensional techniques. However, the two-dimensional techniques don t utilize the separation space as efficiently as one-dimensional techniques do. These theories and simulations utilized circles as the basis function for a two-dimensional zone. This was later relaxed to an elliptical zone shape for a more realistic zone shape (Davis, 2005) with better understanding of the surrounding boundary effects. In addition, Oros and Davis (1992) showed how to use the two-dimensional statistical theory of spot overlap to estimate the number of component zones in a complex two-dimensional chromatogram. 

DEPT Distortionless enhancement by polarization transfer. A useful one-dimensional technique which differentiates methyl and methine carbons from methylene and quaternary carbons. 

The success of PTR-MS triggered interest in further improving its performance. Indeed, PTR-MS is a one-dimensional technique, and ions from a complex headspace, e.g. coffee, can often only be tentatively assigned. Ions from different compounds (parent and fragment ions) can overlap in PTR-MS and prevent an unambiguous identification of VOCs in a complex mixture [198]. There-... 

In fact, two-dimensional solid-state NMR is far superior to one-dimensional techniques for studying structure and dynamics123. In particular, the two-dimensional exchange NMR spectrum124 of a static sample is identical with a two-time distribution function125. Thus a two-dimensional NMR spectrum, which is detected for a fixed mixing time, is an image of the state of the dynamic process under study at that time. 

Chapters 3 and 4 (familiarity with which is assumed) provide us with powerful techniques and methods to elucidate the structures of organic compounds especially when combined with information derived from IR and mass spectrometry. These NMR methods are collectively referred to as one-dimensional techniques. To extend our capabilities, we turn once more to NMR. We will use four compounds as examples ipsenol (see Chapter 3), caryophyllene oxide (a sesquiterpene epoxide), lactose (a j3-linked disaccharide), and a small peptide (valine-glycine-serine-glutamate, VGSE). The structures of these compounds are shown in Figure 5.1. 

As with the advanced one-dimensional techniques discussed in Chapter 12, there is a large and growing family of 2D NMR pulse sequences.1-6 In this chapter we will examine some of the most widely used ones, focusing our attention on the information contained in each type of 2D spectrum. How-... 

Now that we have available the more powerful theoretical approaches of the density matrix and product operator formalisms to augment the still very useful vector picture, we can examine the mechanisms of some common NMR methods in more detail. In this chapter we discuss some one-dimensional techniques but concentrate on 2D experiments. We see that some of the 2D experiments can be extended to three or four dimensions to provide additional correlations and to spread out the crowded 2D peaks that sometimes arise from large molecules. 

A disadvantage of DEPT is that it is a subtraction experiment and, therefore, much more sensitive to certain problems than are typical one-dimensional techniques. One remedy that is used in many experiments that suffer fi om stability problems is the employment of steady-state, or dummy, scans (parameter 2 in the foregoing list Section 2-4i). Poor signal cancelation is generally the result of difficulties in one or more of the following areas ... 

The approach to any structural or mechanistic problem will invariably start with the acquisition of one-dimensional spectra. Since these provide the foundations for further work, it is important that these are executed correctly and full use is made of the data they provide before more extensive and potentially time-consuming experiments are undertaken. This chapter describes the most widely used one-dimensional techniques in the chemistry laboratory, beginning with the simple single-pulse experiment and progressing to consider the various multipulse methods that enhance the information content of our spectra. The key characteristics of these are summarised briefly in Table 4.1. This chapter does not cover the wide selection of techniques that are strictly one-dimensional analogues of two-dimensional experiments, as these are more appropriately described in association with the parent experiment and are found throughout the following chapters. 

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