Practical aspects of 2D NMR

This section introduces the most important experimental aspects relating to two-dimensional data sets, and again uses the COSY experiment to illustrate 

As for ID data, fi quadrature detection requires two data sets to be collected which differ in phase by 90 , thus providing the necessary sine and cosine amplitude-modulated data. Since the fi dimension is generated artificially, there is strictly no reference rf to define signal phases so it is the phase of the pulses that bracket ti that dictate the phase of the detected signal. Thus, for each tj increment two data sets are collected, one with a 90 preparation pulse (ti sine modulation), the other with 90j, (ti cosine modulation), and both stored separately (Fig. 5.17). These two sets are then equivalent to the 

Prior to the advent of the above methods that allowed the presentation of phase-sensitive displays, 2D data sets were collected that were phase-modulated as a function of ti rather than amplitude-modulated. Phase-modulation arises when the sine and cosine modulated data sets collected for each ti increment are combined (added or subtracted) by the steps of the phase cycle, meaning each FID per tj increment contains a mixture of both parts. Here it is the sense of phase precession that allows the differentiation of positive and negative frequencies. This method is inferior to the phase-sensitive approach because of the unavoidable mixing of absorptive and dispersive lineshapes, so is generally only suitable for routine, low-resolution work. 

This section introduces the most important experimental aspects relating to two-dimensional data sets and again uses the COSY experiment to illustrate these. Many of the discussions are simply extensions of what has already been discussed in Chapter 3 for the ID experiment and familiarity with this is assumed. No new concepts are introduced here, although a modified approach to experimental set-up and to data processing is essential if 2D experiments ate to he successful. 

Finally, we note an alternative approach to quadrature detection known as echo-antiecho selection [11], which is applicable only to pulsed field gradient selected 2D methods and which now finds widespread use. As this involves a quite different procedure, it will not be considered further here but will be introduced in Section 5.5.2 after field gradients have been described. 

COSY spectra and concentrate on the aliphatic region only as this is where the interesting correlations most often lie. With modem digital filters, aliased signals in can be eliminated, although those aliased in /i remain since there is no equivalent filtration in the indirect dimension. 

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