Crossed-dispersion mode

In addition, Echelle spectrometers are often used [50], By combination of an order-sorter and an Echelle grating either in parallel or in crossed-dispersion mode, high practical resolution (up to 300 000) can be realized with an instrument of fairly low focal length (down to 0.5 m) (Fig. 94). Therefore, the stability as well as the luminosity are high. By using an exit slit mask with a high number of preadjusted slits, highly flexible and rapid multielement determinations are possible. 

Apart from gratings used at low orders, so-called echelle gratings can be used. Their groove density is low (a up to 1/100 mm) but therefore order numbers of up to 70 can be used [50]. Here, the orders overlap and must be separated by using a second dispersive element (e.g. a prism) either with its axis parallel to that of the echelle grating or in so-called crossed-dispersion mode. In the latter case, the spectrum occurs as a number of dots with a height equalling that of the entrance slit. 

Figure 6 illustrates a block diagram of a crossed-coil variable frequency spectrometer and associated electromagnet. A calibrator circuit 66) is useful for intensity calibration of absorption and dispersion mode signals. A calibrator circuit for the Pound-Knight type of spectrometer is also used... 

The Pound-Knight type of spectrometer has the disadvantage that the dispersion mode is not observed. The dispersion mode is important for the study of solids, since it does not saturate as readily as the resonance absorption in most solids 46), and in some cases (long thermal relaxation time) is the only mode that gives a measurable signal. On the other hand, high temperatures (up to 600°) are more conveniently attained with the Pound-Knight type spectrometer the conventional crossed-coil versions have been somewhat limited (up to 300°) in this respect. 

It may be useful to make some explanations on data processing to prepare good D-HMBC spectral data. Sine-bell window is usually employed for processing of HMBC data to give power-mode spectra as shown in fig. 6(a), because they consist of absorption-mode cosine) and dispersion mode sine) signals for both the t and t2 axes. This procedure causes a considerable loss of signal to noise ratio when cross peaks appear as broad ones and when digital resolution is poor as used for ordinary HMBC measurement. 

Fig. 8.21. Simulated COSY cross peaks originating from a pair of signals dipole-coupled in the presence of cross relaxation with Curie relaxation and in the absence of scalar coupling. The two degenerate components are in antiphase, but they do not cancel out due to their different linewidths. Note that absorption mode phasing (A) and dispersion mode phasing (B) show opposite patterns with respect to the case of scalar coupling (Fig. 8.15A,B).

In the case in which T l is laiger than J, one would think that the antiphase character of the cross peaks in the TPPI mode cancels partially or totally the cross peaks. Indeed, much of the intensity of the signal is lost. The loss can, however, be reduced if the cross peaks are phased in dispersion mode [48] or if the experiments are performed in magnitude mode [36] (Section 8.5). The occurrence of coherence transfer phenomena may give rise to COSY cross peaks even in the absence of scalar coupling (Section 8.8) [35]. As long as this is kept in mind, COSY cross peaks between very broad lines can be looked for, and interpreted as dipolar correlation cross peaks. 

Figure 11.2. Dispersion mode (first-derivative) Lorentzian signal. The position of a dispersion mode signal is where the line crosses its baseline the halfwidth is the horizontal distance between the maximum and the minimum of the signal curve. Compare this signal with the absorption mode Lorentzian signal in Figure 3.18 both were plotted using the same parameter values (see review problem 11.5).

Figure 6-15 Phase-sensitive COSY diagram for two spins, with the diagonal peaks in dispersion mode and the cross peaks in antiphase absorption mode. The ID spectrum is on the right. (Reproduced from F. J. M. van de Ven, Multidimensional NMR in Liquids, VCH, New York, 1995, p. 171.)...

In Kiihni columns mixing zones (with centrifugal mixers) are separated by perforated plates where the opening ratio (free cross-sectional area) may be adjusted to the desired operating conditions. Possible operating are mixer-settler mode and dispersion mode. 

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