Spectral Display

Because we can adjust phasing in both dimensions, we can convert the diagonal peaks to absorption mode, but only at the expense of converting the cross peaks to dispersion, which is quite undesirable. 

By following the coherence pathway described in Section 12.2, we find that there are again eight observable signals, which are similar in form to those in Eq. 12.5 but somewhat different in detail. For example, Eq. 12.6 describing the cross peaks becomes  

The COSY pulse sequence can be modified to provide a 2D spectrum that is decoupled in the a)t dimension while retaining coupling in o2. The simplification in cot can be helpful in interpreting crowded spectra, as the chemical shifts are clearly revealed. Although this experiment is not widely used as such, it serves as a prototype for a building block in other more complex 2D and 3D experiments. 

The pulse sequence is shown in Fig. 12.7. This sequence represents a constant time experiment and differs from those that we have seen in that the total evolution time comprises two variable periods. As , is incremented, one period increases but the other decreases to retain a constant overall evolution time, A. The 180° pulse refocuses chemical shifts at the end of the 1, period, but they then precess as 

FIGURE 12.7 Pulse sequence for Wpdecoupled COSY. The period A is a fixed time, while is incremented as usual in a 2D experiment. 

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Figure 11. Non-echoing commands for spectral display program...

To provide an internal reference, the solvent is sealed in a capillary, which is placed at the bottom of the NMR tube used for the sample solution. For this purpose, a melting-point capillary is closed at one end and a syringe is used to add the reference solution. For aqueous studies, a 2% solution of t-butyl alcohol in water can be used as reference and as solvent for the paramagnetic solute. The shift with respect to the methyl resonance of the r-butyl group is then monitored. With organic ligands such as acetylacetonate (acac) groups, complexes such as Cr(acac)3, Fe(acac)3, and Co(acac)2 are soluble in benzene and the proton resonance of the solvent is a convenient reference. The capillary is filled one-third full, the lower end of the capillary is cooled in ice, and the upper part is sealed off with a small hot flame. Alternatively, the reference solution can be placed in one compartment of a coaxial pair of cyhndrical NMR tubes, which are available commercially. In either case the spectral display should be expanded to permit an accurate measurement of the frequency shift. 

Contour maps and 3-D spectral display, for overall evaluation of the entire sample... 

In any discussion involving relaxation times, remember that 1/Tp 1/T2, etc., have the units of angular velocity (radians/second) and that the usual NMR spectral display is in units of frequency (hertz). Thus, in frequency units, the full widths at half maximum for the Gaussian and the Lorentzian lines are 3.34/(27tT2) and l/nT2, respectively. 

Cinematography. This data acquisition technique is straightforward in principle but tedious in practice. A high-speed movie camera photographs the mass spectral display on the oscilloscope screen. When the camera operates at 400 frames per sec and the mass spectrometer at a repetition rate of 10 kHz, each frame of the film contains the information of 25 mass spectra. This arrangement permits time resolution of 2.5 msec. A 150-m film records data for approximately 50 sec which is adequate for most stress MS experiments. 

SoftLog was probably the first LIMS designed specifically for spectroscopists. It incorporated several important features not typically available in LIMS. These included full and substructure searches, spectral display and search, easy incorporation of... 

In all spectra shown in Fig. 6.2.1, several other features can be attributed to CO2 besides the 667 cm band. These features, only weakly present in the Martian spectrum, include the bands at 961 cm and 1064 cm the distinct Q-branches near 545 cm 791 cm 830 cm and 865 cm and the strong features at 1918 cm and 1932 cm For comparison, a ground-based spectrum of Venus (Fig. 6.2.2) shows the CO2 bands between 750 cm and 1000 cm at a much higher spectral resolution of 0.2 cm (Kunde et al., 1977). If the spectral display of Fig. 6.2.1 extended to higher wavenumbers, then the strong vs-band of CO2 would appear at 2349 cm This is another spectral region where temperature sounding is feasible. Temperature retrieval in the atmosphere of the Earth is also... 

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