The NMR Cell

As discussed below in Sect. 4, the procedure which is the most widely 

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The typical peak width with analytical columns of 4.6 mm i.d. and a 1 ml/min flow rate is of the order of 10-30 s. The acquisition of NMR spectra with a short relaxation delay and an acquisition time of below 1 s allows the acquisition of 8-24 transients for one spectrum during the presence of a peak in the NMR cell. This low number of transients limits the detectable amount of sample to 5-10 xg per compound. 

The second alternative is to direct specific samples to the NMR that are of particular interest. The sample can then be trapped in the cell and data acquired from an adequate number of pulses to provide the required resolution. Subsequently, the sample can be expelled from the cell using solvent supplied directly from the chromatography pump. The third alternative is to direct the eluent from the column to a sample loop where it can be stored until the spectrometer is available to take data. If necessary, a number of solutes can be stored in different loops and they can be examined when convenient. When the data has been acquired from one sample, the solute stored in the next loop can then be displaced into the NMR cell. Samples that have been examined can either be displaced to waste or collected for further examination. A photograph of the Varian flow control device for the LC/NMR system is shown in figure 41. 

Studies of the catalytic reactions in the adsorbed phase require conditions of controlled atmosphere, which become especially challenging if the adsorbate is only weakly adsorbed or catalyst-adsorbate samples are air/moisture sensitive and require preparation on a vacuum line, including loading into the MAS rotor. The NMR cells must thus be carefully sealed. On the other hand, to achieve a sufficiently high rotation speed the NMR ceils must be well balanced (83). 

This is where the LC pump is stopped, and the sample in the NMR cell collected and subjected to static NMR measurements. This enrichment step improves sensitivity (S/N ratio) since the eluted component can be repeatedly scanned or a number of fractions can be combined or preconcentrated to increase sample size/concentration. 

Another way to improve the S/N ratio is the recording of the spectra in stop-flow mode. Operation in the direct stop-flow mode requires that the retention times of the analytes of interest are known or that a sensitive method of detection such as LC/UV or LC/ MS is used prior to LC/NMR to trigger the detection. In practice, one of these detectors is connected online before the NMR instrument and the signal of the analytes of interest passing through this detector is used to trigger a valve that will stop the LC flow exactly when the peak reaches the NMR cell after a calibrated delay (for setup, see Figure 2). 

Among the different experimental procedures which are routinely used in NMR spectroscopy, attention will be focussed on some specific topics concerning the NMR cell, the purification of the sample, the chemical... 

The geometry of the NMR cell should allow flow characteristics that give spectral resolution sufficient to allow spin-coupled multiplets to be resolved. This enables detailed structural elucidation to be carried out. 

Poly(vinyl fluoride) [24981-14-4] (PVF) is a semicrystaltiae polymer with a planar, zig-zag configuration (50). The degree of crystallinity can vary significantly from 20—60% (51) and is thought to be primarily a function of defect stmctures. Wide-line nmr and x-ray diffraction studies show the unit cell to contain two monomer units and have the dimensions of a = 0.857 nm, b = 0.495 nm, and c = 0.252 nm (52). Similarity to the phase I crystal form of poly (vinytidene fluoride) suggests an orthorhombic crystal (53). 

The second choice is a simpler solution. According to Sarko and Muggli,66 all 39 observed reflections in the Valonia X-ray pattern are indexable by a two-chain triclinic unit cell with a = 9.41, b =8.15 and c = 10.34 A, a = 90°, 3 = 57.5°, and y = 96.2°. Ramie cellulose, on the other hand, is completely consistent with the two-chain monoclinic unit cell. Also, there are significant differences between their high-resolution solid-state l3C NMR spectra, indicating that Valonia and ramie celluloses, the two most crystalline forms, reflect two distinct families of biosynthesis. On this basis, the Valonia triclinic and the ramie monoclinic forms are classified69 as Ia and Ip, respectively. It has been shown from a systematic analysis of the NMR spectra by these authors, and from electron-dif-... 

In addition to giving conformational information, solid state NMR relaxation experiments can be used to probe the thermal motion of polymers in the hydrated cell wall (5). The motion of the polymers can give us clues as to the environment of the polymer. When there are both rigid and mobile polymers within a composite material, NMR spin-diffusion experiments can be used to find out how far apart they are. 

Our solid state NMR findings suggest that three distinct groups of pectins differing in mobility on the kHz frequency scale, coexist in at least two different areas of the onion cell wall. 

The alternative method is continuous-flow , in which the reactants flow through the detection coil during data acquisition. Continuous-flow NMR techniques have been used for the direct observation of short-lived species in chemical reactions [4—6]. The main difference between stopped- and continuous-flow NMR is that in the latter the sample remains inside the detection coil only for a short time period, termed the residence time, x [7], which is determined by the volume of the detection cell and the flow rate. The residence time alters the effective relaxation times according to the relationship in Eq. (2.5.1) ... 

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