Water suppression probe

NMR spectroscopy, commonly called magnetic resonance spectroscopy (MRS) in its in vivo application, is continuing to expand its reach in both biomedical research and clinical medicine. Currently, most in vivo MRS utilizes as a straightforward addition to MRI on a clinical scanner. The high sensitivity of permifs relatively small tissue volumes, on the order of 1-8 ml, to be probed in vivo in humans. With water suppression,... 

Because monofunctionalization of fullerenes shows that a single hydrophilic addend is insufficient to prevent the strong hydrophobic interactions among the compounds [89, 90, 93, 94], multiple functionalized derivatives were examined as water-soluble probes. In particular, introduction of a second hydrophilic ligand (e.g., pyrrolidinium salts or carboxylates) to the fullerene core enhances the surface coverage of the hydrophobic fullerene surface. In turn, it was expected that fullerene aggregation might be suppressed. It should be stated that these water-soluble derivatives are important alternatives to the y-CD-incorporated and surfactant-embedded fullerenes. 

A common drawback to LC-NMR analysis derives from the interference of the resonances of the mobile phase, usually much higher than those of the analyte itself. To overcome this problem, the mobile phase coming from the liquid chromatograph is efficiently suppressed with techniques such as water-suppression enhanced-through Ti effects (WET). This allows the separation of analytes in typical RP-HPLC conditions using common solvents such as MeOH or acetonitrile, yet usually replacing water by D2O to achieve better quality spectra however, in the case of analyte signals coincident with the suppressed solvent resonances, those of the analytes will also be suppressed. On the other hand, in spite of the usefulness of LC-NMR for stmcmral elucidation of flavonoids, this is not a sensitive LC detection technique. Therefore, to improve sensitivity, transients can be accumulated by operation in two different modes the on-flow and stop-flow modes. In both cases, the analytes from an LC column pass into an NMR microflow probe that typically has an active volume of 60-120 p.L, comparable with the conventional NMR sample size of 500-600 p,L. 

To probe the possible role of surface hydroxyl groups, addition of 2,6 dimethyl pyridine was conducted, expected to block those sites. It was shown that with increased addition of 2,6 dimethyl pyridine, the water-gas shift rate was suppressed, as shown in Table 125. 

In another study, Nakajima, Konomi, and Kitahara [144] studied the water accumulation in different components of the fuel cell at simulated start-up cycles. Each component was weighed before and after each test once a test was completed, water balance analysis was performed. Through this analysis, the effect of different diffusion layers was probed in detail, and it was concluded that the DLs with higher gas permeability were able to remove water more efficiently. It was also observed that the MPL was effective in improving start-up performance of the fuel cell by suppressing water accumulation at the CL and within the DL. 

Figure 15. H-NMR spectra(2C)0 MHz) of intact banana fruit tissue (A) non-MAS spectrum obtained in a conventional high resolution probe with sample axis parallel to magnetic field (B)MAS spectrum obtained without water peak suppression (C) vertical expansion of (B) (D)MAS spectrum obtained with water peak suppression, the signal-to-noise ratios (S/N) in spectra C and D are 55 and 1137, respectively. The magic angle spinning (MAS) frequency was 1.05 kHz.[Reproduced with permission from Ref.81].

Fig. 9. Proton NMR spectrum of a 75 ng (32 pmol) sample of cryptolepine (1) dissolved in 40 pL of 99.996% d6-DMSO. The data were acquired overnight using a prototype Varian proton Nano-probe with the sample spun at approximately 2 kHz. Both the residual water and protio-DMSO were irradiated to suppress their signals during the data acquisition. The suppressed DMSO signal is plotted on-scale for comparison to the 13C satellite lines from the solvent. Relative to the V-methyl group of the cryptolepine (1) ( 5.05ppm), the unsuppressed 13C satellite lines from the DMSO are observed with approximately twice the intensity of the Y-methyl signal. The line-labeled SSB is a spinning side band. (Reprinted with permission from Ref. 19. Copyright 2002, John Wiley Sons, Ltd.)...

---