Modified inversion recovery

Theoretical expressions for spin-lattice relaxation of 2H nuclei (determined by locally axially symmetric quadrupolar interactions modulated by molecular motions) can be derived for specific dynamic processes, allowing the correct dynamic model to be established by comparison of theoretical and experimental results [34,35]. In addition, T, anisotropy effects, which can be revealed using a modified inversion recovery experiment, can also be informative with regard to establishing the dynamic model [34,35]. 

Polymer Dynamics. 13C spin-lattice relaxation times (Ti) were determined with either an inversion-recovery sequence (16) (for carbons observed by direct polarization) or with a modified cross-polarization experiment (17). 13C rotating-frame relaxation times (Tip(C)) were derived from measurements of the carbon signal that remained after a Tjp(C) hold time of... 

Natural abundance 13c spectra were obtained using quadrature detection modified Bruker HX-270 and HFX-90 spectrometers operating at 67.9 and 22.6 MHz, respectively. Free induction decays were accumulated using 4K/4K data points and a 3kHz spectral window. A fast inversion recovery (FIRFT) pulse sequence (21) was employed to measure T] s T] s were calculated using a nonlinear three parameter fitting procedure(22). 

Due to efficient spin diffusion, homogeneous crystalline solids present a unique relaxation time, regardless of chemical differences. Then different polymorphs can be differentiated by using pulse sequences able to discriminate substances on the basis of their different relaxation properties. In particular cases it has been possible to extract the individual subspectra from a mixture of polymorphs, by modifying the inversion recovery pulse sequence to decompose the spectra [43]. 

The priority phenols (Table 4) in tap and river waters were determined by SPE on line with SEC wi DA-UVD. Tetrabutylammonium bromide was used in the extraction process to increase breakthrough volumes. The mobile phase was CO2 at 40 °C, modified by a gradient of MeOH. LOD was 0.4 to 2 tig L , for 20 mL samples, with good repeatability and reproducibility between days (n = 3) for real samples spiked with 10 [xgL . Seven pollutant phenols, 107a-f and pentachlorophenol, were determined by lEC with a basic SAX resin (styrene-divinylbenzene copolymer with quaternary ammonium groups) and single channel UVD. Resolution of overlapping peaks was carried out by inverse least-squares multivariate calibration. LOD was 0.6 to 6.6 ng, with better than 90% recovery from spiked pure water and 83% from river water. No extensive clean-up was necessary . ... 

Cosmetic products have been the samples for the SFE of their active compounds. Thus, parabens have been extracted from cosmetics, either with pure supercritical CO2 (157) or with 0.05% acetonitrile-modified CO2 (158). SFE of sunscreen agents prior HPLC (159) and micellar electrokinetic capillary chromatography (160) has been carried out, yielding 94.8% and 98.4-101.8% recovery, respectively. An inverse SFE procedure has been developed for the efficient isolation of six xanthene dyes from lipstick matrices (161), which only removed the matrix components, whereas the xanthene colorants remained in the extraction vessel. The target analytes were quantitatively recovered by dispersion of the sample in ethanol under sonication. Finally, cosmetic waxes have been extracted by SFE prior to SFC, using multivariate data analysis for their quantitation (162). 

The use of catalysts based on polymers with inverse temperature solubility, often copolymers of TV-isopropy-lacrylamide, to allow recovery by raising the temperature to precipitate the polymer for filtration,9 was mentioned in Chap. 5. The opposite, if the catalyst is soluble hot, but not cold, has also been used in ruthenium-catalyzed additions to the triple bonds of acetylenes (7.1).10 The long aliphatic tail of the phosphine ligand caused the catalyst to be insoluble at room temperature so that it could be recovered by filtration. There was no loss in yield or selectivity after seven cycles of use. A phosphine-modified poly(A-iso-propylacrylamide) in 90% aqueous ethanol/heptane has been used in the hydrogenation of 1-olefins.11 The mixture is biphasic at 22°C, but one phase at 70°C, at which the reaction takes place. This is still not ideal, because it takes energy to heat and cool, and it still uses flammable solvents. 

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