Phase contrast transfer technique

In parallel with recent developments in GC, multidimensional HPLC (LC x LC) is now also finding application in environmental analysis.33 The combination of two sufficiently different separation dimensions (e.g., NP-HPLC x RP-HPLC or IC x RP-HPLC), however, remains difficult because of the solvent compatibility issues discussed above. Here, too, HILIC may bring about a significant improvement, since its mobile phase requirements are much closer to RP-HPLC than those of other liquid chromatographic techniques.34 In contrast to GC x GC, LC x LC cannot be implemented with a (thermal) modulator that collects the analytes after the first separation dimension and reinjects them into the second column it is most practically realized with a double-loop interface that alternately collects and transfers the analytes from the first to the second dimension (Figure 13.7). Even though the second dimension chromatogram is also very fast, detection is not normally a problem since the peak widths in the second dimension are usually still of the order of 1-2 s. 

In contrast to solution-phase or direct polarization solid-state NMR experiments, CP-MAS is not a truly quantitative technique. As a result of the cross-polarization step, the integral intensities of peaks in CP-MAS spectra do not directly reflect the stoichiometry of nuclei in the molecule. CP-MAS signals originate from protons, and its transfer efficiency varies from carbon to carbon, based on proximity of protons and local mobility of the molecule. In most pharmaceutical applications, only relative concentrations of mixtures of two or more polymorphs need to be determined even in complex dosage forms. The absolute concentration of the drug in formulation is usually known or can be determined by other techniques. 

Calculations of KIEs derived from a classical reaction path (e.g. the MEP) in the presence of a solvent or polar environment typically add quantum corrections to that path [22]. Such a reaction path, however, includes classical motion of the proton, especially near the TS, and thus this technique exhibits no difference in quantum corrections between H and D at the TS for a symmetric reaction (AG])xn=0) [22b], in contrast to the present picture. In variational TS theory for gas phase H atom transfer, the TS significantly deviates from the MEP TS and is isotope-dependent [23]. This feature has been calculated for PT in an enzyme, where the KIE has been diminished because the TS position significantly differs between H and D even in a symmetric case ]22e[. 

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