Selectivity enhancement methods

Pindur U., Lutz G., Otto C. Acceleration and Selectivity Enhancement of Diels-Alder Reactions by Special and Catalytic Methods Chem. Rev. 1993 93 741 761... 

Significant progress in signal enhancement methods for the central transition has been achieved by the implementation of double frequency sweeps (DFS) [62]. The basic idea of DFS, applicable for both static and MAS experiments, is to invert simultaneously the STs so that the populations of the outer spin levels are transferred to the CT energy levels before they are selectively excited (Fig. 4). 

Erythromycin, a macrolide antibiotic, lacks a significant chromophore. Detection sensitivity was enhanced by using a wavelength of 200 nm and selecting an injection solvent of lower conductivity than the BGE. In order to facilitate the separation of erythromycin and its related substances, 35% (v/v) ethanol was incorporated into a 150 mM phosphate buffer pH 7.5. Resolution of all of the compounds was achieved in approximately 45 min. The method was employed as an assay method for erythromycin and for impurity determination. Peptide antibiotics, such as colistin and polymyxin, are mixtures of many closely related compounds. A validated CZE method for impurity analysis of polymyxin B was described, employing 130 mM triethanolamine-phosphate buffer at pH 2.5 to reduce the adsorption of analyte onto the capillary wall. Methyl-/l-cyclodextrin (M-/1-CD) and 2-propanol were found to be necessary for selectivity enhancement. Using similar buffer additives, the same group developed and validated a method for colistin analysis. ... 

Over the past fifteen years a number of different approaches have been taken in an attempt to increase the durability of the metal/polymer interfacial region in the presence of water. These attempts have met with varied degrees of success. However, to date adhesion scientists are still searching for a means of achieving sufficient wet environment durability, so that the enormous potential of metal/polymer adhesion systems can soon be utilized effectively. The authors have selected to discuss a few of the more promising durability-enhancing methods. 

Although not all of the factors that influence homogeneous hydrogenation and hydroboration in sc C02 are fully understood, it is clear that the use of sc C02 can lead to an increase in selectivity for some reactions. Additional work is needed to understand the opportunities for further selectivity enhancements and catalyst separation/recycle strategies. Even sc C02 systems that exhibit similar selectivities to those obtained in organic solvents could offer a practical, environmentally responsible method for the production of many important chiral building blocks. 

A simple, rapid, sensitive, and selective spectrofluorimetric method (2ex/ lem = 345/455nm) has been developed for the determination of zaleplon. Tang et al. have studied the influence of micellar medium on the absorption, fluorescent excitation, and emission spectra character of zaleplon The nonionic surfactant of Triton X-100 showed a strong sensitizing effect for the fluorescence of zaleplon in a pH 5.0 buffer. The possible enhancement mechanism was discussed. Based on the optimum conditions, the linear range was 1.32 x 10 8-1.00 x 10 mol/1. The detection limit was 4.0 x 10 mol/1 with a relative standard deviation (RSD) of 0.06%. The proposed method was successfully applied to the determination of zaleplon in tablets, serum, and urine. 

P.W. Kramer, M.K. Murphy, D.J. Stookey, J.M.S. Henis and E.R. Stedronsky, Membranes Having Enhanced Selectivity and Methods of Producing Such Membranes, US Patent 5,215,554 (June, 1993). 

Finally, new methods of analysis have recently been developed that may allow characterization of single atoms on surfaces such as atomic force microscopy.9 In certain cases, in situ experiments can be done such as the study of electrodes, enzymes, minerals and biomolecules. It has even been shown that one atom from a tip can be selectively placed on a desired surface.10 Such processes may one day be used to prepare catalysts that may enhance selectivity. Other methods that show promise as regards detection of surface catalytic intermediates are temperature programmed desorption techniques.11 Selective poisoning of some surface intermediates with monitoring via temperature programming methods may also allow the preparation of more selective catalysts. 

Zeolites offer possibilities for both catalysis and selectivity enhancement in aromatic substitution reactions, where such effects are much needed to overcome the many problems caused by the traditional methods. Two reaction types where superior methodologies are particularly needed, namely selective nitration of simple benzenoids and 2,6-dialkylation of naphthalene, have been addressed in the present study. 

Resonance Raman and NMR Studies. The major support to the protonation hypothesis is presently based on the recent application of resonance-Raman spectroscopy. (For recent reviews, see refs. 217-219.) The method uses an incident beam which is in resonance with the absorption of the retinyl chromophore. This results in the selective enhancement of the Raman cross sections coupled with the chromophore, relative to the very weak, non-resonant, modes of the opsin. Characteristic spectra are shown in Fig. 6. Early evidence for protonation came from the observation of a close similarity between the C=N vibrational frequency in rhodopsin and in a model protonated Schiff base (220). More conclusive arguments were provided by Oseroff and Callender, who carried out experiments at low temperatures in order to control sample photoability (221). It was observed that deuteration shifts the C=N vibration frequency from 1655 cm- to 1630 cm-- -, both in the pigment and in a model protonated Schiff base. 

The other enhancement method is to use molecular electronic resonances [19, 22, 23]. A most interesting topic in this enhancement method is so-called molecular near-held effect reported by Kano et al. [34]. This effect, explained by intermolecular vibronic coupling, makes HRS spectroscopy highly sensitive to intermolecular interactions one can observe HRS modes of solvent molecules adjacent to the solute dye molecules via the intensity borrowing from the dye molecules. That is, solvent-solute interactions are selectively observed without impairment from signals of the bulk solvent. It is almost impossible to observe a similar effect in resonance RS spectroscopy because of the unavoidable contribution from Franck-Condon type resonance. 

Figure 4 Methods of selectivity enhancement using pulsed field gradients employing (a) selective excitation pulses, (b) a selective refocusing pulse and (c) a double piUsed field gradient spin echo (DPFGSE) sequence with inversion pulses...

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