Enhanced polar selectivity

PCA of Polar Embedded, Enhanced Polar Selectivity, and AQ/Aqua Phases... 

Recently, there has been an increase in the number of commercially available phases which the manufacturers claim to be suitable for chromatography using purely aqueous mobile phases. These have been further classified, by the manufacturers, as polar embedded, enhanced polar selectivity, and Aqua phases. Some of the phases in the latter two classifications have been further described by the manufacturers as polar/hydrophilic endcapped phases, depending on the bonding technology employed. Unfortunately, there is a dearth of information regarding the bonding chemistry employed [2]. 

Those possessing enhanced polar selectivity, as shown by a high hydrogenbonding capacity (i.e., a p values > 1), e.g.. Platinum CjglS EPS, Synergi Polar RP, Zorbax SB AQ, and Aquasil phases (column nos. 75, 105, 134, and 5, respectively) - group A. 

Fig. 6. PC1-PC2 score and loading plots for polar embedded, enhanced polar selectivity and Aqua" phases A = enhanced polar selectivity B = high lipophilic retention C = polar embedded phases...

Solvent extraction has also been used to enhance the selectivity of polaro-graphic determinations. Such measurements are normally carried out in aqueous solutions, and extraction followed by back-extraction has been widely used. However, it may be unnecessary to perform a back-extraction if the organic extractant phase has a sufficiently high dielectric constant to dissolve sufficient background electrolyte for a voltammetric determination or if the organic phase can be diluted with suitable polar solvents, such as methanol or acetonitrile [26]. 

The DEPT experiment, or distortionless enhanced polarization transfer, is a carbon selectivity experiment.29-35 Based on the pulse length selected, one... 

The distortionless enhanced polarization transfer (DEPT) experiment is a carbon selectivity experiment.HO-116 Depending on the pulse length selected, one can selectively observe different types of carbon entities. We recommend setting the DEPT proton pulse length to 135°. In this case, quaternary carbons are suppressed, methylenes are inverted, and methine and methyl carbons... 

The enormous improvement in the nuclear spin polarization achievable by these optical approaches has great potential to impact conventional NMR smdies in a variety of ways. Besides simply increasing the detection sensitivity of NMR, the enhanced nuclear spin polarization may be useful for obtaining enhanced spectral selectivity (e.g., for some spatial, structural, or dynamical feature of a sample) enhanced contrast and resolution in MRI and improved density matrix purity (e.g., for NMR quantum computation). Finally, it is worth noting that the optical fields used in some of these methods can be gated, thereby permitting time-resolved studies that would not be possible with conventional NMR approaches. 

Liquid membrane technology has been applied to a great extent for separation of mixtures of saturated and aromatic hydrocarbons. Investigations reveal that the LSM process offers potential for dearomatization of petroleum streams like naphtha and kerosene to meet product specifications for naphtha cracker feedstock and aviation kerosene, respectively [25, 63, 85, 144-146]. The separation is based on a simple permeation technique and occurs due to the difference in solubility and diffusivity of permeating species through the membrane. Kato and Kawasaki [70] conducted studies on the enhancement of hydrocarbon permeation by the use of a polar additive like sulfolane or triethyl glycol. Sharma et al. [147] enhanced the selectivity of the membrane by several orders with the addition of a carrier. Chakraborty et al. [85] used cyclodextrins to enhance the separation factor and removal efficiency of aromatic compound. 

A considerable number of CE separation methods exist for a wide variety of analytes. However, nitrosamines separation and determination by CE requires additional development for its practical use. " " For the separation of hydrophilic, low molecular weight, neutral, and polar compounds such as nitrosamines, it is necessary to develop CE techniques for enhancing the selectivity. The main reason is that these compounds do not interact strongly with the commonly used surfactants (e.g., sodium dodecyl sulfate, SDS) or other buffer modifiers such as cyclodextrins in electrokinetic chromatography. The separation depends on several factors which must be optimized to reach... 

The chemical properties of a protein fimctional group are strongly influenced by its local environment, such as polarity of the microenvironment, hydrogen bonding effect, field/electrostatic effect, as well as steric and matrix effects. These factors contribute to the selectivity and specificity of chemical modifications. Several strategies can be adapted to enhance the selectivity and specificity ... 

The effect of the reactant is basically a kinetic effect and is hence part of the kinetic analysis considered in Chapter 7. We confine our discussion in this section to the effect of solvent polarity, which has a direct influence on the performance of the catalyst. It is known (Thompson and Naipawer, 1973 Sehgal et al., 1975) that certain functional groups in a substrate can bind to the catalyst surface during a reaction in such a way as to enforce the addition of hydrogen from its own side of the molecule, which is opposite to that expected on the basis of steric hindrance, namely binding occurs from the opposite side. This effect, termed haptophilicity, can be exploited in selecting the right kind of solvent to enhance the selectivity to a desired product. 

Two-phase gas-liquid flow clearly reduces concentration polarization, and this can improve membrane separation. For example, Ghosh et al. [91] assessed the effect of gas sparging on protein fraction with BSA (MW 67,000) and lysozyme (MW 14,100) as model solutes. They reported that a nearly complete separation of these two model proteins was achieved with two-phase flow UF (MWCO 100 kDa), indicating an 18-fold increase in selectivity compared to that without air injection. The enhancement in selectivity was believed to be cansed by the disruption of the concentration polarization so that solnte retentions were closer to the intrinsic values. Although the depolarization decreased transmission for both BSA and lysozyme, the theoretical analysis suggested that air injection affected more the transmission of the more rejected component (BSA) so that high separation efficiency was achieved. 

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