Normal-phase adsorption, solvent

To eliminate these effects, a mobile phase is chosen that is a good solvent for the polymer and whose solubility parameter, 8, is close to that of the packing. Thus the polymer and packing are well solvated and potential adsorptive sites on both are "deactivated". As demonstrated by Dawkins (21,22) and Mori (10,23), if > solvent normal-phase adsorption... 

In normal-phase adsorption chromatography, the stationary phase is composed of strongly polar silica particles whereas the mobile phase is composed of nonpolar solvents such as n-hexane. Analytes are separated due to their varying degree of adsorption onto the solid surface. Polar samples are thus retained on the polar surface of the column packing longer than less polar materials. 

Normal-phase adsorption on silica gel with a relatively less polar mobile phase is the most widely used mode in conventional TLC. To improve separations, silica gel may be impregnated with various solvents, buffers, and selective reagents. Other commercial precoated layers include alumina, florisil, polyamide, cellulose, and ion exchangers. 

The compatibility of electrochemical detection with the various modes of liquid chromatography is limited. For all practical purposes, electrochemical detection is not suitable for use with normal phase adsorption or partition chromatography due to the solvents of low dielectric constant used as the mobile phase. On the other hand, reverse-phase adsorption and partition (including ion-exchange or ion-pairing systems) are highly com-... 

However, in normal phase adsorption systems (or liquid-solid chromatography) the interaction of the mobile phase solvent with the solute is often less Important than the competing Interactions of the mobile phase solvent and the solute with the stationary phase adsorption sites. Solute retention is based upon a displacement mechanism. Multicomponent mobile phases and their combination to optimize separations in liquid-solid chromatography have been studied in detail (31-35). Here, solvents are classified as to their interaction with the adsorption surface (Reference 32, in particular) ... 

Solvent strengths are also measured in terms of polarity, and dielectric constants are generally used to quantify relative strengths (Table 1). A high dielectric constant indicates a polar solvent with a strong power of elution, and a low dielectric constant indicates a nonpolar solvent with a lower ability to elute a component from a sorbent. This elution strength applies to normal phase adsorption chromatography. 

A later study (Aguas 2006) adopted a somewhat different approach to the analysis of acrylamide and extended it to the case of cocoa, which is at least as complex a matrix as coffee. The initial extraction of acrylamide and its C3-intemal standard with water was essentially unchanged, but the aqueous extract was then defatted by hquid-liquid extraction with dichloromethane. The subsequent clean-up strategy was very different from the earlier methods (Tareke 2002 Andrzejewski 2004) in that a normal phase SPE sorbent was used to selectively elute the amide from more polar impurities. The SPE phase chosen for this purpose was of the aminopropyl type that has weak anion exchange properties in addition to the characteristic normal phase adsorption properties. However, such an approach requires that the cocoa extract must be in a relatively nonpolar solvent so that acrylamide is retained on... 

In normal phase LLC the stationary phase is polar and the mobile phase consists of nonpolar solvents. This mode is suited for more polar, water-soluble analytes. The elution order is similar to that in normal phase adsorption chromatography because nonpolar solutes prefer the moving phase and elute with low k values. Solutes preferring the polar stationary phase elute later with high k. ... 

The term polarity refers to the ability of a sample or solvent molecule to interact by combination of dispersion, dipole, hydrogen bonding, and dielectric interactions (see Chapter 2 in reference 5). The combination of these four intermolecular attractive forces constitutes the solvent polarity, which is a measure of the strength of the solvent. Solvent strength increases with polarity in normal phase, and adsorption HPLC decreases with polarity in reversed-phase HPLC. Thus, polar solvents preferentially attract and dissolve polar solute molecules. 

The addition of CO2 to mobile phases in normal phase chromatography using silica gel stationary phases was used as an adsorption-promoting solvent [56], Tetrahydrofiiran or chloroform with 3.5% ethanol was the organic components in... 

Adsorption chromatography on bare silica is an example of normal-phase chromatography, in which we use a polar stationary phase and a less polar solvent. A more polar solvent has a higher eluent strength. Reversed-phase chromatography is the more common... 

Carotenoid separations can be accomplished by both normal- and reversed-phase HPLC. Normal-phase HPLC (NPLC) utilizes columns with adsorptive phases (i.e., silica) and polar bonded phases (i.e., alkylamine) in combination with nonpolar mobile phases. In this situation, the polar sites of the carotenoid molecules compete with the modifiers present in the solvent for the polar sites on the stationary phase therefore, the least polar compounds... 

Adsorption chromatography The process can be considered as a competition between the solute and solvent molecules for adsorption sites on the solid surface of adsorbent to effect separation. In normal phase or liquid-solid chromatography, relatively nonpolar organic eluents are used with the polar adsorbent to separate solutes in order of increasing polarity. In reverse-phase chromatography, solute retention is mainly due to hydrophobic interactions between the solutes and the hydrophobic surface of adsorbent. Polar mobile phase is used to elute solutes in order of decreasing polarity. 

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