Polar compounds, NPLC

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... 

In normal phase HPLC, or NPLC, hexane is a widely used diluent and the common modifiers are one or a combination of dichloromethane, chloroform and ether. Remember, bonded amine derivatives of silica are also used for normal phase separation of sugars and other highly polar compounds. 

NPLC is most suitable for separation of nonionic and moderately polar compounds, especially for lipophilic samples that are too strongly retained by RPLC. Lipids differing in the number and position of double bonds, tocopherol, carotenoids, fat-soluble vitamins, and steroids in pharmaceuticals can be successfully separated by NPLC on silica gel or alumina columns. Mixed lipid classes in the extracts of animal or plant tissues can be analyzed on silica columns or on columns with bonded polyvinyl alcohol using complex solvent gradients. Gradient-elution... 

Normal-phase (NP) and reversed-phase (RP) liquid chromatography are simple divisions of the LC techniques based on the relative polarities of the mobile and stationary phases (Figure 4.10). Both NPLC and RPLC analysis make use of either the isocratic or gradient elution modes of separation (i.e. constant or variable composition of the mobile phase, respectively). Selection from these four available separation techniques depends on many variables but basically on the number and chemical structure of the compounds to be separated and on the scope of the analysis. 

The retention in NPLC increases with increasing polarity (activity) and is proportional to the specific surface area. As, which controls the number of the adsorption sites available for contact with the sample solutes in the column. Water in mobile phases deactivates the adsorbent (decreases oc ), as the sites with strongly held adsorbed water are no longer available for adsorption of moderately polar organic compounds. The elution times of analytes generally increase in the following sequence alkanes < alkenes < aromatic hydrocarbons chloroalkanes < alkylsul-phides < ethers < ketones aldehydes esters < alcohols < amides phenols, amines, carboxylic acids. The... 

Cyclodextrins exhibit chiral recognition characteristics, because the cavities inside are formed from optically active sugars. Cyclodextrins have been introduced as ligands in NPLC as early as 1989 for the separation of sugar alcohols and various saccharides [35]. Risley and Strege [36] used such a stationary phase for the separation of polar chiral compounds, which could not be resolved under nonaqueous NP conditions. 

---