Normal-Phase Chromatography NP HPLC

Normal-phase HPLC explores the differences in the strength of the polar interactions of the analytes in the mixture with the stationary phase. The stronger the analyte-stationary phase interaction, the longer the analyte retention. As with any liquid chromatography technique, NP HPLC separation is a competitive process. Analyte molecules compete with the mobile-phase molecules for the adsorption sites on the surface of the stationary phase. The stronger the mobile-phase interactions with the stationary phase, the lower the difference between the stationary-phase interactions and the analyte interactions, and thus the lower the analyte retention. 

Mobile phases in NP HPLC are based on nonpolar solvents (such as hexane, heptane, etc.) with the small addition of polar modiher (i.e., methanol, ethanol). Variation of the polar modifier concentration in the mobile phase allows for the control of the analyte retention in the column. Typical polar additives are alcohols (methanol, ethanol, or isopropanol) added to the mobile phase in relatively small amounts. Since polar forces are the dominant type of interactions employed and these forces are relatively strong, even only 1 v/v% variation of the polar modiher in the mobile phase usually results in a signihcant shift in the analyte retention. 

Detailed discussion of normal-phase chromatography process, mechanism, and retention theories, as well as types and properties of used stationary phases, is given in Chapter 5. 

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