Dear Ahmad,
I would not neccessarily always use NP chromatography for polar analytes, however this can be an approach (alternative eg. cf. "HILIC").
Simplified: Reversed Phase Chromatography separates mainly by differences in the apolar backbones which interact with the apolar e.g. octadecyl-chains ("C18").
Normal Phase Chromatography however puts the stress on interactions between (polar) functional groups of analyte with polar functional groups of column material.
May I direct your attention to an excellent ressource for deepening your knowledge on chromatography? It is the publication LC-GC which is available for free. Here a chromatography expert named John W. Dolan writes very "hands-on"/practical and good to read articles, which I can definitely recommend for learning!
Good luck!
Holger
http://www.chromatographyonline.com/column-lc-troubleshooting
More than 80% of the drug molecules were in polar nature. If you analyzing polar moieties (drugs) through polar column (because of silanol (Si–O–H) bonding's in the Silica gel) non polar compounds will be eluted first and polar moieties will be eluted later, which causes longer run time for polar compounds.
So it is always better to go with non polar columns like C18 for all polar drug moieties.
In normal phase chromatography, polar molecules have longer retain time thus avoiding come out immidiately without good seperation as in reverse phase chromatography.
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