Types of partition chromatography:
Two types of partition chromatography have been recognized based on relative polarities of stationary phase and mobile phase. In normal phase LC or HPLC, stationary phase consists of highly polar water or triethyleneglycol supported on silica or alumina particles and a nonpolar mobile phase solvent such as hexane is used. In contrast, in the reversed phase chromatography, a stationary phase is nonpolar, frequent a hydrocarbon and the mobile phase is polar such as water, methanol or acetonitrile where most the polar component appears first. Perhaps three quarters of all the HPLC is currently being carried out in columns with reversed phase.
Most commonly, the R group of the siloxane in these coatings is an n-octyl (C-8 chain) or n-octadecyl (C-18 chain). With such preparations, the long chain hydrocarbon groups are aligned parallel to one another and perpendicular to the particle surface, providing a brush or bristle-like structure as illustrated in Figure. The relationship the between polarity of the sample with that of the column packing material and mobile phase is illustrated in Figure. Retention increases with the hydrophobic character of the solute samples. Generally, the lower the polarity of the mobile phase, the higher is its eluent strength. An effect of chain length of the alkyl group upon column performance is illustrated in Figure where it is observed that longer chains produce packings that are more retentive. For example, maximum sample size for a C18 packing is roughly doubles that for a C4 preparation under similar experimental conditions.