Normal-phase liquid chromatography #
NPLC is the oldest form of column LC. It employs a polar stationary phase, such as silica. The main component of the mobile phase is a non-polar solvent, such as n-heptane. Retention is controlled by adding typically small amounts of a more-polar solvent, such as ethyl acetate or dichloromethane.
NPLC exhibits good selectivity for compounds with different functional groups. However, equilibration is slow, which complicates gradient-elution separations. Trace amounts of water may have large effects on retention and selectivity. This threatens the repeatability of NPLC separations.
Due to these disadvantages, the number of NPLC methods in use has become rather limited.
SEE MODULE 3.6.1
Hydrophilic-interaction liquid chromatography #
In HILIC the mobile phase is a water-miscible organic solvent (most often acetonitrile) that contains a small percentage of water. The water adsorbs on the surface of a polar adsorbent, creating a very polar stationary phase.
HILIC has become an LC method of choice for highly polar analytes, such as sugars, which cannot be retained in a reversed-phase liquid chromatography (RPLC) system. This makes the application domain of HILIC complementary to that of RPLC.
HILIC can be used in isocratic and gradient-elution modes.
