Editing High-performance liquid chromatography (section)

===Normal–phase chromatography===

Normal–phase chromatography was one of the first kinds of HPLC that chemists developed, but has decreased in use over the last decades. Also known as normal-phase HPLC (NP-HPLC), this method separates analytes based on their affinity for a polar stationary surface such as silica; hence it is based on analyte ability to engage in polar interactions (such as [[hydrogen-bonding]] or [[dipole-dipole]] type of interactions) with the sorbent surface. NP-HPLC uses a non-polar, non-aqueous mobile phase (''e.g.'', [[chloroform]]), and works effectively for separating analytes readily soluble in non-polar solvents. The analyte associates with and is retained by the polar stationary phase. Adsorption strengths increase with increased analyte polarity. The interaction strength depends not only on the functional groups present in the structure of the analyte molecule, but also on [[Steric effects|steric factors]]. The effect of steric hindrance on interaction strength allows this method to resolve (separate) [[structural isomer]]s.{{citation needed|date=July 2024}}

The use of more polar solvents in the mobile phase will decrease the retention time of analytes, whereas more hydrophobic solvents tend to induce slower elution (increased retention times). Very polar solvents such as traces of water in the mobile phase tend to adsorb to the solid surface of the stationary phase forming a stationary bound (water) layer which is considered to play an active role in retention. This behavior is somewhat peculiar to normal phase chromatography because it is governed almost exclusively by an adsorptive mechanism (''i.e.'', analytes interact with a solid surface rather than with the solvated layer of a ligand attached to the sorbent surface; see also reversed-phase HPLC below). Adsorption chromatography is still somewhat used for structural isomer separations in both column and thin-layer chromatography formats on activated (dried) silica or alumina supports.{{citation needed|date=July 2024}}

Partition- and NP-HPLC fell out of favor in the 1970s with the development of [[reversed-phase chromatography|reversed-phase]] HPLC because of poor reproducibility of retention times due to the presence of a water or protic organic solvent layer on the surface of the silica or [[alumina]] chromatographic media. This layer changes with any changes in the composition of the mobile phase (''e.g.'', moisture level) causing drifting retention times.{{citation needed|date=July 2024}}

Recently, partition chromatography has become popular again with the development of [[Hilic]] bonded phases which demonstrate improved reproducibility, and due to a better understanding of the range of usefulness of the technique.