Glossary Of HPLC-LC Separation Terms – O

The listing should be helpful to those just starting in HPLC but it also can serve as a refresher for long-time users in the field.


Octadecylsilane: The most popular reversed phase in HPLC. Octadecylsilane phases are bonded to silica or polymeric packings. Both monomeric and polymeric phases are available. Abbreviated in column names as C18 and ODS.

Octylsilane: A popular stationary phase in reversed-phase chromatography. Usually provides slightly less retention than the more popular C18. Both monomeric and polymeric phases are available. Abbreviated in column names as C8.ODS: See octadecylsilane.

On-column detection: The column itself serves as the flow cell in HPLC or CE–CEC. Generally, the term used with fused-silica capillary applications. Outer polyimide layer is removed, an optical beam is directed through the capillary, and a measuring device such as a photomultiplier tube is located on the opposite side of the capillary.

On-line preconcentration: A precolumn is placed in front of the separation column to concentrate analytes before their separation. Different mechanisms — hydrophobic interaction, adsorption, or enzymatic reaction — may be used to retain analyte as a function of time. Then concentrated analytes are transferred to the separation column by a displacement process such as solvent elution or pH change.

Open tubular columns: Small inner diameter columns (less than 100 μm) currently being investigated for use in HPLC, supercritical fluid chromatography (SFC), and CE. Stationary phases can be bonded on the internal walls of these small columns. The most frequently used column material is fused-silica tubing. Used very little in routine HPLC or SFC but frequently in CE.

Optically active resin: Incorporation of optically active groups into an ion-exchange resin to allow separation of optically active isomers. Few commercially available resins for HPLC applications.

Organic modifier: Water-miscible organic solvent added to an aqueous mobile phase to obtain separations in reversed-phase HPLC. Common organic modifiers are acetonitrile, methanol, isopropanol, and tetrahydrofuran.

Orthogonality: Of two separation dimensions: such that the elution times in the two dimensions can be treated as statistically independent; ideally, the two dimensions should have totally different retention mechanisms (e.g. reversed phase and normal phase; ion exchange and reversed phase).

Outlet check valve: See check valve.

Overload: In preparative chromatography the overload is defined as the sample mass injected onto the column at which efficiency and resolution begins to be effected if the sample size is increased further. See also sample capacity.


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