Topics - Acetonitrile

Acetonitrile Acetonitrile is basically a polar solvent, miscible with water but, nevertheless, has sufficient dispersive (hydrophobic) properties to elute substances from a liquid chromatography column by dispersive interactions with the solute. Acetonitrile is miscible with water in all proportions so, starting with pure water as the mobile phase, by progressively adding acetonitrile to the water, the mobile phase can be made progressively more dispersive in character and progressively elute more dispersive substances. Acetonitrile does not associate strongly with water and thus, as opposed to methanol, acetonitrile-water mixtures remain binary in character. This simplifies the interactive theory and allows a more simple prediction of retention based on the acetonitrile concentration. Methanol on the other hand, forms a strong associate with water so that at high concentrations of water, the mobile phase behaves as a binary mixture of water and water-methanol associate. At high concentrations of methanol, the converse applies; the mobile phase consists of a mixture of methanol and water-methanol associate. Between these extremes, i.e. between 25 % of methanol and 75% of methanol the mobile phase consists of a complex ternery mixture of methanol, water-methanol associate and water. As opposed to acetonitrile-water mixtures the complex nature of methanol-water mixtures makes solute retention more difficult to predict from the original methanol content of the mobile phase