Why are these two organic solvents used in reserve phase mobile phase?
In chromatography, acetonitrile (MeCN) is generally considered a more polar, aprotic solvent than methanol (MeOH). Acetonitrile’s polarity is due to the strong dipole moment of the C≡N bond, while methanol’s polarity stems from its hydroxyl group (OH) and ability to form hydrogen bonds. This difference in polarity affects their elution strength and suitability for different chromatographic techniques, particularly reversed-phase HPLC.
Acetonitrile is a polar aprotic solvent, meaning it has a strong dipole moment but cannot form hydrogen bonds with other molecules. This characteristic leads to a higher elution strength in reverse-phase chromatography, as it can effectively displace hydrophobic compounds from the stationary phase.
Methanol is a polar protic solvent, meaning it can both be a hydrogen bond donor and acceptor due to its hydroxyl group. This ability to form hydrogen bonds affects its interaction with both the stationary phase and the analyte, influencing its retention and elution behavior.
Acetonitrile is often preferred over methanol in reverse-phase HPLC due to its higher elution strength and ability to promote strong π-π interactions with the stationary phase, especially when using phenyl or aromatic columns.
Acetonitrile’s higher polarity and aprotic nature give it a greater elution strength in reversed-phase chromatography, while methanol’s protic nature and ability to form hydrogen bonds influence its interactions with both the stationary phase and the analyte.
Is this statement at the end accurate? Does it displace hydrophobic compounds to or from the stationary phase?