DL-mevalonolactone is neutral and therefore hydrophobic interaction is the dominant interaction between solute and stationary phase. Because the ODS ligand density is similar for both columns, retention time for DL-mevalonolactone is similar on both columns. In contrast, DL-mevalonic acid is ionized under neutral pH conditions. It elutes early on UK-C18, but is better retained on SM-C18, due to IEX interaction between solute and stationary phase.

Scherzo SM-C18 consists of C18 + anion + cation ligands. SM-C18 can be used in either RP + IEX mode or RP + NP mode. Unison UK-C18 consists of C18 plus polymeric endcapping. The ODS bonding density is similar for both columns - so the difference in elution between the two columns is mostly due to IEX interaction. Using Unison UK-C18 as a point of reference can help to better understand results obtained on Scherzo SM-C18.