Effect of Organic Modifier on the Retention of Low-Molecular-Weight Organic Compounds in Low-Temperature HPLC Using a Liquid CO₂ Mobile Phase and an Octadecyl Stationary Phase

Abstract

The retention behaviors of various low-molecular-weight compounds in low-temperature HPLC with an ODS (octadecyl silica) column and liquid CO₂ mobile phase containing methanol or acetonitrile as modifiers were investigated. The addition of methanol/acetonitrile reduced the retention of xanthone, which was dominated by the liquid CO₂ content, independent of the modifier. Additionally, the retention of polycyclic aromatic hydrocarbons (PAHs) was reduced by the addition of methanol/acetonitrile. Because acetonitrile resulted in a rapid reduction in the retention compared with methanol, it was suggested that the CH- interactions have an important role in the retention of PAHs in the C18 stationary phase. Notably, the retention of alkylbenzenes were only slightly affected by the addition of methanol/acetonitrile. Acetonitrile slightly increased the retention of alkylbenzenes with longer alkyl chains than those with shorter alkyl chains. It is suggested that the interaction of the phenyl moiety in alkylbenzenes with C18 chains was reduced by the increase in the acetonitrile content, and enhanced in the case of the alkyl chain moieties. The retentions of alkyl benzenes were approximately compensated, when the alkyl chain length was relatively short.