A study of two ion interaction reagents (IIRs) viz. n-octadecane sulphonate (C₁₈-sulphonate) and eicosyl sulphate (C₂₀-sulphate) was carried out for the separation of lanthanides by reversed-phase high performance liquid chromatography (RP–HPLC). The objective of the study was to identify a suitable IIR offering long term adsorption onto the RP column, thereby obviating the need to introduce the IIR in the mobile phase during the separation of lanthanides. This avoids the rigorous treatment of purified fractions before their mass spectrometric analysis. Resolution was used for comparing different IIRs for separation of lanthanides under identical chromatographic conditions, employing a-hydroxy isobutyric acid (a-HIBA) as an eluent. The volume and composition of IIR solution required for the modification of the column, as well as their long term adsorption, were studied. Columns coated with C₁₈-sulphonate and C₂₀-sulphate allowed the separation of lanthanides without introducing the IIR in the mobile phase. Between these two IIRs, C₁₈-sulphonate offered higher resolution and provided good long term adsorption stability. A RP column modified with C₁₈-sulphonate, as per the optimised procedure, was used for the separation of lanthanides from a geological reference sample without the need to include IIR into the mobile phase.