Abstract: Baseline noise in high-pressure gradient elution caused by impurities in weak aqueous solvent (solvent A) was improved with an on-line cleanup precolumn placed between the solvent pump A (weak solvent) and the mobile phase mixer. The procedure was implemented by using a sixport valve. With the various TEAP, H₂O/ACN, MeOH combinations examined, results obtained in the work indicate that the cleanup method is sufficiently effective and reproducible.