Acetonitrile to which acid is added in commonly used as a solvent in HPLC analyses. In particular, the widespread use of LC/MS has led to an increase in the number of cases that use acetic acid and formic acid. These products consist of acetonitrile for LC/MS to which high-purity acid has been added, respectively. These solutions meet the specifications for UV and fluorescent substance and are thus ideal as solvents for HPLC analyses.