Fig. 4. HPLC chromatogram showing organic acid peaks at different retention times (min) using an ion exclusion column run at 40°C, 0.75 mL min^–1 flow rate, 10-µL sample volume, and 210 nm detection: (A) organic acid standard mixture showing small unknown ion peak; and (B) soil solution sample showing a large ion peak which may overlap with oxalic acid.