Fig. 1. The chromatogram of standards using 99% 10.0 mmol/L ammonium formate solution at pH 3.6 and 1.0% methanol as mobile phase on a Waters Atlantis T3 (4.6 mm 250.0 mm 5.0lm) column at a flow rate of 1.0 mL/min and UV detection at 254 nm. Peak identification: r guanine; s adenine; t hypoxanthine; u xanthine.