Figure 2a: Comparison of A) 50 mM ammonium acetate, B) 0.1% formic acid and C) 50 mM ammonium formate as buffers for retention of creatinine in male urine using ZIC-HILIC column (2.1 mm × 150 mm × 5 μm) at flow rate of 0.2 mL min-1. The gradient program is described in Table 1.