Retention time measurements were performed using isocratic elution with φMeOH starting at 0.8 and decreasing to 0 by a decrement of 0.1. Samples (1.0 μL) were injected into the column using an autosampler. The flow rate and column oven temperature were set at 0.4 mL/min and 40 °C, respectively. The measurement run was terminated after 2-3 h even if no peak appeared. All peaks were symmetrical and the peak top time was recorded to obtain tr.