A Waters HPLC system, composed of a 600 controller and pump, a 717plus auto sampler, a 2996 photodiode array detector and Empower software, was used. The mobile phase, acetonitrile:methanol (containing 0.05 mol dm_3 ammonium acetate):dichloromethane, 75:20:5, v/v/v, containing 0.1% BHT and 0.05% triethylamine, was filtered through a 0.45 lm PTFE membrane filter and degassed in an ultrasonic bath