Milk sample (4.0 mL) was added into 2.5 mL of 3% (v/v) acetic acid, mixed well and stood for 15 min, diluted to 10.0 mL with water, mixed, and then centrifuged for 10 min at 1500 rpm. If clarification was not achieved, extra drops of glacial acetic acid were added. An aliquot of the supernatant was filtered through a 0.45 lm nylon filter membrane. Samples of orange juice were prepared by freshly squeezing orange fruit in an all glass assembly setup. The injection volume was 20 lL.