Aliquots of 10 mL of the solution containing four
colored compounds, 4% (v/v) Triton-100, and 18% (w/v) Na2SO4 were
shaken. After Na2SO4 was completely dissolved, the two phases were
separated by centrifugation for 10 min at 3500 rpm. The volume of the
surfactant-rich phase was usually 0.1 mL. The supernatant aqueous phase
was carefully removed with a syringe, and the surfactant-rich phase was
left in a centrifugal vial. The HPLC mobile phase was added to the
surfactant-richphase, and the final volume was 0.4 mL. The concentration
factor was 25.