Serum samples were prepared by the addition of 0.5 mL acetonitrile (J.T. Baker)
to 0.2 mL serum.
The tube was mix-vortexed, centrifuged at 16,000 g for 10 min;
100 lL of the supernatant solution were diluted with 100 lL 0.01 M ammonium
formate, pH 3.5 (Sigma–Aldrich).