Internal standards, (i.e. 0.2 mg/ml

Internal standards, (i.e. 0.2 mg/ml ribitol in water, 1 mg/ml ampicillin
in water and 1 mg/ml corticosterone in methanol), were
added into the extraction mixture to give a concentration of
0.86 mg ml
1
, 5.6 mg ml
1
, 7.16 mg ml
1
, respectively in the final
injection. The mixture was then briefly vortexed, centrifuged for
2 min at 14,000 RPM (microcentrifuge 5417R) and the supernatant
was decanted into the new tubes