GC/MS confirmatory method for etorphine in horse urine.

A highly sensitive procedure for GC/MS determination of etorphine in horse urine is described. This assay provides both specificity and reliability and is particularly well suited for the confirmation of radioimmunoassay screening procedures usually used for etorphine. After solvent extraction and purifications, the etorphine is characterized as a pentafluoroacetic derivative (PFAA) by using mass fragmentography. The detection limit is 0.1 ng/mL in urine; the coefficient of variation of the estimations is 10.9%. The procedure has been validated after on-field administration of 5 to 90 micrograms of etorphine to five thoroughbred horses (10 to 180 ng/kg).