Capillary gas chromatography Negative ion mass spectrometry Plasma Estrone, equilin Conjugated estrogens A procedure is described for the analysis of the estrogens equilin and estrone in human plasma following oral administration of conjugated estrogen preparations. After enzymatic hydrolysis of the sulfate conjugates, plasma proteins are precipitated with methanol and the estrogens extracted into ethyl acetate. Derivatization with the reagent flophemesylamine converts equilin and estrone into volatile pentafluorophenyldimethylsilyl ethers ideally suited to capillary gas chromatography-negative ion chemical ionization mass spectrometry. Using a 15 meter dimethyl silicone bonded phase fused silica capillary column separation of the estrone and equilin derivatives is achieved within 9 minutes. Selected ion monitoring of the intense negative molecular ions enables levels of 1 ng.ml-' to be measured with coefficients of variation of 9.3 %and 14.2 % for estrone and equilin respectively. Plasma levels of the compounds are reported in two male volunteers up to 24 hours after dosing with 5 milligrams of PremarinTM. (TM Ayerst Laboratories Inc., New York, USA.) Conjugated estrogen preparations have been in use for many years as replacement therapy for the treatment of symptoms associated with menopause [l]. The pharmaceutical product is formulated from extracts of pregnant mares' urine and is a mixture of the sulfate esters of the closely related compounds estrone, equilin, equilenin, 17 a-estradiol, 17 B-estradiol, 17 a-dihydroequilin, 17 0-dihydroequilin, 17 a-dihydroequilenin, and 17 P-dihydroequilenin [2,3].The relative amounts of each compound present in the formulation can vary according to the time of collection of the mares' urine. However, the major constituents as defined by the United States Pharmacopeia [4] are sodium estrone sulfate (50% -60%) and sodium equilin sulfate (22.5% -32.5%). Other preparations known as esterified estrogens contain different proportions of the individual estrogens but still with the estrone and equilin esters predominant [4,5].