Studies on steroids. CCLIV. Gas chromatographic-mass spectrometric determination of 4- and 6-hydroxylated bile acids in human urine with negative ion chemical ionization detection.

A method for the determination of 4- and 6-hydroxylated bile acids with a vicinal glycol moiety in human urine by gas chromatography-mass spectrometry with negative ion chemical ionization detection is described. The 4 beta-hydroxylated bile acids labelled with 18O and 2H were prepared as internal standards by the exchange reaction of the carbonyl group on the steroid nucleus with H2(18)O, followed by metal deuteride reduction. Bile acids in urine were extracted with a Sep-Pak C18 cartridge and, after solvolysis and hydrolysis of the conjugated forms, transformed into pentafluorobenzyl ester diethylhydrogensilyl ether diethylsilylene derivatives. Subsequent resolution into individual 4- and 6-hydroxylated bile acids was carried out on a cross-linked fused-silica capillary column; a characteristic carboxylate anion, [M-PFB]-, was used to monitor each bile acid in the negative ion mode. The newly developed method was applied to the separation and determination of 4- and 6-hydroxylated bile acids in a urine specimen from a newborn infant.