Analysis of bile acids and bile alcohols in urine by capillary column liquid chromatography-mass spectrometry using fast atom bombardment or electrospray ionization and collision-induced dissociation

Abstract

Solid-phase extraction and group separation by anion exchange chromatography were combined with capillary column liquid chromatography-mass spectrometry (LC/MS) to permit a thorough characterization of bile acids and intact conjugates of bile alcohols in human urine. Groups of compounds were separated according to acid strength and were analysed on a capillary column, 0.25 x 500 mm, packed with 5 microns particles of Chromasil C18, and connected via a fused silica capillary to the continuous-flow fast atom bombardment (CF-FAB) or electrospray (ES) sources of an AutoSpec-TOFFPD hybrid mass spectrometer. Acetonitrile:water mixtures containing 30 mM ammonium acetate pH 7.2 were used as mobile phases, with 5% glycerol added for FAB Ionisation. Bile acids were analysed directly or after derivatization of carboxyl groups with 4-aminobenzenesulphonic acid. Negative-ion spectra (m/z 1000 or 800 to 300 or 100) were recorded using the point detector or, in the case of ES ionization, the focal plane array detector (FPD). Deprotonated molecules of bile acids containing a sulphonic acid group were detected with a spectral signal to noise ratio of 5:1 when about 90 fmol were injected onto the column of the LC/CF-FAB system. The corresponding peak in the reconstructed ion chromatogram gave a signal-to-noise ratio of about 25:1. The sensitivity could be increased 20-50 times by using ES ionization and the FPD. Bile acids without a sulphonic acid group gave about 70% of the signal of sulphonic acids using ES ionization. The capillary column LC/MS systems were evaluated by analyses of urine from an infant with cholestatic liver disease. More than 150 different bile acids and bile alcohol conjugates were detected, some of which were partially characterized using collision induced dissociation (CID) of the deprotonated molecules and B/E linked scans. A number of compounds were detected for the first time, e.g. di-, tri-, and tetra-hydroxycholestanoic acids conjugated with N-acetylhexosamine and cholestenediol, cholestenetriol and cholestanetriol doubly conjugated with sulphuric acid and glucuronic acid. The relative merits of ES and FAB ionization are discussed.