Galactonate determination in urine by stable isotope dilution gas chromatography–mass spectrometry

A stable isotope dilution assay was developed for the sensitive determination of d-galactonic acid. d-[U-13C6]galactono-1,4-lactone was prepared as internal standard. Unlabelled and U-13C-labelled d-galactonic acid species were converted to the N-(1-butyl)galactonamide pentaacetate derivatives and assessed by gas chromatography–mass spectrometry (GC–MS). Positive chemical ionisation and monitoring of the [MH-60]+-ions in the galactonate chromatographic peak at m/z 402 and m/z 408 were used for quantification. The procedure was applied to study the variability of d-galactonate excretion in healthy subjects and galactosemic patients and to monitor the d-galactonate–d-galactitol ratio in human urine.