Analysis of tert.-butyldimethylsilyl [1- 13C]palmitic acid in stool samples by gas chromatography–mass spectrometry with electron impact ionisation: comparison with combustion isotope-ratio mass spectrometry

Abstract

The use of 13C-labelled compounds to study lipid metabolism is increasing. Typically less than 40% of the orally administered label is recovered in breath CO 2. The remainder must be either absorbed and not oxidised or not absorbed and remain in the faeces. Two methods of determining how much tracer passes through the body, and is present in the stool, were compared. Compound specific analysis of tert.-butyldimethylsilyl [ 13C]hexadecanoic acid by gas chromatography–mass spectrometry (GC–MS) with electron impact ionisation was compared with bulk analysis of whole stool and lipid extract by continuous flow isotope ratio mass spectrometry (CF–IRMS) with a combustion interface. The mean difference between the IRMS and GC–MS methods was −0.02 mmol 13C d −1 with a mean excretion of 14.2 mmol 13C d −1. Combustion IRMS is both simpler and cheaper, when the objective is to determine how much administered dose appears in stool, and information about the form of the label is not required.