Urinary organic acid screening by solid-phase microextraction of the methyl esters

We developed a new sample preparation method for profiling organic acids in urine by GC or GC–MS. The method includes derivatisation of the organic acids directly in the aqueous urine using trimethyloxonium tetrafluoroborate as a methylating agent, extraction of the organic acid methyl esters from the urine by solid-phase microextraction, using a polyacrylate fiber with a thickness of 85 μm and transfer of the methyl esters into the GC or the GC–MS instrument. Desorption of the analytes takes place in the heated injection port. The proposed sample preparation is very simple. There is no need for any evaporation step and for the use of an organic solvent. The risk of contamination and the loss of analytes are minimized. The total sample preparation time prior to GC or GC–MS analysis is about 40 min, and therefore more rapid than other sample preparation procedures. The urinary organic acids are well separated by GC and 29 substances are identified by GC–MS.