Quantification of urine organic acids (UOA) through atmospheric pressure chemical ionization gas chromatography mass spectrometry (APCI-GC-MS)

Session: Advances in Mass Spectrometry:  

IEMs (inborn errors of metabolism) cause significant morbidity and mortality when left untreated. A key diagnostic tool when assessing patients for IEMs is urine organic acid (UOA) analysis. The current gold standard for clinical UOA testing utilizes GC-EI-MS (Gas Chromatography-Electron Impact-Mass Spectroscopy). The Waters’ Xevo TQ-S MS in combination with the Agilent 7890 GC allows for an easily interchangeable liquid chromatography (LC)-ESI (Electrospray Ionization) and GC-APCI (Atmospheric Pressure Chemical Ionization) instrument set-up, while maintaining accuracy and sensitivity in both LC and GC applications. Patient urine samples, QC materials, and other test samples were spiked with the internal standard tropic acid, acids were extracted via a liquid-liquid ether extraction, reacted with BSTFA to form trimethylsilyl derivatives, and then run in full scan mode. Initial instrument suitability and method setup was tested and compared on both the current GC-MS and the new APCI-GC-MS. Individual standards were run to determine retention times and spectra. Based on the moisture levels in the source, wet and dry conditions could induce either a protonation mechanism ([M+H]⁺) or a charge transfer ([M+°]⁺), respectively. Wet/dry source condition experiments were conducted on a 1:1 solution of 4-bromo-diphenyl-ether and phenanthrene in hexane to determine source enclosure moisture levels through [M+H]⁺ and [M+°]⁺ peak ratio comparisons. Initial patient sample experiments achieved adequate separation of isobaric compounds. Following the swap over of the instrument from LC-ESI to APCI-GC mode, wet/dry tests concluded that the stabilization of the source conditions required 6 hours to achieve dry mode ionization. The Waters’ Xevo TQ-S micro with Agilent 7890 GC demonstrated promising GC-resolution separation of UOA metabolites using APCI-MS detection. Before clinical use, increased sensitivity and a UOA mass spectra reference library specific for APCI would be needed. The former could be improved using MS-MS in multiple reaction monitoring mode, thus opening another avenue of research.