Travelling wave ion mobility mass spectrometry of 5-aminolaevulinic acid, porphobilinogen and porphyrins

RATIONALE: Human porphyrias, diseases caused by enzyme defects in haem biosynthesis, are characterised by the excessive production, accumulation and excretion of porphyrins and/or 5-aminolaevulinic acid (ALA) and porphobilinogen (PBG). A method for the simultaneous separation, detection and identification of ALA, PBG and porphyrins would greatly facilitate the screening and diagnosis of porphyrias. Such a method would also be invaluable for the biochemical study of the haem, chlorophyll and corrin pathways.

METHODS: An aqueous mixture containing ALA, PBG and type I isomer porphyrins was diluted with acetonitrile and infused (10 mu L/min) into a Waters Synapt G2 high-definition mass spectrometer, equipped with a Z-Spray electrospray ionisation (ESI) source. Mass spectra were acquired in positive ionisation mode and the optimised ion mobility spectrometry (IMS) conditions were as follows: IMS wave height (V), 40; IMS wave velocity (m/s), 648; IMS gas flow (mL/min) 90.40; helium gas flow (mL/min), 182.60.

RESULTS: The IMS drift-time increased with increasing ion mass in the order of ALA, PBG, mesoporphyrin, coproporphyrin I, penta-, hexa-and heptacarboxylic acid porphyrin I and uroporphyrin I. The ESI-IMS-MS spectra shows that PBG could form two different positively charged ions by protonation [M + H](+), m/z 227, or deprotonation [M - H](+), m/z 225. The protonated PBG (m/z 227) easily eliminated ammonia in source and the fragment ion (m/z 210) was monitored instead. Doubly charged ions of porphyrins having different drift times from the protonated singly charged molecules were observed in high abundance, providing further structural characterisation.

CONCLUSIONS: We have shown, for the first time, an analytical method capable of simultaneously separating haem biosynthetic intermediates and metabolites, for a potential rapid clinical screeningmethod for the porphyrias. IMS-MS allowed the separation of doubly charged porphyrin ions, which will be advantageous for the analysis of natural and synthetic tetrapyrrole compounds, while reducing the misinterpretation of contaminants.