Identification of Isomeric Metabolites Using Cold Ion Spectroscopy Add-on to Orbitrap-Based Mass Spectrometer

The identification of chemical structures of metabolites available in trace-level quantities remains a challenging task in analytical chemistry. Although cold ion spectroscopy (CIS) has considerable potential to assist in such identification, the widespread use of the method is currently hindered by the lack of suitable commercial instrumentation. In this study, we present a compact CIS add-on module, which was noninvasively interfaced to an Orbitrap-based host mass spectrometer (MS). This instrument was employed to demonstrate how, in certain cases, high-resolution UV-MS photofragmentation CIS allows for the identification of the chemical structure of a metabolite from a pool of rationally predicted candidates. We show that MS detection of specific "prompt" photofragments indicates the attachment of the associated labile functional group to the chromophores of a metabolite, while a comparison of the UV spectrum of the metabolite with the spectra of chromophore standards determines the exact location of the group. As a proof of principle, we successfully applied this workflow to the blind structural identification of a metabolite of a prospective drug.