Identification of Photochemically Generated Volatile Species of Ruthenium and Osmium Using Direct Analysis in Real Time Mass Spectrometry

Photochemical vapor generation (PVG) was coupled to direct analysis in real time (DART) high-resolution mass spectrometry (HRMS) using N2 as the discharge gas in an attempt to identify unknown volatile carbonyls of Ru and Os generated during the UV photolysis of HCOOH-based photochemical media previously described in the literature. Initial insights into the ambient ionization process in the N2 DART were gained using volatile W(CO)6 and Fe(CO)5, either photochemically generated or introduced as standards from a headspace. In general, significant changes in the original carbonyl structure are observed in both positive and negative ion modes, characterized by the loss of CO group(s), oxidation, hydration, and formation of various adducts derived from N2 used as the discharge gas. Nevertheless, the ions detected under PVG conditions based on dilute or concentrated HCOOH media, preferably in the presence of transition metal mediators, suggest that the generated carbonyls of Ru and Os are mononuclear, contain five carbonyl groups, and are therefore Ru(CO)5 and Os(CO)5. When Co2+ was used as a mediator, some difficulties in identification were encountered because volatile Co(CO)4H was cogenerated with significant efficiency, overloading the DART and HRMS and even resulting in mixed metal carbonyl cluster ions during ionization. Additional experiments with PVG of Os conducted under oxidative conditions using deionized water, dilute HNO3, and dilute H2O2 as the photochemical media confirmed OsO4 as the volatile species. The same volatile species was also identified as the dominant product using dilute CH3COOH with the addition of Fe2+ as a mediator, suggesting the rather oxidative nature of this medium, although some distinct carbonyl/hydrido/methyl or acetato species were also observed. The controversies are discussed as well as other peculiarities of the DART-HRMS technique for the identification of volatile metal carbonyls.