Investigation of uranium and various ligand complexes in the gas phase using electrospray ionization ion trap mass spectrometry

Abstract

The speciation and reactivity of uranium is a topic of sustained interest because species dependent chemistry controls processes ranging from nuclear fuel processing to mobility and fate in the geological surface. Past condensed phase studies have shown uranium a wide range of oxidation states and coordinate ion numbers in the environment. These studies have also suggested the strong interactions such as charge transfer between the uranium cations and solvent molecules cause the latter to behave like equatorial ligands. Studying and understanding intrinsic uranium chemistry is challenging because it is difficult to gain explicit control over the interactions of solvent and non-solvent ligands with uranyl ions in the condensed phase. An attractive alternative, therefore, is to monitor reaction in the gas phase (i.e. a solution free environment) in order to gain control over the chemical species chosen for study and the specific neutral reagents. Recent studies shown that ion-trap mass spectrometry can be applied to the study of intrinsic metal and metal-complex chemistry by generating the metal complexes as ions through electrospray ionization ESI and allowing species to interact with neutral reagents present in the collision gas. Throughout these series of uranyl studies, ESI is used to produce gas-phase ions from solutions containing uranyl nitrate complexes in deionized water. Several studies were conducted to monitor uranium and ligands behavior under different systems by controlling factors such as the numbers of ligands attached to the uranium dioxo cation center, ligand degrees of freedom, and ligand basicities. These studies were designed to gain clear insight to the intrinsic behavior of uranium complexes.