Elucidation of the collision induced dissociation pathways of water and alcohol coordinated complexes containing the uranyl cation

Abstract

Multiple-stage tandem mass spectrometry was used to characterize the dissociation pathways for complexes composed of (1) the uranyl ion, (2) nitrate or hydroxide, and (3) water or alcohol. The complex ions were derived from electrospray ionization (ESI) ϩ . The abundance of the species was greater than expected based on previous experimental measurements of the (slow) hydration rate for UO 2 ϩ when stored in the ion trap. To account for the production of the hydrated product, a reductive elimination reaction involving reactive collisions with water in the ion trap is proposed. T he speciation and reactivity of uranium is a topic of sustained interest because species-dependent chemistry [1] controls processes ranging from nuclear fuel processing [2] to mobility and fate in the geologic subsurface [3,4]. The solution chemistry of uranium is dominated by the uranyl dication, UO 2 2ϩ , which is known to form complexes with a range of ligands [1]. Specific interaction with solvent will significantly influence the physico-chemical behavior of the uranyl ion and its complexes, and this has motivated investigations of complex composition and stability using infrared spectroscopy and extended X-ray absorption fine structure [5][6][7][8][9][10][11]. Unfortunately, explicit control over the interactions of solvent and nonsolvent ligands with the uranyl ion is difficult, which makes the study of species-dependent uranium behavior complicated. To gain a better understanding of the intrinsic interactions between different uranyl species and solvent, we have begun an investigation of uranyl-anion complexes in the gas phase using ion-trap mass spectrometry (IT-MS). Several recent reports have demonstrated that intrinsic metal and metal complex chemistry can be investigated by the (controlled) addition of reagent gas to an ion trap [12][13][14][15][16][17][18][19][20][21][22], or by way of the presence of H 2 O and other small molecule contaminants within the He bath gas used to collisionally cool ions and improve trapping efficiency [23][24][25]. The reactions of uranium ions in the gas phase have been the subject of several earlier investigations. Studies by , and by Schwarz and coworkers [30] have probed the reactions between U ϩ and UO ϩ and organic compounds such as alkanes and alcohols. Armentrout and Beauchamp [31] investigated the oxidation of U ϩ using small molecules such as O 2 , CO, CO 2 , COS, and

Ions generated from uranyl nitrate solutions by electrospray ionization (ESI) and detected with fourier transform ion-cyclotron resonance (FT-ICR) mass spectrometry

“…Interestingly, as shown by the data in Table 2, for ions containing two, three or four uranyl centers, coordinative saturation is achieved with fewer ligands per uranyl center than observed previously for ions with a single uranyl center [18]. This observation clearly supports the presence of bridging ligands in ions containing multiple uranyl centers that are not possible in ions containing a single uranyl center.…”

“…Ions used for calibration are marked with an asterisk in the tables. [18,20,21] and in Chien et al [19], but also more complex positively charged oligomeric species at higher m/z, as well as a number of simple, nitrate-containing, negatively-charged species.…”

“…Given the lack of available information on the solvation of UO 2 2ϩ in dilute solutions containing high concentrations of alcohols, it is difficult to say whether any of the detected ions are actual solution-phase species or if they are simply formed during the electrospray process. The MS/MS fragmentation patterns of the low mass species (Ͻ500 Da) in Figure 1a and b have been previously discussed [18]. The fragmentation patterns of the higher mass species shown in Figure 1c are discussed below.…”

“…The use of a lower operating frequency (965 kHz) with a longer quadrupole trapping pulse length (3.167 ms) results in the detection of more complex adducts (Figure 1b). Here, a hexapole trapping time of 400 ms was used together with a two-step filament trapping in which the plate potential was first decreased from 10 to 4 V and then to 0 V. [18]. Still more complex species were detected with the use of the lower frequency coil (965 kHz), a significantly longer quadrupole trapping pulse length of 3.800 ms, and a hexapole trapping time of 1000 ms ( Figure 1c).…”

Intrinsic hydration of monopositive uranyl hydroxide, nitrate, and acetate cations

“…Recent studies have shown that ion-trap mass spectrometry can be used to probe intrinsic metal and metal-complex chemistry by exposing metal species to reagent gases deliberately added to the ion trap [9 -20], or to adventitious H 2 O present in the He bath gas used to collisionally cool ions and improve trapping efficiency [21][22][23][24]. Further motivating the present study is the fact that ion traps will shortly be used to characterize actinide speciation in samples from radioactive waste disposal sites, and a concise understanding of actinide behavior will be critical for correctly interpreting environmental data [25].We recently demonstrated that electrospray ionization (ESI) and multiple stage tandem mass spectrometry (MS) can be used to generate and characterize "solvated", gas-phase complexes featuring the uranyl ion coordinated by hydroxide, nitrate or a series of alkoxides [26]. In the pilot study reported here, ESI was used to produce gas-phase ions from solutions containing uranyl nitrate or uranyl acetate in deionized H 2 O.…”