Probing uranyl(VI) speciation in the presence of amidoxime ligands using electrospray ionization mass spectrometry

Abstract

This study presents a first look at the solution chemistry of uranyl(VI)-amidoxime complexes using electrospray ionization mass spectrometry. The appearance of previously undescribed solution species suggests that the uranyl-amidoxime system is a rich and relatively complex one, requiring a more in-depth investigation.

Gas‐phase complexes formed between amidoxime ligands and vanadium or iron investigated using electrospray ionization mass spectrometry

“…Peak assignments are given in Table . A pH of 5.5 was selected to make our results easier to compare with those of our earlier investigation of uranyl(VI)–DHIP interactions . At a 1:1 vanadium(V):DHIP mole ratio, a peak that can be assigned to the 1:1 vanadium(V)‐DHIP complex, [VO 2 +DHIP] + , can be seen at m / z 226.2 (Fig.…”

“…In an earlier study using ESI‐MS to study gas‐phase uranyl(VI)–DHIP complexes, 1:1, 1:2 and 2:3 uranyl(VI)–DHIP complexes were detected when the uranyl(VI):DHIP mole ratio in a pH 5.5 solution was 1:5. At this same mole ratio, only 1:2 metal–ligand complexes of either vanadium(V) or iron(III) with DHIP were detected.…”

“…Amidoxime groups on the amidoxime‐functionalized sorbents, which can be modeled using DHIP and DHPD, readily form complexes with both vanadium(V) and iron(III) as well as with uranyl(VI). In an earlier work we characterized gas‐phase uranyl(VI)–DHIP and uranyl(VI)–DHPD species, and are interested here in comparing the ions arising from the vanadium(V) and iron(III) species formed with DHIP and DHPD to those of the uranyl(VI) system. First, we discuss the effects of changing metal:ligand mole ratio and pH on the mass spectra acquired from solutions containing vanadium(V) and each ligand in order to gain insight into the number and types of vanadium(V) complexes that can form with DHIP or DHPD.…”

“…This is in contrast to the iron(III)/DHIP system, where the relative abundance of the only iron(III) complex formed, [Fe+2(DHIP–H)] + ( m / z 340.1), was unaffected by increases in the DHIP concentration. Similarly to the uranyl(VI)/DHPD system, iron(III) appears to form gas‐phase complexes with DHPD less effectively than with DHIP.…”

“…Vanadium(V) has also been reported to form strong complexes with amidoxime ligands, and we expected vanadium(V) to be stable under the conditions used in our study (aqueous solutions exposed to atmospheric oxygen). We have used ESI‐MS previously to examine the gas‐phase interactions between DHIP, DHPD, and uranyl(VI) . Although there is debate as to whether the gas‐phase ions observed are a true representation of the solution species due to solution perturbations that occur during the ESI process, ESI‐MS is known as a fast and accurate method of determining the stoichiometries of metal–ligand complexes and examining their chemical interactions in the gas phase, We believe that ESI‐MS results, carefully interpreted, can provide certain insights into the types of complexes that may exist in solution.…”