Correlation of ESI-MS ions with concentration and pH of solution

Using a semi-quantitative approach (see Table 2-2), the relative peak intensities of ESI-MS ions were examined in correlation with the pH of beryllium solutions injected into the ESI-MS. The ESI-MS representation of the hydrolyt ic tendencies of beryllium ions with change in solution pH is shown in Figure 2-10.

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Figure 2-10 ESI-MS speciation diagram showing the pH-dependent hydrolytic trend of beryllium ions in a 2.2 x 10-3 _{mol L}-1 _{solution. (Deduced from the peak intensities of}

representative ESI-MS ions correlated to the beryllium hydroxido cores of the species in solution ignoring H2O, SO42- ions and other adducts)

Signal intensities from the illustrative ESI mass spectra at pHfeed 2.5, 4.5 and 6.0 shown in Figure 2-4 have further been summarised in Table 2-2. Data from the relative abundance of species in the mass spectra clearly confirms that the predominant species in beryllium hydrolysis is the beryllium trimer [Be3(OH)3]3+.

In acidic solutions of pH less than 3, beryllium exists as the monomeric tetraaqua coordinated dicationic species [Be(H2O)4]2+ and this is consistent with the

observation of its representative monomeric ESI-MS ion [Be(HSO4)(H2O)2]+ m/z

142 as the base peak at pHfeed 2.5 (see Figure 2-4). The higher concentration of protons at this pH also resulted in the abundance of ESI-MS ions containing the hydrogen sulfate species [HSO4]- such as [Be2OH(HSO4)2(H2O)2]+ at m/z 265 and

[Be3(OH)3(HSO4)2(H2O)]+ at m/z 290.

Upon increasing the pH of the beryllium solution, the onset of polymerisation in solution can be seen in the distribution of the ESI-MS ions at pHfeed 4.5 and 6.0. At pHfeed 4.5, the beryllium trimer is already the most dominant species in solution but is now observed in the ESI mass spectra with the sulfate

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anion as the species [Be3(OH)3SO4(H2O)3]+ at m/z 228. Figure 2-4 also reveals that

the ESI spectra of beryllium sulfate solution at pHfeed 4.5 were the most complicated of all the pHs examined. This is because the trimer [Be3(OH)3]3+ (which is

predominant at pHfeed 4.5) yielded a variety of aggregates in the gas phase. Potentiometric measurements of aqueous beryllium solution at pH6.0 have pointed out the predominance of a Be(OH)2 species alongside a decline in the trimer

[Be3(OH)3]3+ abundance in solution.20, 29 However, ESI mass spectra of the

beryllium solution at pHfeed 4.5 and 6.0 showed the trimeric ion [Be3(OH)3SO4(H2O)3]+ m/z 228 as the base peak. This may be related to the pH

decreasing during droplet evaporation such that the actual pH in the droplet (which of course cannot be controlled) will be lower than the starting pH.47 _{Nevertheless,}

since Be(OH)2 is neutral and cannot be adequately represented in the mass spectra

(except as adducts with pre-existing charged species), the trimer [Be3(OH)3]3+

remained the most abundant solution species suitably charged to yield corresponding ESI-MS ions at both pHfeed 4.5 and 6.0. Adducts which likely contained the Be(OH)2 species included [Be3(OH)3(HSO4)Be(OH)2(H2O)3]2+ m/z

136 and [Be3(OH)3(HSO4)2Be(OH)2(H2O)]+ m/z 333. The emergence of these

species and increase in their relative abundance from pHfeed 4.5 to pHfeed 6.0 supports the existence of Be(OH)2 in solution prior to precipitation, most likely as

colloidal dispersed species due to the low concentration in this study. Furthermore, at pHfeed 6.0 the relative intensity of the dimeric species declined to 16% from an intensity of 60% at pHfeed 4.5 while the monomeric species reduced to 6% from 100% at pHfeed 2.5 (Table 2). This is consistent with the formation of polynuc lear hydroxido species with increasing pH in solution up to 6.0.

The potential of ESI-MS as a sensitive technique for the detection of Be speciation is evident not only by its ability to illustrate the existence of the beryllium hydroxido species [Be5(OH)6]4+ and [Be6(OH)8]4+ but also to further provide

insightful quantitative data over their relative abundance in solution. These species are known to exist at low abundance in beryllium solutions and are often undetectable at lower concentrations.20 _{In a beryllium solution of concentration 10}- 3 _{mol L}-1_{, ESI-MS data suggest that the [Be}

6(OH)8]4+ derived species exist in about

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trimer species [Be3(OH)3]3+. The formation of polymeric hydroxido species in

beryllium hydrolysis is equally dependent on the solution concentration. At a concentration of 10-4 _{mol L}-1_{, the ratio of the signal intensity for the monomer ic,}

dimeric and trimeric ESI-MS ions is 1:14:4 showing the diminished significance of the trimeric species in solution as the beryllium concentration reduces. At concentrations of 10-6 _{mol L}-1 _{the only species expected in solution are species}

derived from the mononuclear complexes [Be(H2O)4]2+ and Be(OH)2 (the latter is

by mass spectrometry). However, the poor electrospray properties of pure water47

coupled with the low ionisation efficiency of species in this study limited their observation due to an increased signal to noise ratio that was noted at lower concentrations (<10-5 _{mol L}-1_).

While correlating the relative peak intensities of ESI-MS ions with the abundance of beryllium hydroxido species in solution, the inclusion of the beryllium oxide adducts such as [Be3(OH)3(BeO)n(H2O)n]+ revealed a better

correlation with the abundance of the trimer in solution. However, it resulted in an underestimation of monomeric species in solution which were the likely origin of the beryllium oxide adducts (BeO)n. Likewise, the shrinking of the droplets during

the electrospray process can segregate among species transferred into the gas phase depending on their solvation energies and this further distorts speciation data from ESI-MS. Unfortunately, standardisation of peak intensity is difficult to achieve because of the complex equilibria between the hydroxido species in solution and the complicated ESI-MS spectra obtained. Nevertheless, ESI results revealed an impressive representation of the trend in beryllium hydrolysis probably due to the interaction of the solution species with the sulfato ligands to yield species of similar charge density in the gas phase.