As part of a collaborative investigation of fundamental chemical processes taking place at deep-sea hydrothermal vents, a recent focus of Professor Doug Ridge's research group has been the elucidation of metal sulfide cluster formation using mass spectrometry (MS). Metal salt clusters have been observed by mass spectrometry using electrospray ionization (ESI) of metal salt solutions, but it has proved difficult to establish that the clusters reflect nucleation in solution and not electrospray processes. Recent observations in their laboratory, however, seem to be the first example of detecting clusters by ESI-MS that unambiguously reflect solution phase nucleation. In the standard ESI-MS experiment, aqueous solutions are diluted to 50% methanol to provide the surface tension required for stable ESI conditions.
The Ridge group, whose members are Jeffrey M. Spraggins, Kaitlin Papson, Una Kim and Douglas P. Ridge*, have found that the distribution of salt clusters observed in the mass spectrum of the ESI sampled solutions shifts with time following the addition of methanol to the sample solution. The effect is illustrated in the figure below. Although Cd(OAc)2 is soluble in aqueous solution, it begins to form clusters as soon as methanol is added. The methanol water mixture has a lower dielectric constant than water allowing a divalent electrolyte like Cd(OAc)2 to begin to associate. A similar phenomenon is observed in supercritical water. At high temperature and pressure the dielectric constant of water drops and it begins to behave like an organic solvent. Direct observation of association of electrolytes in supercritical water has proved difficult. The present results are therefore promising and could be useful in helping them understand the behavior of supercritical water. Undersea hydrothermal vents produce water at high temperature and pressures so that their results may actually pertain to the problem that motivated the study in the first place.
Time-dependence of relative abundances of anionic cadmium acetate clusters as a function of time following dilution of solutions with methanol. Determined by ESI-MS. (A) Sampling was done using separate aliquots for each spectrum and (B) a single aliquot was continually injected as spectra were continuously taken.
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