Basic Study of Lanthanide Nitrate Species in Solution by Electrospray Ionization Mass Spectrometry

Lanthanide nitrate (Ln(NO3)3) solutions were analyzed by electrospray ionization mass spectrometry (ESI-MS) to characterize the solution states of the lanthanides. The following monomer species were observed: [Ln(OH)(H2O)j]2+, [Ln(OH)2(H2O)k]+, [Ln(NO3)(OH)(H2O)l]+ and [Ln(NO3)2(H2O)m]+ (j,k,l,m: numbers of adducted H2O). The peak intensity ratio of each Ln species was calculated from the peak intensity of the Ln species divided by the total peak intensity of all the Ln species. The change in the relative peak intensities of [Ln(OH)(H2O)j]2+ and [Ln(OH)2(H2O)k]+ was consistent with changes in the hydration number of Ln (La to Tb: 9, Tb to Lu: 8). The behavior of the relative peak intensity of [Ln(NO3)(OH)(H2O)l]+ against the atomic number of Ln was similar to those of the stability constants of the lanthanides and the nitrate group. ESI-MS is expected to be a useful technique for examining lanthanide reactions in solution.