First application of the depth profile of silica species as a tracer by fast‐atom bombardment mass spectrometry: investigation of the circulation of seawater and silica uptake by diatoms

Silica, represented by SiO₂, is the general name for the compounds composed of Si, O and H with their derivative complexes. Silica forms various chemical species in aquatic solutions, such as a monomer (Si(OH)₃O⁻), dimer (Si₂(OH)₅O), and others. These species are known to vary in their relative abundances in solution depending on the chemical and physical conditions. Silica species dissolved in seawater have been examined by fast-atom bombardment mass spectrometry (FAB-MS) to elucidate the behavior of silica and its circulation as a novel tracer reflecting the chemical and physical conditions of seawater and the bioactivity of diatoms, which take up silica. In the seawater of Tokyo Bay, silica species such as Si(OH)₂O₂Na]⁻ (monomer–Na]⁻), Si₂(OH)₅O₂]⁻ (dimer]⁻), Si₂(OH)₄O₃Na]⁻ (dimer–Na]⁻), Si₄(OH)₇O₅]⁻ (cyclic tetramer]⁻), Si₄(OH)₆O₆Na]⁻ (cyclic tetramer–Na]⁻), Si₄(OH)₉O₄]⁻ (linear tetramer]⁻) and Si₄(OH)₈O₅Na]⁻ (linear tetramer–Na]⁻) were observed and assigned by FAB-MS. To investigate the suitability of silica species as a tracer, the relative peak intensity ratios of silica species observed in the mass spectra, i.e. the profiles of the ratio of the linear tetramer to the cyclic tetramer (m/z 329/311) and the ratio of the dimer to the cyclic tetramer (m/z 173/311) against depth, were examined to determine the annual changes and reproducibility of the depth profiles. In particular, the depth profile of the relative ratio of the linear tetramer to the cyclic tetramer, 329/311, exhibits critical changes depending on the seawater budget. These changes in the relative ratios were identified by an experiment involving a simple sodium chloride solution system. Our measurement is expected to elucidate the dynamics of silica and its role as ‘food’ for diatoms, and we showed that speciation using mass spectrometry is a powerful tool for examining elemental behavior in nature and environmental changes. Our results suggest that a silica tracer is useful for investigating the behavior of seawater in small coastal regions and the uptake of silica by diatoms. Copyright © 2009 John Wiley & Sons, Ltd.