Determination of rare earth elements in seawater samples by inductively coupled plasma tandem quadrupole mass spectrometry after coprecipitation with magnesium hydroxide

Coprecipitation with magnesium hydroxide was investigated as a pretreatment method for the determination of rare earth elements (REEs) in seawater samples, both for separation of the salt contents and for the enrichment of REEs. The precipitate of magnesium hydroxide was collected on a syringe filter and then subjected to the on-line measurement of REEs by an inductively coupled plasma tandem quadrupole mass spectrometer (ICP-MS/MS). Coprecipitation resulted in a separating rate of over 99% for salt contents, while the on-line measurement of REEs in the magnesium hydroxide provided an enhancement factor of 130-fold for the transient peak height of signal intensities. Mass-shift mode was applied to the measurement of REEs by ICP-MS/MS, where each isotope of the REEs was permitted to pass the first QMS and to react with oxygen gas in the reaction cell so as to form a monoxide ion, which was permitted to pass the second QMS and then measured by the detector. Mass-shift to monoxide ion effectively removed the spectral interferences in the measurement of REEs and permitted the correct determination of REEs in seawater samples. Detection limits were calculated based on the results of 10-set of the replicated blank test, which were low enough for the determination of REEs in natural seawater samples. The effectiveness of the present method was confirmed by the analysis of three certified reference materials of seawater, i.e. CASS-4, CASS-5, and NASS-6.