Development of an automatic pH-adjustment system for solid phase extraction prior to the determination of REEs in seawater by ICP-MS

Abstract

An automatic pH adjustment instrument was developed for solid phase extraction (SPE) of rare earth elements (REEs) using chelating resins. The automation of pH adjustment was carried out by using a spectrometer, an input/output (I/O) board, an automatic-switching valve, and a personal computer with LabVIEW® programming platform. Contactless monitoring of the pH was carried out by measuring the intensity of transmitted light at 550 nm that passed through a sample solution with an added pH-indicator, i.e. methyl red. A reagent for adjusting the pH, i.e. an aqueous ammonia solution, was sprayed into the sample solution using a nebuliser, which permitted the addition of a small amount of the reagent and the precise adjustment of the pH. The pH value of the sample was monitored by measuring the intensity of the transmitted light using the spectrometer, while the open-time of the valve to add the reagent was automatically controlled simultaneously with the measurement of the transmitted light. The precision of the pH adjustment was tested and confirmed by using a seawater sample. The blank values and detection limits of REEs were given as the analytical figures of merits. The usefulness of the present method was confirmed by analysing seawater samples from the Nikkawa Beach, the Kashima Bay, and the Ishigaki Island. The results of REEs in these samples were discussed with shale normalised REE distribution pattern.