Accurate determination of rare earth elements in small volumes of porewater from marine sediments by laser ablation solution sampling ICP-MS

Abstract

Rare earth elements (REEs) in porewater can be useful proxies in the studies of marine geochemistry. The main challenges for the determination of REEs in porewater samples from marine sediments lie in three aspects—low concentration, high salinity, and limited sample sizes (usually only a few millilitres). In this study, a method based on laser ablation (LA) solution sampling ICP-MS for REEs analysis of small volumes (2–4 mL) of porewater is established. With the help of matrix removal and REEs preconcentration from natural liquids by a miniaturized Fe(OH)₃ co-precipitation method, a high enrichment factor (10–20 fold) was achieved with only 2 mL of the original liquid, which was preconcentrated to be as small as 100–200 μL. Ultratrace (sub-pg mL⁻¹) analysis was realized through the use of high laser ablation pulse frequency (up to 250 Hz) and based on the high transport efficiency of aerosol particles by the LA system. Due to the almost “dry” ICP condition generated by LA and nitrogen addition (5 mL min⁻¹) to the plasma, the CeO⁺/Ce⁺ ratio was reduced to approximately 0.05%, and the total yields of BaO⁺ and BaOH⁺ ions were reduced to 0.003%, which means that the potential biases induced by oxide and hydroxide interferences could be negligible. Method validation was achieved through the analysis of the seawater reference material NASS-7 and a spike recovery test with quantitative recoveries of 94–101% for REEs. The application results of porewater samples characterized by no or slight Ce anomalies together with significantly positive Eu anomalies may be indicative of highly reducing and neutral to basic pH conditions. Reagents consumed by the LA-ICP-MS method were significantly reduced, making it a more environment-friendly method. Moreover, the simplicity in sample preparation and lack of need for special reagents or devices give the proposed method great potential for wide applications in the determination of REEs in natural water samples.