Modified enrichment and purification protocol for dissolved silicon-isotope determination in natural waters

The preparation of dissolved silicon in natural waters for the determination of silicon isotopes by MC-ICP-MS often involves pre-concentration and purification steps. In this study, we present a simplified magnesium co-precipitation step to enrich the dissolved silicon by substituting concentrated ammonia for NaOH solution and increasing the co-precipitation time. Silicon recovery using single co-precipitation protocol is quantitative and less affected by the volume of ammonia. The separation of silicon from other cations is achieved using a cation exchange resin and ensures complete yields. A low procedural blank of ∼14.0 ng Si was achieved by purifying all the reagents. No detectable Si isotope fractionation is observed using the optimized single co-precipitation followed by a cation exchange-resin purification step. Suitability of the application of this method in various natural waters is demonstrated, including river saline and seawater. The modified single co-precipitation method presented in our study is more convenient than the previous multiple co-precipitation protocol and has a broad application range.