Phosphate in three seawater certified reference materials developed in previous work was not certified because it was only determined by continuous flow analysis (CFA). To obtain certified values for phosphate, we developed a novel method for the analysis of phosphate in seawater via inductively coupled plasma sector field mass spectrometry (ICP-SFMS) combined with anion exchange ion chromatography (IC). With use of a Jet-sampler cone and an X-skimmer cone on the ICP-SFMS instrument, the 31 P sensitivity derived from phosphate was enhanced by a factor of 18 compared to that of standard cones. To reduce the severe saline-generated matrix effect, an IC with a switching-valve system that separates phosphate from saline was employed. By adding an internal standard element (As) to the seawater samples, time-dependent fluctuation of the 31 P sensitivity was corrected and a better calibration curve was obtained. The developed method was applied to the analysis of three types of seawater. Although NMIJ CRM 7601-a could not be determined because its concentration after dilution for the IC separation is lower than the detection limit (0.005 μmol kg À1 ), the analytical results for NMIJ CRM 7602-a and 7603-a are in good agreement with the values obtained via CFA, considering their expanded uncertainties with the coverage factor of 2. The developed method was validated by analyzing an existing certified reference material (MOOS-3). This method enabled the measurement of sub-μmol kg À1 levels of phosphate with high accuracy and sensitivity.
For accurately determining nutrients in seawater by continuous flow analysis (CFA), the characteristic of the calibration curve was examined in detail. Under absorbance below 0.8, the calibration curve and the bracketing methods showed more accurate results that the bias fell below 0.5%. The analytical results of dissolved silica in seawater from the nutrient maximum layer of the Pacific Ocean obtained by the proposed methods showed good agreement with those obtained by an ion exclusion chromatography postcolumn absorption spectrophotometry (IEC-postcolumn) and an ion exclusion chromatography isotope dilution ICP mass spectrometry (IEC-ID-ICP-MS). From the results, the analysis of nutrients in seawater could be accurately carried out by CFA with an expanded uncertainty of below 1% using both the calibration curve and the bracketing methods with an appropriate absorbance range.
The determination of dissolved silica in seawater was investigated by an ion-exclusion chromatography with a post-column derivatization/silicomolybdenum yellow method. To determine dissolved silica in seawater accurately, the experimental conditions, such as the volume of a reaction coil, the eluent concentration and the sample dilution factor, were optimized. This is the first report that has examined the details of the experimental condition for the measurement of dissolved silica in seawater by ion-exclusion chromatography with silicomolybdenum-yellow detection. The developed method was compared with the widely used continuous flow analysis and an ion-exclusion chromatography isotope dilution inductively coupled plasma mass spectrometry, which is a primary method of measurement. The analytical results by the three methods were in good agreement, considering the expanded uncertainty with a coverage factor, 2. The validity of the three methods was confirmed with each other. The developed method can give a quantitative value with less than 1% of the expanded uncertainty, and can be used for the determination of dissolved silica in candidate reference materials having a seawater matrix.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.