Direct chromatographic separation and quantification of calcium and magnesium in seawater and sediment porewaters

Direct analysis of Ca²⁺ and Mg²⁺ is required for accurate determination of metastable carbonate mineral phase saturation states (ΩCaCO₃; ΩMgCO₃) in seawater, sediment porewaters, and other high ionic strength brines. To this end, we have implemented a method using High Performance Chelation Ion Chromatography (HPCIC) in which metal ion complexation at the stationary phase renders separation efficiency insensitive to high ionic strength matrix effects common to other ion chromatography (IC) methods. This method, using direct automated on-column injection, vastly increases sample throughput capacity in comparison to current titration methods. Calcium and magnesium ions in IAPSO standard seawater were selectively separated using a monolithic silica column (100 × 4.6 mm ID) activated with a covalently bonded iminodiacetic acid (IDA) chelator. The colored ion complexes resulting from post-column reaction (PCR) of the ions with a metallochromic indicator, in this case 4-(2-pyridylazo)-resorcinol (PAR), were detected spectrophotometricaly at 510 nm. Optimization of flow rate, eluent concentration, pH, and sample injection volume allowed baseline separation of Mg²⁺(0.05474 mol kg^–1) and Ca²⁺ (0.01065 mol kg^–1) in less than 8 min using 2 μL seawater sample injections. At a flow rate of 1 mL min^–1, peak elutions occurred respectively at 4 and 5 min, using an eluent containing 0.1 M potassium chloride and 1 mM nitric acid adjusted to pH 2.5. Retention time variability below 0.5% for both metals following more than 200 injections indicates long-term stability of the derivatized monolithic silica column. Method application to marine sediment porewaters is discussed.