Direct chromatographic separation and quantification of calcium and magnesium in seawater and sediment porewaters
Melendez, M and Nesterenko, EP and Nesterenko, PN and Corredor, JE, Direct chromatographic separation and quantification of calcium and magnesium in seawater and sediment porewaters, Limnology and Oceanography: Methods, 11, (September) pp. 466-474. ISSN 1541-5856 (2013) [Refereed Article]
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.
calcium and magnesium, seawater, chelation ion chromatography