Determination of trace magnesium and strontium in calcium carbonate and calcareous skeletons of marine planktonic organisms using high performance chelation ion chromatography

Abstract

A new high performance chelation ion chromatography method for the simultaneous determination of trace magnesium and strontium in various calcium carbonate samples was developed. Separations were performed on a monolithic silica column (Chromolith Si, 100 × 4.6 mm I.D.) chemically modified with hydroxyethyliminodiacetic acid functional groups. At a flow rate of 1.0 mL min⁻¹, an eluent containing 80 mM NaCl and 20 mM picolinic acid at pH 5.30 was found to provide complete separation of Ni²⁺, Cu²⁺, Mg²⁺, Cd²⁺, Sr²⁺ and Ca²⁺ in 15 minutes, in matrices with 20 000 fold excess of Ca²⁺. Two post-column reagents, o-cresolphthalein complexone (o-CPC) and ZnEDTA-PAR, were compared for post-column reaction based photometric detection at 570 and 490 nm, respectively. This method provides sensitive detection of Mg²⁺ (LOD 20 μg L⁻¹ and 5 μg L⁻¹ for o-CPC and ZnEDTA-PAR, respectively) and Sr²⁺ (LOD 200 μg L⁻¹ and 39 μg L⁻¹ for o-CPC and ZnEDTA-PAR, respectively). Using o-CPC, the linear range was from 0.5 to 24 mg L⁻¹ for Mg²⁺ and from 1.0 to 32 mg L⁻¹ for Sr²⁺. For ZnEDTA-PAR, the linear range was from 0.5 to 4 mg L⁻¹ for Mg²⁺ and 0.1 to 32 mg L⁻¹ for Sr²⁺. The method was applied to the analysis of a variety of calcium carbonate samples, including laboratory reagents, limestone NIST certified reference material, and the calcite based shells of marine microorganisms. The accuracy of the method was confirmed using inductively coupled plasma mass spectrometry.