Determination of metal ions is greatly demanded for various scientific fields, like medical, geologic and environmental studies. For example, the limestone and dolomite diagenetic fabrications are associated to salinity and the Mg/Ca ratio [1]. In addition, the trace metal chemistry of calcite based fragments of marine microorganisms, such as foraminiferal calcite and coccolithophore calcite, can be used to reconstruct the nature of ancient oceans and environments [2]. The evaluation of the changes in marine environment is associated with global warming and large increase in CO₂ emission [3].

However, the powerful and sensitive spectroscopic methods, such as ICP-MS, AAS and neutron activation have limited possibilities for the analysis of samples which have the strong ionic strength and high complexity as a matrix. As a rule for the analysis of these samples the additional time-consuming sample pre-treatment is required. The recent development of high performance chelation ion chromatography (HPCIC) is a possible alternative for the determination of metal ions [4]. The separation mechanism of HPCIC includes the formation of kinetically labile complexes between chelating ion-exchange groups and the separated metal ions, so the retention of metals is less affected by the ionic strength of the samples.

The current work presents the results of determination of alkaline earth ions (Mg²⁺, Sr²⁺ and Ca²⁺) in seawater, calcium carbonate, limestone and in calcite based shells of marine microorganisms, such as coccolithophores. Separations are performed on silica monolithic column (Chromolith Si, 100 x 4.6 mm I.D.) chemically modified with 2-hydroxyethyliminodiacetic acid functional groups. The complete separation of Ni²⁺, Cu²⁺, Mg²⁺, Cd²⁺, Sr²⁺ and Ca²⁺ in 15 min at flow rate of 1.0 mL/min was obtained in the eluent containing 80 mM NaCl, 20 mM picolinic acid (pH 5.3) and these conditions are used for the determination of trace metals in calcium carbonate, Two colour-forming ligands, o-cresolphthalein complexone (o-CPC) and ZnEDTA-PAR are tested as reagents for spectrophotometric detection. The optimum detection of alkaline earth metals was achieved after post-column reaction (PCR) using o-CPC at 570 nm or ZnEDTA-PAR at 490 nm. This method allows quantitative determination of low concentrations of Mg²⁺ with 20 ppb and 5 ppb LODs for o-CPC and ZnEDTA-PAR respectively. For ZnEDTA-PAR PCR, the linear range of the determination is from 0.02 to 4 ppm and from 0.1 to 32 ppm for Mg²⁺ and Sr²⁺, respectively. For the seawater analysis, eluent was composed of 80 mM KCl and 4 mM picolinic acid (pH 5.0). It allowed the complete separation of Mg²⁺, Sr²⁺ and Ca²⁺ in 12 minutes at flow rate of 1.0 mL min^-1. Spectrophotometric detection at 510 nm after PCR with PAR is suitable for accurate direct determination (without any sample pre-treatment) of Mg and Ca in seawater. The linear range of the determination is from 2 to 800 ppm and from 1 to 300 ppm for Ca²⁺ and Mg²⁺, respectively.