A new method for the determination of free calcium concentration in human plasma was developed by online coupling capillary electrophoresis (CE) with inductively coupled plasma optical emission spectrometry (ICP-OES). Baseline separation of calcium-containing species was achieved by CE-ICP-OES in a 120-cm-long capillary with 100-microm internal diameter, at 20 kV applied voltage, with a 30 mmol/L Tris-HCl buffer at pH 7.4. A total of eight calcium-containing species were found in human plasma; the concentration of free calcium ion was found to be 41.9 mg/L. The concentrations of calcium for other seven calcium species, estimated from the calibration against Ca(2+) standard, were 3.14-15.6 mg/L. The precision (RSD, n = 10) ranged from 1.2 to 2.7% for the migration time and 2.8 to 3.9% for the peak area. The developed method was also applied to analyze plasma samples with recovery ranged from 94.5 to 102% for samples spiked with 40 mg/L free Ca(2+) ion.

A new method for speciation analysis of magnesium species and quantification of free magnesium concentrations in rat plasma was developed by on-line coupling of CE with inductively coupled plasma-atomic emission spectrometry (ICP-AES). Baseline separation of seven magnesium species was achieved by using a 120 cm (100 mm internal diameter) fused-silica capillary, a 20 kV separation voltage and a solution of 50 mmol/L NaAc-HAc (pH 5.5) as electrolyte buffer. CE-ICP-AES analysis of a rat plasma sample showed the presence of seven magnesium species, one of which was identified as free Mg 21 ion by spiking a Mg 21 standard; the migration time of the Mg 21 peak in the standard and the spiked sample matched with each other. One protein-bound magnesium species in rat plasma is associated with albumin, and the other three species are combined with globulin. The concentration of free magnesium in the plasma was 14.0 mg/L. The other six magnesium species were estimated to be 4-15 mg/L. RSDs of migration time and peak area for the magnesium species from ten replicates were less than 5%. The developed method was also applied to speciation analysis of magnesium species in spiked plasma samples. The recoveries of the free magnesium species in four samples ranged from 95.8 to 103.8%.

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