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Paper

J. Anal. At. Spectrom., 2007, 22, 1067 - 1075, DOI: 10.1039/b703812g

Determination of free iron and iron bound in metalloproteins via liquid chromatography separation and inductively coupled plasma-optical emission spectroscopy (LC-ICP-OES) and particle beam/hollow cathode-optical emission spectroscopy (LC-PB/HC-OES) techniques

Tim M. Brewer and R. Kenneth Marcus

The abilities of liquid chromatography-particle beam/hollow cathode-optical emission spectrometry (LC-PB/HC-OES) and liquid chromatography-inductively coupled plasma-optical emission spectroscopy (LC-ICP-OES) techniques are demonstrated for the quantitative determination of free iron and bound iron in metalloproteins. Separations were performed in the size exclusion and reversed-phase chromatographic modes. Myoglobin, holo-transferrin and iron(II) chloride were separated by size exclusion chromatography and the eluent species were detected by both PB/HC-OES and ICP-OES through the Fe I 259.9 nm and S I 180.7 nm optical emission. Sulfur optical emission was monitored as means of protein identification through the Fe/S empirical formula differences, with the absence of S emission used to identify the free Fe. Both techniques demonstrated detection limits for triplicate injections on the nanogram level for iron (0.9 ng mL–1 ICP-OES and 41.9 ng mL–1 PB/HC-OES) over the concentration range of 0.1 ng mL–1–100 µg mL–1 for iron and iron metalloproteins. The corresponding values for sulfur in the proteins were more similar for the two techniques, being of the order of 20 ng mL–1. The SEC retention times and peak shapes of the three analyte peaks for both techniques are similar to those determined by UV absorbance at 220 nm (Fe-containing metalloproteins) and conductivity (inorganic iron), demonstrating the ability of both techniques to preserve the integrity of the separation. While the LODs in bulk sampling were lower with ICP-OES, the signal-to-noise values were comparable for both techniques when sampling chromatographic eluents in real-time. In addition, a reversed-phase separation of the same analyte mixture was carried out with Fe I 259.9 nm and S I 180.7 nm optical emission detection by the PB/HC-OES, demonstrating the compatibility of the PB/HC-OES with a wide range of solvent polarities, its ability to detect non-metals and act as a protein specific detector for liquid chromatography of proteins.

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