Evaluation of different column types for the hydrophilic interaction chromatographic separation of iron-citrate and copper-histidine species from plants

Hydrophilic interaction chromatography (HILIC) has emerged as a very useful separation method for polar analytes, including non-covalent metal species. Several types of stationary phases are available for HILIC applications, differing mainly in their chemical functionalities that supply additional interaction modes and alternative selectivities for the separation of special analytes. With regard to the separation of metal species only few of these stationary phases have been applied to date, and it is not completely clear what are their differences with respect to the chromatographic separation of metal species, but also with respect to species stability during chromatography. Here, a comparison of different column types for the HILIC separation of iron citrate and copper histidine species is presented and the results are discussed with respect to retention mechanisms and chromatographic stability of these metal species. It is shown that different stationary phases display very different separation patterns. In particular, three types of HILIC columns enable successful separation of iron citrates and copper histidine at pH 5.5, namely a crosslinked diol phase, a zwitterionic phase, and an amide phase. Two groups of iron-citrates are separated on all three columns, consisting of a species of 3:3 stoichiometry and another one of mainly 3:4 stoichiometry (plus 1:2 and 2:2 species). For copper-histidine only one stable species is found based on the 1:2 stoichiometry. Detection and unambiguous identification of the different species is possible by employing electrospray mass spectrometry in the negative ionization mode. Species found in standard solutions are consistent with species found in spiked plant samples. Also in unspiked solutions iron citrate of 3:4 stoichiometry (plus 1:2 and 2:2) is detectable, but no species of 3:3 stoichiometry. Significant differences of related species patterns are found in real plant samples.