Screening and identification of aluminium-containing biomolecules by column-switched LC-ICP-MS and LC-ESI-MS/MS

Column-switching liquid chromatography followed by low resolution ICP-MS was evaluated as a tool for speciation analysis of aluminium-containing biomolecules. The strategy was applied on siderophores, small organic molecules (M_r < 1500) which normally act as strong iron chelators. The drawbacks normally encounterd with aluminium detection using low resolution ICP-MS are the formation of polyatomic ions causing isobaric overlaps and space-charge effects. When adding a carbon rich solvent, such as methanol or acetonitrile, the ¹³C¹⁴N⁺, ¹²C¹⁵N⁺ and ¹²C¹⁴N¹H⁺ with the same mass as ²⁷Al⁺ will form in the plasma. The nitrogen is either entrained from the surrounding atmosphere or added with the constituents in the mobile phase. These disadvantages were successfully counteracted by the use of nitrogen free organic modifier in the mobile phase and the use of cool plasma conditions. Detection limits for standard solutions of aluminium-chelated ferrichrome in sub-nanomolar range were obtained by monitoring the aluminium-27 isotope. The combined use of LC-ICP-MS and LC-ESI-MS/MS was also evaluated as a tool to identify unknown metal complexes, here siderophores, in field soil solution samples. Two aluminium-chelated siderophores, Al-desferrichrom and Al-desferricrocin, were identified and quantified. Both aluminium-siderophore complexes were present in the low nanomolar range (1.1 and 0.7 nM, respectively).