Identification of complexes involving thallium(I) and thallium(III) with EDTA and DTPA ligands by electrospray ionization mass spectrometry

Thallium is considered to be an environmental threat; however, its hazardous properties depend on its oxidation state. Tl(III) is approx. 1000-times more toxic compared with Tl(I), therefore identification of each species is essential in order to properly evaluate the associated health hazard. Electrospray ionization mass spectrometry (ESI-MS) allows determination of speciation in solution due to its soft mode of ionization while selective complexation with ligands can distinguish the Tl species. Selective complexation of Tl(I) and Tl(III) ions requires the use of two selective complexing agents and selection of appropriate conditions for this process.Tl(I) and Tl(III) ions as well as two ligands (EDTA and DTPA) were used to form binary (single ion + single ligand), ternary (one ion + both ligands) and quaternary systems (both ions and both ligands) under different pH conditions (7 and 8). These mixtures were subjected to the determination of Tl species using ESI-MS operating in positive and negative ion mode.Tl(I) complexes with DTPA were identified at pH 7 and 8, whereas, in the case of EDTA, the complexes were detected only at pH 8. In contrast, Tl(III) formed distinct complexes with EDTA at pH 7 and 8, while with DTPA the complexes were detected only at pH 8. Analysis of the quaternary system (which contained both ions and both ligands) revealed that Tl(I) formed complexes with EDTA, while Tl(III) formed complexes with DTPA at pH 7 and 8.The obtained results confirmed that the increase in the solution complexity allowed simultaneous identification of different complexes in solutions containing both Tl species. The initial analyses carried out for binary and ternary solutions facilitated the simultaneous determination of specific complexes (Tl(I) with EDTA and Tl(III) with DTPA) in the quaternary system.