Evaluation of Dissolved Species of Lanthanum in the Solutions Containing Different Amino Acids by Cation-Exchange Chromatography Coupled with Electrospray Ionization Mass Spectrometry

Abstract

Because the separation and purification methods of rare-earth elements (REEs) are well developed, REEs have recently been widely used in many fields of industry and agriculture. The widespread application of REEs as additives in fertilizers, especially in China, 1 inherently leads to their residues in the environment, accumulation in organisms, and entering into the food chain. Moreover, the bioavailability and toxicity of REEs mainly depend on its dissolved species, which can be partly assimilated by plants and animals. Thus, the speciation analysis of REEs in the environment and organisms is becoming ever more important today. The study of the interaction of REEs with humic substances and proteins is a useful way to understand their behavior in the environment or during the biological process of an organism.Electrospray ionization (ESI), as a soft source for mass spectrometry (MS), has recently been used to evaluate the chemical species of dissolved metal ions; it provides information on the interaction between metal ions and organic ligands, which forms the basis of many essential biochemical processes. [3][4][5][6][7][8][9][10] On the other hand, because of the complexity of a real sample, the chromatographic separation of the target metallic element species was usually necessary for its speciation. [11][12][13][14][15] In this study, the interaction of lanthanum with ethylenediamine tetraacetic acid (EDTA), which has donor atoms of nitrogen and oxygen, and may be considered as a model ligand existing in the environment, and with Ltryptophen (Try) as the basic unit of a protein, were studied by the cation-exchange HPLC coupled with ESI-MS. The stability of La species during a cation-exchange chromatographic separation process is also discussed in this paper for obtaining accurate information on lanthanum speciation. Experimental Reagents and chemicalsLanthanum oxide (La2O3) was obtained from Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences; purity >99.9999%. La stock solution (1.00 mg cm -3 ) was prepared by dissolving an appropriate amount of La2O3, which had been ignited at 850˚C for 4 h, in 2 mol dm -3 HNO3, and then diluted with doubly deionized Milli-Q water to 100 mL in a polyethylene bottle. Ligand stock solutions (5×10 -3 mol dm -3 ) were prepared by dissolving EDTA-Na2 (A. R., Shanghai Reagent Factory) with doubly deionized Milli-Q water, and Try (purity >99%, Shanghai Bo-Ao Bio-Technology Co.) with a 1:1 water-methanol solution in acid-washed polyethylene bottles. HPLC-grade methanol was obtained from Shanghai Wusong Chemical Factory. An ammonium acetate-acetic acid (A. R., Shanghai Reagent Factory) solution (1×10 -3 mol dm -3 , pH 5 -6) was used as a buffer. Formic acid (A. R., Shanghai Reagent Factory) and α-hydroxyisobutyric acid (α-HIBA, Hunan Institute of Rare-earth Metals of China, A. R.) solutions were used as mobile phases for HPLC separation. Samples were prepared by combining the calculated amount of stock and buffer solutions to give an La concentration of 1×10...

Electrospray mass spectrometry (ESI‐MS) in the study of metal–ligand solution equilibria

“…Similar comparisons were performed also in the previously cited studies of Xu, Zhang, & Yergey (1996), Blanc et al (1997), and Deery et al (1998); while Blanc and Deery found the quantitative ESI-MS data to be very similar to those expected, Xu noticed a poor agreement. In the study of Wang et al (2000), who observed the complexes of La(III) with EDTA or with L-tryptophan in aqueous pH-buffered solutions, ESI-MS distribution of the species agreed with previous equilibrium calculations.…”

Evaluation of electrospray ionisation mass spectrometry as a technique for the investigation of competitive interactions: A case study of the ternary Th‐Mn‐EDTA system

“…investigated competitive interactions in systems containing either one metal and two siderophores or two metals and one siderophore. The technique also tolerates a wide range of solvents (from water to organic solvents) and an extended pH range; thus it has been widely used to determine the stoichiometry of metal‐ligand complexes in environmental, biological and chemical systems, including radionuclide complex speciation . Where radionuclide speciation from ESI‐MS spectra has been compared with external information, e.g., with chemical equilibrium calculations, observations of complementary techniques or previous experimental work, the results have generally been consistent, demonstrating the suitability of ESI‐MS for the study of radionuclide complexation.…”

Failure of ESI Spectra to Represent Metal-Complex Solution Composition: A Study of Lanthanide–Carboxylate Complexes

“…Nonetheless, a third mechanism also appears in the literature that provides no explanation for the transfer of the bulk solution phase ions to the gas phase but assumes that all ions produced by ESI are the same, unaltered ions present in solution . This simplistic method has resulted in multiple publications attempting to directly correlate the abundance and nature of the ion observed by ESI with known or assumed equilibrium concentrations of species in the droplet solution. ,,,− The goal of this work is to better define the limits of ESI-MS for studying metal–complexation speciation.…”