Isotachophoretic separation behavior of rare-earth EDTA chelates and analysis of minor rare-earth elements in an iron ore by bidirectional isotachophoresis–particle-induced X-ray emission

Mobilities of 16 anions of rare-earth–EDTA 1:1 chelate (RE–EDTAs) were isotachophoretically measured by using two leading electrolytes (pH 3.6 and 6.0) in order to assess their separation behavior. The leading electrolyte was 20 mM hydrochloric acid. The pH of the solution was adjusted to 3.6 by adding β-alanine and to 6.0 by adding histidine. The obtained mobilities were very close to each other in the range 20.1·10−5–21.9·10−5 cm2 V−1 s−1 with the minimum mobilities for Pr–EDTA and Nd–EDTA for pH 3.6 and 6.0, respectively, and pH dependence was hardly observed. On the basis of the above knowledge, minor rare-earth elements in a standard iron ore sample were determined as RE–EDTAs by bidirectional isotachophoresis–particle-induced X-ray emission (PIXE), where the Fe(II) matrix digested by alkali fusion was separated as Fe(II)Phen32+ (phen=1,10-phenanthroline). Since 5% of the total iron was still detected as Fe(III)EDTA− and might disturb PIXE analysis of RE–EDTA−, itaconic acid was used as the spacer for Fe(III)EDTA− and RE–EDTA−. The fractions of RE–EDTA− were successfully analyzed off-line by a multielemental analytical method, PIXE [analytical result (3.62% (w/w) as RE2O3]; the nominal value was 3.37% (w/w) as RExOy.