Advances in ion chromatography: speciation of μ 1 −1 levels of metallo-cyanides using ion-interaction chromatography

Abstract

Fe(CN) 4− 6, Cu(CN) 3− 4, Co(CN) 3− 6, Fe(CN) 3− 6, Ni(CN) 2− 4 and Cr(CN) 3− 6 are determined by ion-interaction chromatography using a C 18 column and methanol-tetrahydrofuran-10 mM phosphate buffer (pH 7.9) (25 + 1 + 74, v/v/v) containing 5 mM tetrabutylammonium hydroxide as mobile phase, with spectrophotometric detection at 214 nm. Detection limits are in the range 0.01–0.5 mg 1 −1. In an alternative approach, an automated on-line sample preconcentration technique is used wherein a 2-ml volume of sample containing metallo-cyanides is loaded onto a C 18 precolumn which has been equilibrated with the above mobile phase. The bound solutes are then eluted from the precolumn to a C 18 analytical column where they are separated using the same mobile phase as employed to equilibrate the precolumn. Detection limits are in the rate 0.08–1.58 μg 1 −1 and calibration graphs are linear up to 200 μg 1 −1. The preconcentration step is shown to give quantitative recoveries for all species except Fe(CN) 4− 6 and (CN) 3− 4. The iron(II) complex does not bind quantitatively to the precolumn, and extensive studies with the copper complex suggested that low recoveries were due to dissociation and ligand-exchange reactions occurring during the chromatographic separation process. Negative interference effects were observed for Cl − and SO 2− 4 when present at a level of 250 mg 1 1−, and UV-absorbing anions such as Br −, SCN −, NO − 2 and NO − 3 caused positive interference when present at concentrations as low as 1 mg 1 −1. The negative interferences could be reduced by diluting the sample and the positive interferences could be eliminated by incorporating an additional step in the preconcentration process, in which UV-absorbing anions bound to the precolumn after sample loading were eluted selectively using an eluent consisting of 10 mM NaCl in phosphate buffer (pH 6.7).