Application of experimental design for the characterisation of a novel elution system for high-capacity anion...

A novel elution system for the application of high-capacity anion exchangers with suppressed conductivity detection in ion chromatography is presented. The ternary elution system is based on perchloric acid, sodium hydroxide and sodium carbonate. The novel elution system was applied to a self-made high-capacity anion-exchange column (Q = 453 mu equiv. Cl-). A central composite design with 20 experiments was used to investigate the influence of the eluent compounds, which varied from 0.2 to 1.0 mM HClO4, 20 to 100 mM NaOH and 0 to 20 mM Na2CO3, on the retention factor k' of seven common anions. A quadratic model including interactions was postulated. The model equations were used to estimate retention factors at known eluent compositions. No significant differences of calculated and experimental retention factors were found. Further statistical analysis was done by analysis of variance. The results showed that the three eluent compounds have completely different effects on the retention behaviour of the anions investigated. The elution of soft and weakly hydrated anions like bromide and nitrate is strongly influenced by the content of perchloric acid, whereas strongly hydrated anions like fluoride and sulphate are mainly affected by the hydroxide or carbonate content of the eluent. The results can qualitatively be explained comparing the relation of ionic radii to charge for eluent and analyte anions. As a consequence, the retention of the anions investigated can easily be manipulated by varying the contents of the eluent compounds. The usefulness of the novel elution system and high-capacity anion chromatography is demonstrated by the determination of trace anions in phosphate and fluoride matrices.