Use of temperature programming to improve resolution of inorganic anions, haloacetic acids and oxyhalides in drinking water by suppressed ion chromatography

Temperature programming was used to improve selectivity in the suppressed ion chromatographic (IC) separation of inorganic anions, haloacetic acids and oxyhalides in drinking water samples when using NaOH gradient elution. The programme exploited varying responses of these anions to changes in temperature. Heats of adsorption (ΔH, kJ/mol) for 17 anionic species were calculated from van’t Hoff plots. For haloacetic acids, both the degree of substitution and log P (log of n-octanol–water partition coefficient) values correlated well with the magnitude of the temperature effect, with monochloro- and monobromoacetic acids showing the largest effect (ΔH = −10.4 to −10.7 kJ/mol), dichloro- and dibromoacetic acids showing a reduced effect (ΔH = −6.8 to −8.4 kJ/mol) and trichloro-, bromodichloro- and chlorodibromoacetic acids showing the least effect (ΔH = −4.7 to −2.4 kJ/mol). The effect of temperature on oxyhalides ranged from ΔH = 8.4 kJ/mol for perchlorate to ΔH = −9.1 kJ/mol for iodate. The effectiveness of two commercial column ovens was investigated for the application of temperature gradients during chromatographic runs, with the best system applied to improve the resolution of closely retained species at the start, middle and end of the separation obtained using a previously optimised hydroxide gradient, in a real drinking water sample matrix. Retention time reproducibility of the final method ranged from 0.62 to 3.18% RSD (n = 30) showing temperature programming is indeed a practically important parameter to manipulate resolution.