Investigation of the chromatographic behaviour of some selenium species—Comparison with their octanol-water partitioning

The retention behaviour of selenites (Se(IV)), selenates (Se(VI)), seleno-dl-methionine (Se-Met), selenocystine (Se-Cyst), selenocystamine (Se-CM) and selenourea (Se-U) was investigated using a Discovery end-capped reversed-phase column as stationary phase and different mobile phase conditions. Extrapolated to 100% aqueous mobile phase retention factors (log k_w) of the investigated Se species, determined using different methanol fractions (φ) as organic modifier, were compared with the corresponding actual values. The proper operation of this column even at 100% aqueous phase proved to be valuable for the accurate determination of log k_w values of Se-CM and Se-Cyst, presenting a convex curvature log k = f(φ) at low MeOH fractions, often neglected in the extrapolation procedure. The effect of the presence of n-decylamine as well as saturation of the mobile phase with n-octanol was also studied. For ampholytic Se-Met and Se-Cyst the effect of n-decylamine in retention reflected the predominance of zwitterionic nature in the case of Se-Met in contrary to the non-zwitterionic species found in the case of Se-Cyst, in accordance with our previous findings concerning partitioning experiments in the n-octanol/water system. Finally, an attempt was made to correlate log k_w values with the logarithm of n-octanol/water distribution coefficient, log D, of the investigated Se species and an indicative log D value of Se-U was derived.