Static dielectric constants of acetonitrile/water mixtures in the whole composition range and within the temperature range from (15 to 60) °C have been measured. The values were fitted to a unique equation as a simultaneous function of temperature and acetonitrile composition. Densities of these mixtures at temperatures from (5 to 55) °C and different compositions have been collected from the literature. The complete data set was critically analyzed, the outliers were excluded, and the remaining values were also fitted to a unique equation as a function of temperature and acetonitrile composition. From the dielectric constants and densities, the activity coefficients of ions can be calculated by means of the Debye−Hückel approach, allowing the study of the ionic equilibria and determination of reference pH values or acidity constants in any acetonitrile/water mixture at any temperature within the studied range.

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The use of flow splitters between the two dimensions in on-line comprehensive two dimensional liquid chromatography (LC×LC) has not received very much attention in comparison to their use in GC×GC where they are quite common. In principle, splitting the flow after the first dimension column and performing on-line LC×LC on this constant fraction of the first dimension effluent should allow the two dimensions to be optimized almost independently. When there is no flow splitting any change in the first dimension flow rate has an immediate impact on the second dimension. With a flow splitter one could for example double the flow rate into the first dimension column and do a 1:1 flow split without changing the sample loop size or the sampler’s collection time. Of course, the sensitivity would be diminished but this can be partially compensated by use of a larger injection; this will likely only amount to a small price to pay for this increased resolving power and system flexibility. Among other benefits, we found a 2-fold increase in the corrected 2D peak capacity and the number of observed peaks for a 15 min analysis time by using a post first dimension flow splitter. At a fixed analysis time this improvement results primarily from an increase in the gradient time resulting from the reduced system re-equilibration time and to a smaller extent it is due to the increased peak capacity achieved by full optimization of the first dimension.