Theory of Gradient Elution Liquid Chromatography with Linear Solvent Strength: Part 2. Peak Width Formation

The process of formation of the width (σ _b) of a solute band migrating along a column and its effect on the width (σ) of a corresponding peak in a chromatogram are quantified along with the extra-column contributions (Δσ _b and Δσ) to these parameters due to insufficiently narrow injection plugs. Previously unknown expressions for σ _b and Δσ _b as functions of the band migration distance and time were found. The negative gradients in the solvent strength cause the fronts of the solute bands to travel slower than their tails. This compresses the bands (reduces their widths). Previously unknown expressions describing the band compression process as functions of the band migration distance and time are found. The band compression tends to narrow the peaks. However, as shown here, the gradients that compress the bands also reduce their elution speeds. This tends to broaden the peaks (typically ignored phenomenon) and, as shown here, can cause a slight net peak broadening under normal conditions (in spite of general expectations that the gradients should narrow the peaks). On the other hand, as shown here, the gradients can significantly suppress the harmful effect of the extra-column peak broadening.