Normalized solvent-strength sensitivity for gradient separations: Comparing effective column lengths across...

The linear solvent strength (LSS) model provides a robust framework for predicting retention in reversed-phase liquid chromatography (RPLC), but its extension to ion-exchange chromatography (IEX) requires normalization of the solvent-strength coordinate. In this work, we introduce a dimensionless normalization for salt and pH gradients, enabling direct comparison of retention sensitivity across chromatographic modes. We define the universal solvent-strength sensitivity (S_U) as the product of the slope parameter and the gradient span in its respective coordinate, capturing the total change in retention factor across the gradient. This descriptor allows prediction of effective column length (L_eff) and supports rational design of ultrashort columns for high-throughput separations. Using previously published and new experimental data, we demonstrate the applicability of S_U to monoclonal antibodies (mAbs), double-stranded DNA fragments, and single-stranded oligonucleotides and mRNA. Overall, the proposed normalization provides a unified approach to retention modeling across gradient modes and biomolecular classes, facilitating method transfer and accelerating analytical workflows.