This technical article introduces the solvent-strength nomogram, a tool to pick the (approximately) equal strength of one solvent from another.
During method development in HPLC, one of the powerful variables that can be used to change selectivity is to switch from one solvent to another. For example, the B-solvent can be changed from acetonitrile (ACN) to methanol (MeOH) or tetrahydrofuran (THF). As a general rule, making such a solvent change will change the peak spacing, or selectivity. Hopefully this will improve the separation, but there is no guarantee of this.
Most of us are familiar with using MeOH and ACN as organic components of the mobile phase for reversed-phase separations, but THF may not be so familiar. Besides differences in selectivity, these different solvents also have different elution strengths.
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