Snyder selectivity triangle
Group solvents by how they interact — not just how strong they are — to change which peaks move.
Compiled by Hemant RawatLast reviewed July 2026How we verify
Two solvents with the same polarity index can produce very different separations. Snyder explained this by scoring each solvent on three interaction types — proton acceptor (xe), proton donor (xd), and dipole (xn) — and plotting them on a triangle. Solvents cluster into eight selectivity groups (I–VIII), each dominated by one kind of interaction.
The eight groups
| Group | Character | Examples |
|---|---|---|
| I | Aliphatic ethers, trialkylamines | diethyl ether, MTBE |
| II | Aliphatic alcohols | methanol, ethanol, 2-propanol |
| III | Pyridines, THF, sulfoxides, amides | THF, DMSO, DMF |
| IV | Glycols, acetic acid, formamide | ethylene glycol, acetic acid |
| V | Dichloromethane, dichloroethane | dichloromethane |
| VI | Ketones, esters, dioxane, nitriles, sulfones | acetone, ethyl acetate, acetonitrile, dioxane |
| VII | Aromatic & halo-aromatic hydrocarbons, nitro compounds | toluene, benzene |
| VIII | Water, chloroform, fluoroalcohols | water, chloroform |
How to use it in method development
The practical recipe: first set strength (retention) using the polarity index — adjust the water/organic ratio until your peaks elute in a reasonable time. Then, to fix selectivity (peaks that co-elute), swap the organic solvent for one in a different group at the same strength. The three classic reversed-phase modifiers sit in different groups — methanol (II), acetonitrile (VI), THF (III) — which is exactly why trying all three is the standard way to resolve a stubborn pair.
Look up any solvent's group on its property page or in the polarity index table.
Sources
- Stenutz / L. R. Snyder — Solvent polarity index (P′) and selectivity groups
- USP <621> — Chromatography — general chapter
Values are compiled from public references and were last verified July 2026. See ourmethodologyfor how we source and verify. Always confirm critical values against primary references and the SDS.