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RP-HPLC · Method Scouting

Chromatogram Predictor

Enter the SMILES of the molecules you want to separate — your API and its impurities or degradants. MolWard scouts a standardized grid of columns, mobile phases and pH, ranks the conditions by resolution of the critical pair, recommends a UV wavelength, and — using the 817-column HSM database — flags peak-tailing risk and suggests orthogonal columns when peaks co-elute.

Molecules to Separate
Up to 25 molecules. Put the parent first. Load a paracetamol example
Enter SMILES and click predict.
Predicted Method Scouting
Your predicted chromatogram, the ranked condition grid, and column recommendations will appear here.

How reversed-phase retention is predicted

In reversed-phase HPLC, how long a compound is retained depends on its hydrophobicity and on the mobile phase. Understanding a few published relationships lets you anticipate elution order and scout conditions before running a gradient.

Solvent strength and hydrophobicity

Over a useful range, the log of the retention factor falls linearly as the organic fraction (φ) rises — the linear-solvent-strength relationship:

log k = log kw − S·φ

and log kw tracks the analyte's hydrophobicity (log P / log D). More hydrophobic compounds retain longer; adding organic modifier elutes everything sooner.

Why pH changes the picture

For ionisable analytes, pH sets the charged fraction through Henderson–Hasselbalch, and the effective hydrophobicity is log D, not log P. A charged molecule is far less retained than its neutral form, so pH can reorder the chromatogram entirely.

Resolution improves with retention (k), selectivity (α) and efficiency (N)

Worked example

A neutral compound and a weak acid co-elute at low pH (the acid is protonated and hydrophobic). Raising the mobile-phase pH above the acid's pKa deprotonates it; its log D drops, it elutes earlier, and the two peaks separate — a pH change, not a stronger column, solved the co-elution.

Reference: Snyder, Kirkland & Dolan, Introduction to Modern Liquid Chromatography, 3rd ed.

Frequently asked questions

What is the linear-solvent-strength model?

An empirical relationship stating that log k decreases linearly with the organic fraction of the mobile phase; it underlies gradient method development and lets retention at one composition predict another.

Why does mobile-phase pH matter for method scouting?

For ionisable analytes, pH controls the charged fraction and therefore effective hydrophobicity (log D). Scouting pH often changes selectivity and elution order more powerfully than changing the organic modifier.

What resolution is "good enough"?

A resolution (Rs) of 1.5 gives essentially baseline separation of two peaks; 2.0 is a common robustness target for stability-indicating methods to keep the separation reliable across normal variation.