Reverse-phase HPLC separates most peptides well, but it struggles at one end of the range. Di-, tri- and tetrapeptides built mainly from polar or charged residues have so little hydrophobic surface that they pass through a C18 column almost unretained, eluting with the solvent front alongside salts and system disturbances. Hydrophilic interaction liquid chromatography, usually shortened to HILIC, was developed for exactly this kind of molecule. The name was introduced by Andrew Alpert in 1990, and the technique is now a standard option for small polar analytes.
Recognizing a peptide that C18 cannot hold
Retention is more reliably judged by the retention factor rather than by minutes: k = (tR − t0) / t0, where t0 is the time an unretained compound takes to reach the detector. Suppose t0 is 1.0 min and a short polar peptide elutes at 1.3 min. Its k is 0.3. At values that low, small changes in the sample solvent or the system can shift the peak, early-eluting impurities have nowhere to separate, and anything unretained in the sample co-elutes with the analyte. Most chromatographers want k comfortably above 1 before trusting a peak for quantitation.
Warning signs in a reverse-phase chromatogram include a main peak sitting on or just after the void disturbance, a peak shape that changes with the amount of sample, and a retention time that barely moves when the gradient changes.
How HILIC retention works
HILIC uses a polar stationary phase with a mobile phase that is mostly acetonitrile, commonly starting somewhere around 80 to 95% organic. Under those conditions a water-enriched layer forms on the polar surface. Polar analytes partition from the organic-rich mobile phase into that layer and are retained; increasing the water content of the mobile phase weakens retention and elutes them. The gradient therefore runs in the opposite direction from reverse phase, from high organic toward more aqueous.
Partitioning is not the only mechanism. Hydrogen bonding with the surface and electrostatic interaction with charged groups on the stationary phase contribute as well, which is why buffer type and ionic strength have a strong effect. Ammonium formate and ammonium acetate are common choices because they control ionic interactions and remain volatile for mass spectrometry.
The acid chosen for the mobile phase matters in a way reverse-phase users may not expect. Trifluoroacetate pairs with the positive charges on a peptide and makes it effectively less polar, which reduces HILIC retention, although it can sharpen peaks. Formic acid and ammonium formate leave the charges more exposed, so electrostatic attraction to the surface contributes more. On bare silica in particular, raising the buffer concentration weakens that ionic component and shortens the retention of basic peptides, while neutral polar peptides change far less. Analysts use this difference deliberately: adjusting ionic strength is one of the simplest ways to move a charged impurity away from a main peak without changing the column or the gradient.
Stationary phase options in HILIC
| Phase | Character | Notes for peptides |
|---|---|---|
| Bare silica | Polar, with ionizable silanols | Strong cation-exchange contribution; basic peptides can be retained very strongly |
| Amide-bonded | Neutral, strongly hydrophilic | Retention dominated by partitioning and hydrogen bonding; widely used for peptides |
| Diol-bonded | Neutral, moderately polar | Milder retention; fewer ionic effects |
| Zwitterionic (for example sulfobetaine) | Carries both positive and negative groups | Balanced weak electrostatic interactions for charged analytes |
The choice follows the same logic described for reverse-phase ligands in HPLC column chemistry for peptide separation: a different surface produces a different selectivity, even for the same molecule.
Elution order and orthogonality to reverse phase
As a rough rule, HILIC reverses the reverse-phase order. The most polar and most highly charged peptides elute last, and anything with a significant hydrophobic region elutes early. The reversal is not exact, because electrostatic effects add their own selectivity, but the two separations depend on sufficiently different properties that they are considered largely orthogonal.
That orthogonality is valuable beyond simply holding a polar peptide. A deletion impurity missing a serine or an aspartate may sit almost on top of the main peak in reverse phase, yet shift noticeably in HILIC because the lost residue was a polar one. Running a sample in both modes is a recognized way to check that a purity result is not concealing a co-eluting component, an idea developed in orthogonal methods for peptide identity.
Alternatives before switching modes
HILIC is not the only way to retain a polar peptide, and it is not always the simplest.
- A stronger ion-pairing reagent. For short peptides with several basic residues, replacing TFA with heptafluorobutyric acid can add enough retention on C18, as explained in HFBA and PFPA ion-pairing reagents. It helps little for acidic sequences.
- Aqueous-stable reverse-phase columns. Polar-embedded or polar-endcapped C18 phases tolerate a starting mobile phase with little or no organic solvent without the loss of retention that conventional C18 can show under fully aqueous conditions.
- Porous graphitic carbon. Retains very polar compounds through a different surface interaction, though its behavior can be harder to predict.
Practical characteristics and pitfalls
- Sample solvent. Because water is the strong solvent in HILIC, a sample dissolved in a largely aqueous medium can distort or split peaks. Methods therefore specify the sample diluent carefully, and the solubility of very polar peptides in high-organic solutions can itself be a limitation.
- Equilibration. The water layer takes longer to reestablish than a reverse-phase surface, so HILIC methods need longer equilibration between gradient runs for reproducible retention.
- Mass spectrometry. The high organic content favors efficient desolvation in electrospray, and volatile ammonium buffers avoid the signal suppression associated with TFA. HILIC coupled to MS is often more sensitive for small polar peptides than reverse phase.
- Detection. UV detection at low wavelength works, but acetate buffers raise background absorbance. For peptides lacking a useful chromophore, ELSD and CAD detection are alternatives.
Most products we supply are analyzed by reverse phase, and each has its independent HPLC result published on its listing. Vials are identified by crimp and cap color, without lot codes, so the trace a reader sees is the one that corresponds to that product.
Frequently asked questions
Why does a short polar peptide elute in the void on C18?
It has too little hydrophobic surface to partition into the bonded alkyl phase, so it moves with the mobile phase and reaches the detector with unretained material.
Is the HILIC gradient the reverse of a reverse-phase gradient?
Yes. HILIC starts at high acetonitrile and increases the water content, because water is the stronger eluting solvent in this mode.
Can HILIC and reverse-phase purity results be compared directly?
Not peak for peak. The elution order and resolution of impurities differ, so each result is interpreted within its own method, although agreement between the two strengthens confidence.
Research use only. All products supplied by Battle Born Peptides are laboratory reference materials for in-vitro research and analytical use by qualified professionals. They are not drugs, foods, dietary supplements, cosmetics or medical devices; they are not approved by the FDA or any other regulator for use in humans or animals; and they are not intended to diagnose, treat, cure, mitigate or prevent any disease, or to affect the structure or any function of the body of humans or animals. Nothing in this article is preparation, handling or dosing guidance. See our full research-use terms.