Methods / Enantiomeric purity
Enantiomeric purity
Is the material the stated enantiomer, or its mirror image?
| Stationary phase | Polysaccharide derivative on silica, coated or immobilised |
| Mode | Normal phase, polar organic or reversed phase, per compound |
| Detection | Ultraviolet at the compound maximum |
| System suitability | Racemate injection demonstrating resolution |
| Deliverable | Enantiomeric excess, limit of quantitation, chromatogram |
An enantiomer has the identical molecular formula, the identical exact mass and, on an achiral column, the identical retention time. A sample that is 50 percent the wrong enantiomer therefore reports as 100 percent pure by mass spectrometry and by conventional reversed-phase chromatography together. Enantiomeric purity is measured on a chiral stationary phase and on nothing else, and it is reported as its own figure.
Applied to: Small molecules, Peptides with D-amino acids. Reference: Chiral stationary phase, ICH Q6A decision tree 5.
Mass spectrometry measures mass. Two enantiomers have the same atoms in the same numbers and therefore the same mass to any number of decimal places, so a mass spectrum cannot distinguish them and a mass accuracy figure quoted against them is meaningless.
Reversed-phase chromatography separates on interaction with an achiral surface. Both enantiomers interact identically with it, so they co-elute as one peak and area normalisation counts them as one substance. This is not a resolution problem that a shallower gradient solves; the separation does not exist at any gradient.
The consequence is specific and worth stating plainly. Material that is a racemate, half of it the inactive or differently active mirror image, passes an identity determination and a purity determination without either registering anything. A certificate showing identity confirmed and purity 99 percent is fully consistent with that material.
| Determination | Separates enantiomers | Reports the impurity |
|---|---|---|
| LC-MS identity | No | No, identical mass |
| RP-HPLC purity | No | No, co-elutes as one peak |
| Quantitative NMR | No | No, identical spectrum |
| Chiral HPLC | Yes | Yes, as enantiomeric excess |
A chiral stationary phase presents an asymmetric surface, most commonly a polysaccharide derivative such as amylose or cellulose carbamate coated or immobilised on silica. The two enantiomers form transient complexes of different stability with it, which is the only reason they elute at different times.
Method development is compound-specific and is not transferable. A phase and mobile phase that resolves one aryl propanamide will frequently fail on its close neighbour, so the determination is developed against authentic material of both enantiomers where available, or against the racemate where only one enantiomer can be obtained.
The racemate is what proves the method works. A single-enantiomer standard injected alone gives one peak whether or not the column is separating anything, so a chiral method validated without a racemic injection has demonstrated nothing.
Enantiomeric excess, as a percentage, with the chromatogram showing both peaks and their resolution. Resolution below baseline is stated rather than smoothed, because partially resolved peaks are integrated with an assumption and the assumption belongs on the certificate.
Where the minor enantiomer is not detected, the figure reported is a limit rather than a zero, expressed against the demonstrated limit of quantitation for the method as run. A report stating 100 percent enantiomeric excess without a limit is stating that nothing was found, in language that reads as though something was measured.
| Chemical impurities that are not stereoisomers. Chiral phases are poor at general purity work and this determination does not replace an achiral purity method. |
| Diastereomers in molecules with more than one stereocentre may or may not resolve on the same phase; each pair is a separate development question. |
| Anything about content in milligrams. Enantiomeric excess is a ratio between two forms of the same substance. |
| Chiral inversion occurring after the determination, in storage or in solution. |
Every method has a blind spot, and the blind spots are why a certificate lists what was not determined alongside what was. A reader who sees only results will read silence as reassurance.
Can mass spectrometry tell enantiomers apart?
No. Enantiomers have identical molecular formulae and identical exact masses, so no mass measurement at any resolution distinguishes them. The separation has to happen before the detector.
Does a 99 percent purity figure include the wrong enantiomer?
No. On an achiral column the two enantiomers co-elute as a single peak and are counted together, so material that is half the mirror image can still report 99 percent chromatographic purity.
Which compounds need this determination?
Any compound supplied as a single enantiomer. That includes most non-steroidal androgen receptor modulators and any peptide containing D-amino acids, where the achiral methods used for the L-sequence give the same answer for both.
Why does the method need a racemate?
Because a single enantiomer injected alone produces one peak whether or not the column resolves anything. The racemic injection is what demonstrates that the separation exists.
References
- ICH Q6A, Specifications, decision tree 5 on chiral substances
- USP <621>, Chromatography
Sources
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Updated 2026-09-01