Mass spectrometry answers a different question, properly

The reason it matters is that co-elution is the failure mode a single method cannot detect. A chromatogram showing one sharp, symmetrical peak and a purity of 99.3 per cent is consistent with a clean preparation and equally consistent with a preparation in which a related species elutes under the parent. Peak-purity assessment from a diode-array detector helps and is not conclusive. A second separation on a different principle is the practical answer, and it is cheap: the sample is already in the vial, the second run costs instrument time.

Mass spectrometry, and the limits of a matching mass

Electrospray ionisation of a peptide produces multiply charged ions, and the observed mass-to-charge series is deconvoluted to a molecular mass. Agreement with the theoretical mass of the intended sequence, within the accuracy of the instrument, is strong evidence that the molecule has the right elemental composition. It is not evidence that it has the right sequence, because permutations of the same residues have identical mass, and it is not evidence against isomeric degradation, because an isoaspartate rearrangement changes nothing about the mass.

Fragmentation closes most of that gap. Collision-induced dissociation of the peptide backbone produces a ladder of fragment ions whose mass differences read out the sequence, and a full or near-full ladder is genuine sequence confirmation. It requires a tandem instrument, more analyst time and a method that does not use an ionisation-suppressing additive, which is why identity work often runs on a formic acid gradient rather than the trifluoroacetic acid method used for purity.

The practical reading of a certificate follows. Identity confirmed by mass means the elemental composition matches. Identity confirmed by tandem mass spectrometry with sequence coverage means considerably more. Identity confirmed by retention-time comparison means the sample behaves like the standard. Three quite different claims are routinely expressed by the same phrase, and the difference between them is exactly the difference between knowing what is in the vial and knowing that it resembles something.¹

The second method, and what makes it orthogonal

Orthogonality is not a synonym for repetition. Two runs of the same method differ only in random variation. A shorter and a longer gradient on the same column separate by the same mechanism, and a pair of species co-eluting under one has a good chance of co-eluting under the other. Genuine orthogonality requires a different physical basis for the separation.

For peptides the practical options are well established. Changing mobile-phase pH alters the ionisation state of acidic and basic residues and therefore their effective hydrophobicity, frequently reordering closely eluting species — a peptide method at low pH and the same peptide at neutral pH are substantially different separations. Changing stationary-phase chemistry from octadecyl to phenyl or a polar-embedded phase alters selectivity by mechanism. Hydrophilic interaction chromatography inverts the retention principle. Ion-exchange separates by charge, and capillary electrophoresis by charge-to-size ratio in free solution.

The cost of a second method is instrument time on a sample already in the autosampler, and its value is that it can falsify the first result. Where the two agree, confidence rises substantially. Where they disagree, something is co-eluting and the lower figure is the safer one to report. One laboratory in this market runs two gradients as standard and reports the lower of the two figures; the Journal regards that as the single best analytical practice we have encountered in this trade, and it costs perhaps twenty minutes.²

Five things a purity figure cannot tell you

First, how much peptide is in the vial. Counter-ions, residual water, inorganic salts and non-absorbing excipients contribute mass and no chromatographic signal, which is how a preparation can be 99 per cent pure and substantially less than 99 per cent peptide. Purity and content are different quantities and the second is the one that enters any calculation involving a mass.

Second, whether anything is aggregated. Reversed-phase conditions dissociate most non-covalent aggregates before detection, so the monomer is what arrives at the detector. Only a size-based separation reports high molecular weight species.

Third, whether the sequence is correct. Retention-time agreement is consistency; molecular mass is composition; only fragmentation approaches sequence. Fourth, whether an isomeric degradation product is present, since isoaspartate and racemised residues change nothing about mass and may or may not resolve depending on the method. Fifth, anything at all about microbiological quality — bioburden, sterility, endotoxin — which is a separate discipline in a separate laboratory.

Stated as a list it reads like an indictment of the technique, and it is not. Reversed-phase chromatography answers its own question superbly. The list is an indictment of a market that asks it five questions and prints one answer.

A note on method and sourcing

The compendial and regulatory material in this piece is taken from the current general chapters on chromatography and on validation of compendial procedures, from the European Pharmacopoeia chapters on liquid chromatography and on chromatographic separation techniques, and from the harmonised guidelines on analytical validation, on impurities and on specifications for biotechnological products, all read in the original. The separation science is drawn from the chromatography literature, with the peptide-specific behaviour cited where it differs from small-molecule practice.

Where the Journal reports a number it obtained, it states the number of vials, the number of laboratories, whether the vials came from one lot, whether the laboratories knew, and what method parameters were disclosed to us. Where we quote a figure from a certificate we state whether the method was disclosed on it. Where a laboratory or a company answered our questions we distinguish an answer from a refusal and a refusal from a non-response.

Nothing in this department is a recommendation to buy, use or avoid anything. The compounds referred to are sold for research use only and are not approved for human use in any jurisdiction. Corrections and disputes go to standards@compoundjournal.com. Readers with certificates or chromatograms they would like read should write to letters@compoundjournal.com; we do not identify the source of anything sent to us, and we do not publish a reader’s name without permission.

Dissolution, pH and what happens before the injection

A peptide in a vial exists in whatever state the manufacturer left it. Reconstituted in water, a hydrophobic sequence may not dissolve completely and the chromatogram will show particles or aggregates. Reconstituted in an acidic buffer, the same sequence dissolves and the chromatogram shows monomer. The purity figure—and the mass balance—changes accordingly. Sample preparation is not one of the twelve values that belong on a method disclosure, and yet it is one of the most consequential, because it determines what population the peptide is actually in when the injection happens.³

The trade addresses this by assuming that samples are dissolved in the mobile phase or its aqueous component, and the assumption is sometimes true and sometimes false. A certificate that states how the sample was dissolved, at what concentration, in what solvent and after what incubation time, is one that can be repeated. A certificate that does not is one that will produce different results if the receiving laboratory uses different dissolution practice, which is particularly consequential for poorly soluble sequences.

Readers who take one habit from this piece should take the second method. A single separation cannot detect its own co-elution, and a second run on a different principle costs instrument time on a sample already in the autosampler. Where two orthogonal figures agree, a purity claim has survived an attempt to break it. Where they disagree, the lower number is the one to write down.