What orthogonal actually means, and what it does not

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.

Injection load, linearity and the flattened peak

A detector responds linearly to concentration over a defined range and then stops. Overload the column or saturate the detector with too much sample and the main peak flattens at the top, its apex broadens, and its integrated area no longer represents the quantity present. Since the main peak is the numerator and dominates the denominator, distorting it distorts the purity figure — usually downwards, because the flattened peak loses area relative to a properly loaded one.

There is a competing pressure, and it is the reason overloading happens. Small impurities near the reporting threshold need adequate signal-to-noise to be integrated at all, and the way to raise their signal is to inject more sample. A laboratory hunting for 0.05 per cent impurities is tempted towards a load that compromises the main peak. The correct answer in regulated practice is two injections: a small load for the main peak and a larger one for the related-substances profile, with the results combined.

Column overload is a separate phenomenon from detector saturation and produces a characteristic asymmetric fronting peak. Both are visible on the chromatogram to anybody who is shown it, which is one of several reasons the Journal asks for the trace rather than the number. A purity figure calculated from a distorted main peak is arithmetically correct and analytically meaningless, and the only way to know is to look.¹

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.²

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.³

The response-factor problem is the honest limitation on all of this and it is worth stating plainly. Area per cent approximates a mass fraction only if every species absorbs alike, and they do not. Correcting for it properly requires isolated impurity standards that nobody in this trade holds.

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.

Readers occasionally ask why this publication does not simply publish a league table of purity figures. Because a table of numbers produced by undisclosed methods is a table of methods, and ranking suppliers on it would be ranking their chromatography departments rather than their chemistry.

The technique is not on trial here and never was. Reversed-phase chromatography can resolve species differing by a single methyl group, and the laboratories running it for this market are, on the evidence we have gathered, largely competent and entirely willing to describe what they did when somebody asks. What is on trial is a document format that omits the four values needed to compare one figure with another.