The change that weighs nothing at all

The useful way to think about a mass spectrometer is as an instrument that answers one question extremely well and is silent on several adjacent ones. Deamidation of an asparagine residue converts an amide to a carboxylic acid and adds nine hundred and eighty-four thousandths of a dalton. Oxidation of a methionine adds one oxygen atom, fifteen point nine nine five daltons. Inversion of a single stereocentre from the L to the D configuration adds nothing whatever. Those three numbers — roughly one, roughly sixteen, and zero — describe the three most consequential things that go wrong with a synthetic peptide, and an instrument’s ability to detect them falls off in exactly that order.

Which source a laboratory chooses, and why it should say so

The choice between the two techniques is not a matter of quality but of question. A synthesis chemist watching a coupling proceed wants a fast, salt-tolerant check that the chain has grown by the expected residue, and MALDI on a bench instrument answers that in minutes. An analytical laboratory asked whether a submitted vial contains the labelled compound and nothing closely related to it needs the resolving power and the accuracy that electrospray into a high-field analyser provides, coupled to a chromatographic separation so that species which co-elute can at least be assigned to retention times.

Both appear in this market, and reports rarely distinguish them. That matters because the two techniques have different blind spots. MALDI can induce loss of labile modifications during desorption, so a phosphorylated or otherwise fragile species may be under-represented. Electrospray suppresses ionisation of some analytes in the presence of others, so a minor component of a mixture may be absent from a spectrum in which it is genuinely present.

A certificate stating the source therefore tells a reader which class of error to consider. The Journal has stopped asking suppliers for more testing and started asking them for this line instead, on the grounds that it costs nothing and changes what the existing test can be said to support.

Calibration, drift, and the internal standard

Every mass spectrometer is calibrated against a mixture of compounds of known exact mass, and every mass spectrometer drifts away from that calibration afterwards. The rate depends on the analyser type, on ambient temperature stability, and in trapping instruments on the number of ions in the trap: space-charge effects shift apparent masses in a manner that depends on how much sample was injected.

External calibration means the calibrant was run separately, before or after the samples. It is simple, it is what most routine work uses, and it is vulnerable to everything that happens between the calibration and the sample. Internal calibration means a compound of known mass was present in the same spectrum as the analyte, so the correction is applied to the measurement rather than to the instrument. Internal calibration is the reason sub-part-per-million figures are achievable at all, and it is the difference between a stated accuracy and a demonstrated one.

None of this is exotic or contested; it is ordinary laboratory practice, described in accreditation requirements as part of metrological traceability and in the pharmacopoeial chapters as part of system suitability.¹ The reason it belongs in an article aimed at buyers is that it explains why two competent laboratories analysing the same vial on the same class of instrument can differ by tens of parts per million, and why the honest response to such a difference is to ask about calibration rather than about honesty.

Tandem mass spectrometry, and how an instrument spells

Tandem mass spectrometry selects an ion of a particular mass-to-charge ratio, breaks it, and measures the masses of the pieces. Collision-induced dissociation and its higher-energy variant fragment the peptide predominantly at the amide bonds, producing two complementary series: b ions retaining the N-terminal portion and y ions retaining the C-terminal portion. The mass difference between consecutive members of either series is the residue mass of one amino acid, so reading the series in order reads the sequence.

Electron-transfer dissociation fragments differently, producing c and z ions, and preserves labile modifications that collisional methods tend to strip. Between them the two approaches cover most of what a peptide chemist needs. The nomenclature for these fragment series was fixed decades ago and is stable enough that a spectrum annotated in it can be read by anybody in the field.²

Two limitations should be stated because they are routinely elided. Fragmentation is not uniform along a chain: proline residues and basic residues bias cleavage, and stretches of a sequence can go unrepresented, which is why coverage is reported as a percentage rather than asserted as complete. And leucine and isoleucine remain indistinguishable under collisional fragmentation because their residue masses are identical; separating them requires side-chain fragmentation under specialised conditions, which almost nobody performs outside a research context.

Isobaric and near-isobaric substitutions

Two species are isobaric if their masses are identical to the precision of the measurement, and the term covers two quite different situations. True isobars have identical elemental compositions: leucine and isoleucine are structural isomers of one another, as are the aspartate and isoaspartate products of deamidation, and no mass measurement at any resolving power will separate them. Near-isobars have different compositions that happen to give similar masses, and these are resolvable given sufficient performance.

The canonical near-isobaric pair in peptide work is glutamine against lysine, differing by 0.036 daltons — nine parts per million on a four-thousand-dalton peptide, and therefore a discrimination that requires an orbital trap or better. A second is the classic composition ambiguity in which a combination of light elements substitutes for a heavier one at nearly the same nominal mass; the mass defect of hydrogen relative to the heavier elements is what makes these separable at high resolving power and indistinguishable at low.³

The reason this matters commercially is narrow but real. A synthesis error that substitutes one residue for another may be invisible on a low-resolution instrument, present at a few per cent, and chromatographically unresolved from the parent under a fast gradient. The combination of a twelve-minute purity method and a unit-resolution identity check is not a conspiracy; it is simply a pair of tests neither of which is looking in that direction.

Deamidation: 0.98 daltons, and where it hides

Deamidation of asparagine proceeds through a five-membered succinimide intermediate formed by nucleophilic attack of the following residue’s backbone nitrogen on the asparagine side-chain carbonyl. Hydrolysis of the intermediate yields aspartate or isoaspartate, in a ratio typically favouring the isoaspartate form. Glutamine deamidates by an analogous but slower route. The rate depends strongly on pH, temperature, and the identity of the residue immediately following the asparagine, with glycine and serine accelerating it markedly.⁴

The analytical difficulty is threefold. The mass increase is 0.984 daltons, which requires only modest resolving power to see at low molecular weight and becomes demanding as the peptide gets larger. The aspartate and isoaspartate products are exactly isobaric with one another, so distinguishing them requires either a chromatographic separation that happens to resolve them or a specific enzymatic assay. And deamidated species often elute close to the parent under reversed-phase conditions, so a fast gradient may not separate them either.

The result is a degradation product that is common, that has real consequences for biological activity, that accumulates in storage, and that a certificate produced by a unit-resolution instrument on a twelve-minute gradient is structurally unable to detect. When the Journal describes a certificate as silent on stability, this is a large part of what is meant.

Oxidation: sixteen daltons, and the one that gets caught

Methionine oxidises to the sulphoxide with a mass increase of 15.995 daltons, and on to the sulphone at a further 15.995. Tryptophan and histidine oxidise by related routes. The chemistry is driven by dissolved oxygen, by trace peroxides in excipients and in some grades of polysorbate, by light, and by transition-metal contamination, and it proceeds in lyophilised material as well as in solution, though more slowly.

Analytically this is the easy case, and it deserves to be described as such in an article otherwise concerned with what cannot be seen. A sixteen-dalton shift is resolvable on essentially any instrument, and the oxidised species is usually chromatographically distinct enough to appear as a separate peak under a reasonably shallow gradient. A spectrum showing a plus-sixteen satellite at a few per cent of the parent intensity is unambiguous evidence of oxidation, and its absence is meaningful evidence of the opposite.

Which is why the Journal’s standing request to laboratories in this market is for the spectrum rather than the verdict. A reproduced spectrum, even at the modest resolution of a routine instrument, allows a reader to look for the plus-sixteen satellite themselves. A conformance statement does not, and cannot be made to. This is the cheapest available improvement to identity reporting in the trade and it consists of printing a picture the laboratory has already produced.

The inversion that weighs nothing

Amino acids other than glycine are chiral, and peptide synthesis is performed with L-configured building blocks. Racemisation during synthesis — most commonly at cysteine, histidine and aspartate residues, and promoted by prolonged base exposure during coupling and deprotection — produces a peptide containing one or more D residues. The resulting molecule has the same elemental composition, the same monoisotopic mass, the same average mass, and the same fragmentation masses as the intended product.

Mass spectrometry cannot detect it. This is not a limitation of any particular instrument; it is a consequence of what the technique measures. Reversed-phase chromatography sometimes separates diastereomeric peptides, and where it does the epimer appears as a shoulder or a satellite peak of unassigned identity — which is one reason a chromatogram with an unexplained minor peak deserves more attention than a purity percentage does. Where the epimer co-elutes, no routine analysis in this market would find it.

Deliberate detection requires chiral amino-acid analysis after total hydrolysis, or digestion with a stereospecific protease that fails to cleave across a D residue, or in some cases ion-mobility separation. None of these is offered as a standard service to private buyers by any of the four testing services this market relies on, and the Journal’s position is that this is a genuine gap rather than a failing on their part: nobody has ever been asked to price it.

The standing rule in this department

This publication applies one rule to every identity claim it reports, and it is worth stating in isolation because it governs the rest. A mass measurement supports a statement about composition. Only a fragmentation or mapping experiment supports a statement about sequence. Where a source says identity was confirmed, we report that a mass was measured, unless we have seen evidence of the second kind.

The rule has consequences we accept. It makes our coverage read as more sceptical than the underlying documents, because the documents claim more than they establish. It occasionally irritates laboratories which have in fact done sequence-level work and have simply not printed it, and the remedy there is a two-line email which we are glad to receive. And it means we cannot describe any research-grade vial in this market as sequence-confirmed, because on the evidence available to us almost none are.

What the rule is not is an accusation. Nothing in this article suggests that vendors are selling material other than what they label, and the Journal has no evidence of that in respect of any company it covers. The claim is narrower and, we think, harder to argue with: the documentation in general circulation does not have the discriminating power that the language on it implies, and the gap between the two is where every avoidable dispute in this market begins.

If this market spent one more pound on identity, where should it go

A fair question, and the Journal’s answer has changed. Our first instinct was to argue for sequence confirmation on every lot, and the arithmetic does not support it: peptide mapping on every batch would raise the analytical cost per vial by a multiple, and the failure mode it protects against — a wholly substituted or permuted sequence — is not the one we see evidence of.

The better allocation, on our present assessment, is orthogonal. Identity by high-resolution intact mass on every lot, at a resolving power sufficient to resolve a one-dalton shift at the parent mass, with the spectrum reproduced. Sequence confirmation once per synthesis campaign rather than once per lot, on the reasoning that the sequence is a property of the process and the lot-to-lot risk is degradation rather than misconstruction. And a chromatographic method shallow enough to separate the deamidated form, because that is the change most likely to have occurred between the certificate and the buyer.

That package is not expensive. Two of the twenty companies in our dossier programme already do something close to the first item, and one has told us it is costed for the second. Whether any of it happens depends on whether buyers ever ask, which is a market question rather than a scientific one and is therefore the harder of the two.

Readers who take one thing from this piece should take the arithmetic. Isotope spacing is one over the charge. Deamidation is one dalton, oxidation is sixteen, and stereochemical inversion is nothing at all. A tolerance of ±1 dalton on a four-thousand-dalton peptide is two hundred and forty parts per million and excludes almost nothing worth excluding. Those four facts are sufficient to read most of the identity claims in circulation, and they fit on the back of an envelope.