Tandem mass spectrometry, and the difference between weighing and reading

Consider the arithmetic. A peptide of thirty residues built from twenty available amino acids has on the order of ten to the thirty-ninth possible sequences. The number of those sequences with an identical elemental composition — and therefore an identical mass to any achievable precision — is not one. Any permutation of the same residues weighs exactly the same. Any exchange of leucine for isoleucine weighs exactly the same. Any inversion of stereochemistry weighs exactly the same. An intact mass measurement is blind to all of it, by construction rather than by inadequacy.

A mass that matches, and the space of things it does not exclude

Suppose a laboratory reports an observed monoisotopic mass within two parts per million of the theoretical value for the labelled peptide. What has been established is that the sample contains a species whose elemental composition is either identical to the target or differs from it in a way that happens to conserve mass to within that tolerance. This is genuinely strong evidence, and it is not identity.

The set of molecules consistent with that observation includes every permutation of the target sequence, every substitution of leucine for isoleucine and vice versa, every inversion of stereochemistry at any of the chiral centres, every migration of aspartate to isoaspartate, and — at tolerances above roughly ten parts per million — every glutamine-for-lysine exchange. It also includes any unrelated molecule of coincidentally matching composition, though in practice the chromatographic retention time excludes most of those.

The proteomics literature has spent two decades formalising exactly this problem under the heading of identification confidence, developing false-discovery-rate frameworks precisely because a matching mass is a weak identifier and a matching fragmentation pattern is a strong one.¹ The research-peptide trade has borrowed the instrument from that field and not the epistemology, and the result is a market in which the word confirmed is applied to the weakest available evidence.

Peptide mapping: the test that reads the chain

Peptide mapping is the standard method by which the primary structure of a peptide or protein product is verified. The material is digested with a protease of defined specificity — trypsin cleaving after lysine and arginine, Lys-C after lysine alone, chymotrypsin after aromatic residues — and the resulting fragments are separated by reversed-phase chromatography with mass detection. Each fragment’s observed mass is matched against the masses predicted from the expected sequence, and the fraction of the chain accounted for is reported as sequence coverage.

A digest that returns every predicted fragment at the predicted mass is a far stronger identity statement than an intact mass, because it constrains the order of residues in a way that an intact measurement does not: a permuted sequence generally produces different cleavage products. It is not complete on its own, because a fragment mass is subject to the same permutation ambiguity in miniature, which is why serious mapping proceeds to a second stage of mass analysis on the fragments themselves.

For synthetic peptides of thirty to forty residues, mapping is straightforward chemistry and unremarkable chromatography, and the reason it does not appear on certificates in this market is cost and turnaround rather than difficulty. The regulatory expectation for a peptide product characterised as a biotechnological article treats structural confirmation of this kind as a matter of routine rather than of specialism.²³

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.⁴

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.

Sequence coverage as a reported number

Where a peptide map is performed, the headline output is a coverage figure: the percentage of residues in the expected sequence accounted for by identified fragments. Ninety-five per cent coverage sounds close to complete and is worth interrogating, because the five per cent that is missing is not randomly located. Very short fragments elute in the solvent front and are lost. Very hydrophobic fragments retain on the column. Regions between closely spaced cleavage sites produce peptides too small to identify unambiguously.

The consequence is that the uncovered fraction tends to sit in the same places for a given protease and a given sequence, which means a laboratory reporting ninety-five per cent coverage in run after run has ninety-five per cent coverage of a specific ninety-five per cent. A second digest with a different enzyme is the conventional remedy, and a report that used two orthogonal proteases is doing something a report using one cannot.

For a reader assessing a document, the useful questions are which enzyme, what coverage, and whether the uncovered residues are identified. A map that names the missing stretch has told you where the residual uncertainty lives. A map that reports a percentage alone has told you a number whose meaning depends on information it withheld — which is, in a different guise, the same complaint this department makes about purity figures reported without a gradient.

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.

How an identity result travels once it leaves the laboratory

Follow a mass spectrum through the market and its meaning changes at every step. A laboratory issues a report to whoever submitted the sample, stating what was observed on a named instrument on a named date. The submitter — a vendor, in most cases — extracts a figure and a verdict onto a certificate of analysis for the lot. A reseller reproduces the certificate, or a portion of it. A listing page distils the whole chain into a phrase: identity verified.

Nothing dishonest need happen at any step for the final phrase to support far more than the original report does. The instrument’s resolving power is lost at step two. The convention behind the theoretical mass is lost at step two or three. The date, the batch and the submitter’s identity survive unevenly. By the time the claim reaches a buyer it has become a property of the product rather than a record of a measurement on one vial from one lot on one day.

This is the structural reason the Journal reports identity claims by asking for the underlying laboratory report rather than the certificate. When a supplier supplies it, the claim usually holds up and often turns out to be stronger than the certificate suggested. When a supplier cannot locate it, that is itself information about how far back the documentary chain reaches, and we report that too, without inferring anything about the material.

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.

A short glossary, because the terms are used loosely

m/z — mass-to-charge ratio, the quantity a mass spectrometer actually measures. Monoisotopic mass — mass calculated using the lightest stable isotope of each element. Average mass — mass calculated using standard atomic weights. Nominal mass — the integer sum of integer isotope masses; adequate for small molecules, useless here.

Resolving power — m divided by peak width at half height; the ability to separate nearby masses. Mass accuracy — deviation of a measurement from the true value, in parts per million. Mass defect — the difference between an exact mass and its nominal value, and the property that makes near-isobars separable.

Adduct — an ion formed by association with something other than a proton, commonly sodium or potassium. Charge-state envelope — the family of differently charged ions from one compound. Isobaric — of identical mass at the achieved precision. Isomeric — of identical composition and different structure. b and y ions — the complementary fragment series produced by amide-bond cleavage.

Precision in these terms is not decoration. Several disputes this department has been asked to adjudicate turned out, on inspection, to be disagreements about whether the word mass meant monoisotopic or average.

The uncomfortable conclusion of all this is not that identity testing in this market is worthless. It is that identity testing here is doing considerably less work than the language attached to it suggests, and that the shortfall is documentary rather than analytical. The instruments are capable. The laboratories are competent. What is missing is six lines on a page, and the reason they are missing is that nobody has ever declined a purchase for want of them.