What “LC-MS: conforms” tells a reader, which is almost nothing

What the pharmacopoeias require here is instructive, because the requirement is not onerous. A compendial identity test states the technique, the reference against which the article is compared, and the acceptance criterion. That is the whole architecture: technique, reference, criterion. A certificate reading “LC-MS: conforms” supplies a technique in the loosest possible sense, no reference, and no criterion. It is not a false statement. It is a statement with no content.

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.

The charge-state arithmetic, worked

For a peptide of neutral monoisotopic mass M observed as a protonated ion carrying z protons, the mass-to-charge ratio is (M + z × 1.00728) divided by z, where 1.00728 is the mass of a proton — the mass of a hydrogen atom less the mass of an electron, a distinction that matters at parts-per-million accuracy and not at all below it.

Run this for a peptide of average mass 4113.58. The singly protonated ion appears at 4114.59. The doubly protonated ion appears at 2057.80, the triply at 1372.20, the quadruply at 1029.40 and the quintuply at 823.72. All five describe the same molecule. A reader shown only the fourth of those figures, without a charge assignment, would reasonably conclude the vial contained a peptide of about a thousand daltons.

Inverting the calculation is how the neutral mass is recovered: multiply the observed m/z by the charge and subtract z proton masses. Doing this for two or three charge states from the same spectrum and finding agreement to within the instrument’s stated accuracy is the standard internal consistency check, and it is the check that catches a misassigned charge. A single m/z with a single assumed charge has no such redundancy, which is one reason electrospray with a visible charge-state envelope is more informative than a single MALDI peak even when both instruments are equally well calibrated.

Mass accuracy, and what a tolerance ought to be

Mass accuracy is the difference between the measured mass and the true mass, expressed in parts per million of the measured value. It depends on calibration, on the stability of the instrument’s electronics and temperature, on the number of ions arriving at the detector, and on whether an internal calibrant was co-analysed with the sample. It is not a fixed property of an instrument; it is a property of a measurement made on an instrument on a particular day.

Certificates in this market seldom state a tolerance at all. Where they do, the figure is usually expressed in daltons rather than parts per million and is generous: ±0.5 or ±1.0 dalton is common, which at incretin molecular weights corresponds to 120 to 240 parts per million and is achievable on almost any instrument sold in the last thirty years. A tolerance that no plausible measurement could fail is not an acceptance criterion. It is a formality.

What a meaningful criterion looks like is not mysterious. State the theoretical mass and its convention, state the observed mass, state the deviation in parts per million, and state the limit above which the result would have been reported as non-conforming. Four numbers, all of them already known to the analyst. The compendial framework for validating an analytical procedure asks for exactly this kind of specificity about what a test can discriminate, and the framework predates this market by decades.¹

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.² 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.

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.

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.³

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.

Six lines that would make an identity claim checkable

The Journal has settled on a short list, arrived at by writing to laboratories and asking what they could supply without additional work. Six lines. The ionisation source and mode. The analyser, named by class at minimum and by model preferably. The theoretical mass, with the convention stated as monoisotopic or average. The observed mass, with the charge state from which it was derived. The deviation, expressed in parts per million. And the acceptance criterion that was applied.

Every one of those is in front of the analyst at the moment the report is generated. None is commercially sensitive. Together they convert a verdict into a measurement, because they allow a reader to determine what the test could have detected and what it could not. A document carrying those six lines can be assessed by somebody who has never seen the sample; a document reading “MS: conforms” cannot be assessed at all, by anybody, including the person who wrote it.

The compendial approach to identity testing is built on the same three elements — a technique, a reference, and a criterion — and asks for them to be stated because a test whose discriminating power is undocumented has not been validated in any meaningful sense.⁴ We are not asking this market to become a regulated one. We are asking it to print what it already knows.

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.

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.

One correction of emphasis, since this article has spent most of its length on what cannot be seen. Mass spectrometry is a triumph of twentieth-century instrumentation and it answers the question it is asked with a precision no other technique approaches. The problem examined here is not the method. It is the practice of asking it one question and printing the answer to a larger one.