Quadrupole, time-of-flight, orbital trap: what each class can and cannot see

The single hardest discrimination in routine peptide identity work is glutamine against lysine. The two residues differ in elemental composition by one carbon-hydrogen-hydrogen group swapped for one nitrogen-hydrogen, and the resulting mass difference is thirty-six thousandths of a dalton. On a peptide of four thousand daltons that is nine parts per million. An orbital trap or Fourier-transform instrument will separate it without difficulty. A quadrupole will not see it at all, and neither will most of the identity confirmations sold in this market.

The isotope pattern, and how it declares the charge

Because carbon-13 is present at roughly 1.1% natural abundance, a peptide containing one hundred and ninety carbon atoms will exist substantially as molecules containing one, two or three carbon-13 atoms. In a spectrum this appears as a series of peaks above the monoisotopic peak, separated in mass by approximately 1.00336 daltons and distributed in intensity according to the binomial statistics of the composition.

Two things follow, and both are practically useful. First, the spacing between adjacent isotope peaks in a charge-state cluster is one over the charge: a spacing of 0.5 on the m/z axis means the ion is doubly charged, 0.333 means triply, 0.25 means quadruply. This is the simplest charge assignment available and it requires no assumptions about the sample at all. Second, the relative intensities of the isotope peaks are predictable from the elemental composition, so a cluster whose shape departs markedly from the calculated envelope is evidence that two species are overlapping.

Both observations require an instrument capable of resolving the isotope peaks at the relevant m/z, which is where resolving power stops being a specification-sheet number and becomes the thing that determines whether a spectrum can be interpreted at all. Below roughly ten thousand resolving power, a multiply charged peptide envelope collapses into a single broad hump that carries neither the spacing nor the shape information.

Resolving power, defined and then converted

Resolving power is conventionally defined as m divided by Δm, where Δm is the width of the peak at half its maximum height. An instrument quoted at 30,000 resolving power at m/z 1000 produces peaks roughly 0.033 wide at that position, which is sufficient to separate the isotope peaks of a triply charged ion. The same instrument at m/z 4000 may deliver rather less, because resolving power is not constant across the mass range and the figure on a specification sheet is quoted at whichever mass flatters it.

The number that matters for identity work is whether the instrument can separate two species whose masses differ by the amount you care about. To distinguish a deamidated peptide from its parent at four thousand daltons requires separating peaks 0.98 apart, which is a resolving power of roughly four thousand — modest. To distinguish a glutamine-for-lysine substitution requires separating peaks 0.036 apart at the same mass, which is a resolving power above one hundred thousand. Those two requirements differ by a factor of twenty-five and both are described in the trade by the same phrase, high resolution.

The Journal’s practice, adopted after an exchange with a laboratory that pointed out we had been sloppy about it, is to state the discrimination rather than the specification: not “high-resolution MS”, but “sufficient to resolve a 0.98-dalton shift at the parent mass”. It is longer and it says something.¹

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.²

The instrument classes, and what each can support

A single quadrupole mass filter provides unit resolution and mass accuracy of a few tenths of a dalton. It is entirely adequate to confirm that a sample is broadly the compound expected and to detect large modifications, and it is the analyser in most low-cost LC-MS systems. It cannot resolve an isotopic envelope at peptide molecular weights and therefore cannot assign charge from spacing.

Time-of-flight analysers separate ions by the time they take to traverse a flight tube. A linear tube gives modest resolving power; adding a reflectron and delayed extraction raises it into the tens of thousands, and modern quadrupole time-of-flight hybrids achieve low single-figure parts-per-million accuracy with routine calibration. Orbital trapping instruments measure the frequency of ion oscillation in an electrostatic field and convert it by Fourier transform, delivering resolving powers from sixty thousand to several hundred thousand and sub-part-per-million accuracy with internal calibration. Fourier-transform ion cyclotron resonance remains the highest-performing class and the least common outside academic facilities.

What this hierarchy means for a reader of certificates is that the instrument named on the document sets a ceiling on what the document can claim, independent of the laboratory’s competence. An unnamed instrument leaves that ceiling unknown, which is why the Journal now treats the absence of an instrument name as a material omission rather than a stylistic one.

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.³ 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.

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.⁴

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.

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.

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.⁵ We are not asking this market to become a regulated one. We are asking it to print what it already knows.

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.

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.