What high resolution actually buys you

Parts per million are a flattering unit, and they need converting before they mean anything. Five parts per million on a peptide of four thousand daltons is a tolerance of two hundredths of a dalton, which is excellent and sufficient to exclude most compositional substitutions. One hundred parts per million on the same peptide is four tenths of a dalton: enough to confirm the compound is broadly the right one, not enough to exclude a deamidated form. Five hundred parts per million, an ordinary figure for a linear time-of-flight instrument at this mass, is two daltons, a tolerance inside which most of the interesting questions disappear.

Electrospray, and why it produces a family of ions

Electrospray ionisation works by pumping a solution of the analyte through a fine capillary held at a potential of a few kilovolts relative to the instrument’s entrance. The liquid emerging from the tip forms a cone and then a jet of charged droplets. As solvent evaporates the droplets shrink, the charge density on their surfaces rises, and at the point where electrostatic repulsion exceeds surface tension they fission into smaller droplets. Repeat this enough times and what is left is a bare, charged analyte ion in the gas phase.

Because the charge is acquired in solution and retained through desolvation, a peptide with several basic residues will carry several protons, and the population of ions reaching the analyser is distributed across charge states. This is the defining characteristic of electrospray spectra and the reason they look bewildering to a first-time reader: a single pure compound produces four or five prominent peaks, none of them at the molecular weight.

The distribution is not noise. It carries information about the number of accessible basic sites and about the conformational state of the molecule in solution, and it shifts predictably with mobile-phase composition and pH. It also has a practical advantage that matters for identity work: dividing the mass by three or four brings a large peptide into the range where instruments achieve their best resolving power and accuracy.¹

MALDI, the matrix, and the singly charged ion

Matrix-assisted laser desorption ionisation takes a different route. The peptide is mixed with a large molar excess of a small organic compound that absorbs strongly at the laser wavelength — α-cyano-4-hydroxycinnamic acid and sinapinic acid are the usual choices for peptides and proteins respectively — and the mixture is dried on a metal target. A pulsed ultraviolet laser strikes the crystal, the matrix absorbs the energy, and a plume of matrix and analyte is ejected into the vacuum with the analyte largely intact and mostly singly protonated.

Two consequences follow. First, MALDI spectra are simple: one predominant ion per compound, at the molecular weight plus one proton, which makes them easy to read and easy to print on a certificate. Second, MALDI is markedly more tolerant of salts, buffers and heterogeneous samples than electrospray, which is why it survives in routine synthesis monitoring where electrospray would require a chromatographic clean-up first.

The trade-offs are equally real. The matrix produces intense chemical background below roughly a thousand daltons, which obscures small fragments. Ion yield varies between compounds and between spots on the same target, making MALDI a poor quantitative technique. And the achievable mass accuracy on a linear instrument at peptide molecular weights is measured in hundreds of parts per million unless a reflectron and delayed extraction are in use.²

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.

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.³

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.⁴

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.⁵ 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.

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.⁶⁷

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 next piece in this department takes the document rather than the instrument as its subject: what a certificate of analysis contains, what it systematically omits, and how to check one in the time it takes to drink a coffee. Identity is one line on that page, and by the standards of the rest of it, one of the better-behaved ones.