Mass spectrometry confirmation: what an expected mass tells you

Last reviewed 9 August 2026

A certificate of analysis usually carries two numbers: a purity figure, and a mass — printed either as a calculated value beside the word conforms, or as an observed value against a calculated one. This page is about the mass: which of several a molecule has, why they differ, how to compute the right one from a sequence, and what agreement between measured and calculated figures establishes.

The short answer to the last is that it establishes elemental composition — narrower than the word identity implies, and the distance between the two is where the useful reading happens.

A molecule does not have one mass

Yergey and colleagues set this out in 1983, calculating molecular ion distributions across 1,000, 10,000 and 100,000 amu and asking which of four quantities is useful at which size. The four are still in use.

Nominal mass is bookkeeping, and most abundant mass matters only where it stops coinciding with the monoisotopic mass. The working pair is monoisotopic and average. A document reporting a mass without saying which it means has left a systematic difference unresolved — 0.58 Da here, more than a hundred times the accuracy a high-resolution instrument is specified to.

Where the difference comes from

Elements occur as mixtures of stable isotopes. Monoisotopic mass takes one nuclide per element and uses its exact mass; average mass takes the natural mixture and weights by abundance.

Monoisotopic mass is therefore a property of one molecular species — the one in which every atom is the light isotope. Average mass is a property of a population.

The average mass is a softer number than it looks

Standard atomic weights are not constants. IUPAC publishes several as intervals, because the isotopic composition of normal terrestrial materials varies between sources: carbon is [12.0096, 12.0116], sulfur [32.059, 32.076].

Those intervals propagate coherently rather than cancelling, because every carbon atom in one preparation shares one isotopic composition. The single sulfur in Semax spans 0.017 Da, about 21 ppm of an 814 Da molecule; the 62 carbons in BPC-157 span 0.124 Da, about 87 ppm. An average mass quoted to two decimal places is quoted past the precision its definition carries — which is why two calculators disagree in the second decimal on one formula: one holds the 2021 atomic weights, the other a table from the 1990s.

Monoisotopic mass has no such ambiguity. Nuclide masses are known to parts per billion, and the isotope is specified rather than sampled.

Why the choice changes with molecular size

The height of the first isotope peak relative to the monoisotopic peak is close to the sum of the abundance-weighted contributions of every atom that has a heavier isotope one mass unit up:

A+1 / A ≈ (nC × 0.0107) + (nN × 0.00364) + (nH × 0.000115) + (nO × 0.00038) + (nS × 0.0075)

For Semax that gives about 0.45; for BPC-157, with 62 carbons, about 0.74. Carbon dominates the expression, and a peptide carries roughly one carbon per 22 Da, so the ratio reaches 1 near 84 carbons — somewhere around 1,800 to 1,900 Da. Above that the monoisotopic peak is no longer the tallest in the envelope, and it keeps shrinking.

Senko, Beu and McLafferty addressed the consequence: for large biomolecules the monoisotopic peak may be absent from the spectrum altogether, and assigning it by eye produces errors of about ±1 Da even when the decimal places look convincing. Their answer was to fit the observed envelope against a modelled one rather than pick a peak. Hence average mass for proteins, monoisotopic mass for peptides, and a certificate naming neither has not finished the job.

Computing an expected mass from a sequence

The arithmetic is addition. Each amide bond forms with the loss of one water molecule, so:

peptide mass = sum of the residue masses + one water

Water is 18.010565 monoisotopic and 18.015 average. The residue masses are fixed:

Two worked examples

Semax is Met-Glu-His-Phe-Pro-Gly-Pro. Summing the monoisotopic column gives 795.33740; plus one water, 813.34796. The average column plus 18.015 gives 813.92. The implied composition is C₃₇H₅₁N₉O₁₀S; computing directly from that formula returns 813.34796, which is also the figure PubChem records for CID 9811102. Two independent routes to one value is what a check is for.

BPC-157 is Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val, C₆₂H₉₈N₁₆O₂₂. Residue sum plus water gives a monoisotopic mass of 1418.704. The average mass is 1419.5 — and to two decimal places it is 1419.54 or 1419.56 depending on which atomic-weight table the software carries, which is the earlier point arriving in practice.

Modifications move the total by fixed amounts

A terminal or side-chain modification adds or removes a defined group, so its effect is a constant added to the residue sum. Unimod compiles these values from elemental compositions; the compositions are given below, so each figure can be re-derived from the nuclide table.

The last six are not decoration. D’Hondt and colleagues catalogue the impurities arising in solid-phase synthesis, naming incomplete side-chain deprotection, side-chain oxidation and unwanted counter-ions such as trifluoroacetate. An unexplained peak 56 or 252 Da above the target is a specific hypothesis, not noise.

From a neutral mass to a number on the spectrum

The masses above are for the neutral molecule. A mass spectrometer measures mass-to-charge ratio, and in positive-ion electrospray a peptide is usually observed with one or more added protons:

m/z = (M + n × 1.007276) / n

The constant is the mass of a proton, not of a hydrogen atom. The difference is one electron, 0.000549 Da — negligible at low resolution, and 0.7 ppm at 800 Da, which is not negligible against a claimed accuracy of 5 ppm. For Semax the series runs 814.355 at one charge, 407.681 at two and 272.123 at three.

Fenn and colleagues described in 1989 how electrospray produces coherent sequences of multiply charged ions differing by one charge; Mann, Meng and Fenn set out the interpretation the same year. Charge is read from isotope spacing: consecutive isotopologues differ by 1.00336 Da, so they sit that value divided by the charge apart in m/z — about 1.00 at one charge, 0.50 at two, 0.33 at three. A reported figure is therefore either a raw m/z at a stated charge or a deconvoluted neutral mass, and the two are not interchangeable.

Three cases in this catalogue:

• A permanently charged molecule is not protonated. 5-Amino-1MQ is a quaternary quinolinium cation, already charged in the solid state, so positive-ion electrospray detects the intact cation near m/z 159.09 rather than a protonated species one unit higher. An analyst expecting the higher figure records a failed confirmation that has not failed.
• The counter-ion is invisible. Iodide, trifluoroacetate and acetate contribute to the mass on the balance, not to the cation signal — half of why a mass spectrum says nothing about how much peptide is in a vial. Residual water is the other half; both belong to net peptide content rather than to identity.
• An envelope can carry an element signature. Copper’s two stable isotopes lie 1.998 Da apart in roughly 69:31 abundance, so a copper-containing species such as the complex described in the GHK-Cu entry shows a characteristic pair of peaks at that ratio. The shape of an envelope is evidence in its own right.

What agreement establishes

Chrone, Lorentzen and Højrup describe mass spectrometry as an optimal method for evaluating the authenticity and integrity of a synthetic peptide, because the sequence is already known and the analysis is directed at confirming it. A conforming mass is confirmation against a hypothesis, not identification from first principles.

What it confirms is that the material giving rise to that ion has the elemental composition expected of the target sequence, to whatever tolerance the laboratory applied — and the tolerance is the part usually left off the page. Brenton and Godfrey distinguish exact, accurate and measured mass and set out how deviation should be reported; Zubarev and Mann separate anecdotal from statistical mass accuracy, and argue for the statistical definition. USP’s general chapter opens by describing the technique as measurement of the mass-to-charge ratio of ionic species related to the analyte under investigation — related to, the same caution in pharmacopoeial phrasing. A certificate reading only MS: conforms has used the word without disclosing which sense it means.

What agreement does not establish

Sequence order

Composition is order-independent: transposing two residues leaves the formula and both masses identical, so every permutation of a sequence is isobaric with it. Order comes from fragmentation — tandem mass spectrometry, in which a selected ion is dissociated and the fragment series read — which Steen and Mann review as the technology underlying mass-spectrometric protein work. A single-stage measurement is not that experiment.

Stereochemistry

Substituting a D-amino acid for its L-form changes no atoms. D’Hondt and colleagues record that Fmoc deprotection can cause racemisation and hence diastereomeric impurities; Lian and colleagues regard structural isomers and peptide epimers as a distinct analytical problem requiring dedicated methods. As Yang and Zubarev put it of a related case, isomerisation does not change elemental composition by definition and is therefore silent in the mass spectrometric sense.

Leucine against isoleucine

Both are C₆H₁₁NO as residues, 113.084064 monoisotopic. No mass measurement at any resolution separates them, and neither does ordinary tandem mass spectrometry, since their immonium ions are identical too. Armirotti, Millo and Damonte assessed ten synthetic peptides differing only in Ile/Leu position and used consecutive low-energy ion-trap stages to dissociate the shared immonium ion into two distinguishable products — a deliberate extra experiment, not a by-product of the identity check.

The one-dalton region

Several real differences live within about a dalton of zero, which is exactly where a misread isotope peak also lives.

• Glutamine and lysine differ by 0.036 Da as residues — resolvable, but only on an instrument with the resolving power for it.
• Deamidation adds 0.984 Da, the difference between −NH₂ and −OH, as Yang and Zubarev state; C-terminal amidation subtracts the same 0.984. The same arithmetic in opposite directions.
• Misassigning the monoisotopic peak in a partly resolved envelope shifts the answer by a whole dalton with the decimals still looking correct — the failure Senko and colleagues built their fitting method to remove.

How much of it there is

Peak height is not proportional to molar quantity. Cech and Enke review the fundamentals: electrospray response tracks chargeability and surface activity, so two species at equal concentration ionise unequally. Annesley describes ion suppression, in which co-eluting material alters droplet formation or evaporation and reduces the signal reaching the detector. A species that ionises poorly can be present and nearly unseen; one that ionises well can dominate a spectrum at low abundance. Quantity is a chromatographic and gravimetric question, handled on the certificate page. A mass result is equally silent on residual water and cake condition, and on endotoxin and sterility, which are separate tests with separate methods.

Reading a reported mass, in order

1. Monoisotopic or average? Unstated, the figure cannot be compared with a calculated one below about a dalton of tolerance.
2. m/z or neutral mass? If m/z, at what charge, and is the added constant a proton or a hydrogen atom?
3. What is the acceptance tolerance? Conforms without a window is an assertion, not a measurement.
4. Free peptide or salt? Counter-ions are weighed and not observed.
5. Does the molecule’s size fit the figure reported? A monoisotopic value much above 10 kDa invites a question about how it was assigned.
6. Does the sequence reproduce the number? Residue sum plus water. Two routes, one answer.

Where this appears on these pages

Some entries in the compound library print the expected mass in both forms, and several add the calculated ions at the charge states an electrospray spectrum is most likely to show. Without the monoisotopic figure the average value is the only number available, and it is the softer of the two.

NovoVita’s published position on purity is one sentence — Third-party tested at greater than 99% purity.

References

1. Yergey J, Heller D, Hansen G, Cotter RJ, Fenselau C. Isotopic distributions in mass spectra of large molecules. Anal Chem. 1983;55(2):353–6. DOI 10.1021/ac00253a037. Computational and analytical study.
2. Senko MW, Beu SC, McLafferty FW. Determination of monoisotopic masses and ion populations for large biomolecules from resolved isotopic distributions. J Am Soc Mass Spectrom. 1995;6(4):229–33. PMID 24214167. DOI 10.1016/1044-0305(95)00017-8. Analytical method.
3. Zubarev R, Mann M. On the proper use of mass accuracy in proteomics. Mol Cell Proteomics. 2007;6(3):377–81. PMID 17164402. DOI 10.1074/mcp.M600380-MCP200. Methodology.
4. Brenton AG, Godfrey AR. Accurate mass measurement: terminology and treatment of data. J Am Soc Mass Spectrom. 2010;21(11):1821–35. PMID 20650651. DOI 10.1016/j.jasms.2010.06.006. Methodology.
5. Fenn JB, Mann M, Meng CK, Wong SF, Whitehouse CM. Electrospray ionization for mass spectrometry of large biomolecules. Science. 1989;246(4926):64–71. PMID 2675315. DOI 10.1126/science.2675315. Analytical method.
6. Mann M, Meng CK, Fenn JB. Interpreting mass spectra of multiply charged ions. Anal Chem. 1989;61(15):1702–8. DOI 10.1021/ac00190a023. Analytical method.
7. Chrone VG, Lorentzen A, Højrup P. Characterization of synthetic peptides by mass spectrometry. Methods Mol Biol. 2024;2821:83–9. PMID 38997482. DOI 10.1007/978-1-0716-3914-6_7. Methods chapter.
8. Steen H, Mann M. The ABC’s (and XYZ’s) of peptide sequencing. Nat Rev Mol Cell Biol. 2004;5(9):699–711. PMID 15340378. DOI 10.1038/nrm1468. Review.
9. Armirotti A, Millo E, Damonte G. How to discriminate between leucine and isoleucine by low energy ESI-TRAP MSⁿ. J Am Soc Mass Spectrom. 2007;18(1):57–63. PMID 17010643. DOI 10.1016/j.jasms.2006.08.011. Analytical method, synthetic peptides.
10. Yang H, Zubarev RA. Mass spectrometric analysis of asparagine deamidation and aspartate isomerization in polypeptides. Electrophoresis. 2010;31(11):1764–72. PMID 20446295. DOI 10.1002/elps.201000027. Review.
11. D’Hondt M, Bracke N, Taevernier L, Gevaert B, Verbeke F, Wynendaele E, De Spiegeleer B. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2–30. PMID 25044089. DOI 10.1016/j.jpba.2014.06.012. Review.
12. Lian Z, Wang N, Tian Y, Huang L. Characterization of synthetic peptide therapeutics using liquid chromatography-mass spectrometry: challenges, solutions, pitfalls, and future perspectives. J Am Soc Mass Spectrom. 2021;32(8):1852–60. PMID 34110145. DOI 10.1021/jasms.0c00479. Review.
13. Cech NB, Enke CG. Practical implications of some recent studies in electrospray ionization fundamentals. Mass Spectrom Rev. 2001;20(6):362–87. PMID 11997944. DOI 10.1002/mas.10008. Review.
14. Annesley TM. Ion suppression in mass spectrometry. Clin Chem. 2003;49(7):1041–4. PMID 12816898. DOI 10.1373/49.7.1041. Review.
15. Creasy DM, Cottrell JS. Unimod: protein modifications for mass spectrometry. Proteomics. 2004;4(6):1534–6. PMID 15174123. DOI 10.1002/pmic.200300744. Database description.
16. Prohaska T, Irrgeher J, Benefield J, Böhlke JK, Chesson LA, Coplen TB, et al. Standard atomic weights of the elements 2021 (IUPAC Technical Report). Pure Appl Chem. 2022;94(5):573–600. DOI 10.1515/pac-2019-0603. Standards body technical report.
17. IUPAC Commission on Isotopic Abundances and Atomic Weights. Standard atomic weights, Abridged standard atomic weights and Isotopic compositions of the elements, ciaaw.org. Reference data.
18. National Institute of Standards and Technology. Atomic weights and isotopic compositions with relative atomic masses, NIST Standard Reference Database 144. Reference data.
19. United States Pharmacopeia. General Chapter ⟨736⟩ Mass Spectrometry. USP–NF. Pharmacopoeial general chapter.
20. National Center for Biotechnology Information. PubChem compound summaries for CID 9811102 (Semax) and CID 9941957 (BPC-157). Database records.