Mass spectrometry for peptides: how identity is confirmed

Q: Why does the observed mass differ slightly from the expected mass?

Every instrument has a measurement error. A larger gap of about 0.06% can appear if one value is monoisotopic and the other is average.

By the PeptideLabCheck team · Last reviewed 14 November 2026 · 8 sources

Short answer

Mass spectrometry turns the molecules in a sample into charged particles and measures their mass-to-charge ratio. A lab works out the mass of the main compound from that measurement and compares it with the mass expected for the named peptide. A match is strong evidence that the right compound is present. On its own it does not show how much is present or how pure the sample is, and it cannot tell apart molecules that have the same mass.

What does a mass spectrometer actually measure?

A mass spectrometer does not weigh molecules directly. It first converts them into ions, which are molecules that carry an electric charge. It then sorts the ions by their mass-to-charge ratio, written m/z, and records how many ions arrive at each value ^{1, 3}.

The result is a mass spectrum. This is a plot of m/z values against signal strength. Each peak stands for an ion with a particular m/z ¹.

Because the instrument measures m/z and not mass, the charge matters. One university facility gives this example: a peptide of mass 2,000 with one charge and a peptide of mass 4,000 with two charges both give a single peak at m/z 2,001 ². The lab has to work out the charge before it can state the mass.

How is the measured mass compared with the expected molecular weight?

Every peptide has an expected mass. It is calculated from the atoms in its chemical structure ⁴. This number is often called the theoretical mass. Masses of molecules are given in daltons (Da) ⁴.

The lab compares the theoretical value with the value it observed. A paper by authors at the United States Pharmacopeia (USP), an organisation that publishes quality standards for medicines, shows what this looks like. For the peptide leuprolide, the theoretical m/z was 1209.6533 and the measured m/z was 1209.6515. The authors took this as confirmation of the molecular mass ⁵.

Two details affect the comparison:

Monoisotopic or average mass. Atoms occur naturally as isotopes, which are versions of the same element with slightly different masses. A peptide therefore gives a small cluster of peaks, not one. The monoisotopic mass is the first peak of that cluster. The average mass is the centre of the whole cluster. For peptides the two differ by about 0.06%. One example peptide has an average mass of 1086.28 and a monoisotopic mass of 1085.55 ⁴. The expected and observed values must be the same type.
Instrument accuracy. No measurement is exact. One university facility reports an average error below ±0.05 mass units for peptides of 100 to 2,500 Da on its instrument, and notes that the error for any one sample can be higher or lower ². The acceptable gap depends on the instrument and method.

What do LC-MS, ESI and MALDI-TOF mean?

These terms describe how the sample is prepared, ionised and measured.

Term	Stands for	What it does
LC-MS	Liquid chromatography–mass spectrometry	A liquid chromatography step is joined to a mass spectrometer, so the two tests run as one procedure ^{5, 6}.
ESI	Electrospray ionisation	A "soft" way to make ions that does not badly damage the molecule. It adds or removes one or more protons ³. It usually gives several peaks for one compound, each with a different number of charges ².
MALDI	Matrix-assisted laser desorption/ionisation	The sample is mixed with an energy-absorbing compound called a matrix and hit with a laser. It mainly gives ions with a single charge, which makes the spectrum simpler to read ^{2, 3}.
TOF	Time of flight	A type of analyser. It finds m/z by timing how long an ion takes to travel down a flight tube ³.
MS/MS	Tandem mass spectrometry	The peptide is broken into fragments and the fragments are measured. This is used to check the order of the amino acids, called the sequence ⁵.

With ESI, each of the several charged peaks gives its own measurement of the molecular mass. Software converts them into a single mass value ².

Why is identity testing separate from purity testing?

The two tests answer different questions.

Identity asks: is this the compound named on the label?
Purity asks: how much of what the detector saw is that compound, and how much is something else?

Purity is usually measured by HPLC (high-performance liquid chromatography), a method that separates the parts of a sample. Impurities are reported as a percentage of the total detected area ⁵.

International guidance on specifications lists identification, assay (how much is present) and impurities as separate tests. It also says an HPLC retention time on its own is not specific enough to identify a substance, while a combined test such as HPLC/MS is generally acceptable ⁶. So a purity figure does not prove identity, and a mass match does not prove purity. USP's own work on peptide reference standards uses several independent methods together to confirm identity ⁵.

What can a mass match rule out, and what can it not?

A mass match can rule out a compound whose mass is clearly different from the expected one. That includes a peptide that is missing an amino acid or carries an extra one, because the mass changes. Such deletions and insertions are known impurity types in peptides made by chemical synthesis ⁷.

A mass match cannot settle these points by itself:

How much is present. The amount of peptide is established by a separate assay, and by measuring water, counter-ions (the salt partner of the peptide) and other non-peptide material ⁵.
Leucine or isoleucine. These two amino acids have the same chemical formula and the same mass ⁸. USP describes using amino acid analysis when the two must be told apart ⁵.
Mirror-image forms. Amino acids exist in L and D forms with identical mass ⁸. A D form in place of an L form is a known impurity type ⁷. Checking for it needs chiral testing, a method that separates mirror-image forms ⁵.
Sequence. The intact mass does not show the order of the amino acids. MS/MS fragment data is used to confirm sequence ⁵.
Other substances in the sample. A peak at the right mass shows the compound is there. It does not show what share of the sample it makes up.

What should I look for on a report?

The name of the compound and its expected (theoretical) mass.
The observed m/z values, with the charge of each ion, or the mass calculated from them.
Whether the masses are monoisotopic or average.
The method used, such as LC-MS with ESI, or MALDI-TOF.
The spectrum itself, not only a pass or fail statement.
The sample or lot identifier and the test date.
A separate purity result, usually from HPLC.

If expected and observed values are both shown, you can check the comparison yourself.

Key facts

A mass spectrometer measures the mass-to-charge ratio (m/z) of ions, not mass directly ^{1, 2}.
A peptide of mass 2,000 with one charge and a peptide of mass 4,000 with two charges give the same m/z of 2,001 ².
For peptides, average mass and monoisotopic mass differ by about 0.06% ⁴.
In a published USP example, leuprolide had a theoretical m/z of 1209.6533 and a measured m/z of 1209.6515 ⁵.
ICH guidance says identification by a single chromatographic retention time is not regarded as specific, while a combination such as HPLC/MS is generally acceptable ⁶.
Leucine and isoleucine share the formula C6H13NO2 and the same monoisotopic mass of 131.0946 Da ⁸.
Peptide-related impurities from chemical synthesis can include amino acid deletions, insertions, truncations, D forms and side chain changes ⁷.
MALDI mainly produces singly charged ions, while electrospray usually produces a range of multiply charged ions ^{2, 3}.

Common questions

Does a mass match mean the peptide is pure?

No. It shows that a compound with the expected mass is present. Purity is a separate test, usually HPLC ^{5, 6}.

Does mass spectrometry show how much peptide is in a vial?

An identity check by mass does not. The amount is set by a separate assay that also accounts for water and counter-ions ⁵.

Why does my report show several peaks for one peptide?

Electrospray usually gives several ions of the same compound with different numbers of charges, and each appears at a different m/z ². Isotopes also create a small cluster of peaks around each one ⁴.

Why does the observed mass differ slightly from the expected mass?

Every instrument has a measurement error ². A larger gap of about 0.06% can appear if one value is monoisotopic and the other is average ⁴.

Can two different peptides have the same mass?

Yes. Swapping leucine for isoleucine, or an L form for a D form, does not change the mass ⁸. Other tests, such as amino acid analysis, chiral testing or MS/MS, are used to check these points ⁵.

Sources

"What is Mass Spectrometry?", Broad Institute. https://www.broadinstitute.org/technology-areas/what-mass-spectrometry — Used for what mass spectrometry measures (m/z), ionisation, sorting of ions, and what a mass spectrum shows.
"UNM Mass Spectrometry Tutorial", Mass Spectrometry Facility, Department of Chemistry and Chemical Biology, University of New Mexico. https://massspec.unm.edu/ms.html — Used for the m/z example with one and two charges, multiply charged ions in electrospray, singly charged ions in MALDI, processing of multiply charged spectra, and typical mass accuracy for peptides.
Singhal N, Kumar M, Kanaujia PK, Virdi JS. "MALDI-TOF mass spectrometry: an emerging technology for microbial identification and diagnosis." Frontiers in Microbiology 6:791 (2015). https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2015.00791/full — Used for the definition of mass spectrometry, soft ionisation in ESI and MALDI, the MALDI matrix and laser, singly charged ions, and how a time-of-flight analyser measures m/z.
"Accuracy & resolution" (Mascot help), Matrix Science. https://www.matrixscience.com/help/mass_accuracy_help.html — Used for monoisotopic and average mass, the 0.06% difference for peptides, and the example peptide masses.
McCarthy D, Han Y, Carrick K, et al. "Reference Standards to Support Quality of Synthetic Peptide Therapeutics." Pharmaceutical Research 40(6):1317 (2023), Springer (open access; read through Europe PMC, PMC10338602). https://doi.org/10.1007/s11095-023-03493-1 — Used for the leuprolide theoretical and measured m/z, LC-MS/MS for mass and sequence, HPLC impurities as percent of detected area, amino acid analysis for leucine and isoleucine, chiral testing for D forms, counter-ions and water in content assignment, and the use of several methods for identity.
"Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products: Chemical Substances Q6A" (Step 4 version, 6 October 1999), International Council for Harmonisation (ICH). https://database.ich.org/sites/default/files/Q6A%20Guideline.pdf — Used for identification, assay and impurities as separate tests, and for the statements on retention time and HPLC/MS.
De Groot AS, Mattei A, Gabriel J, et al. "Immunogenicity of Generic Peptide Impurities: Current Orthogonal Approaches." Pharmaceutical Research 42(5):805 (2025), Springer (open access; read through Europe PMC, PMC12159091). https://doi.org/10.1007/s11095-025-03843-1 — Used only for the list of peptide-related impurity types that can arise in solid phase peptide synthesis.
PubChem compound records for L-leucine (CID 6106), L-isoleucine (CID 6306), L-phenylalanine (CID 6140) and D-phenylalanine (CID 71567), U.S. National Library of Medicine. https://pubchem.ncbi.nlm.nih.gov/rest/pug/compound/cid/6106,6306,6140,71567/property/Title,IUPACName,MolecularFormula,MolecularWeight,MonoisotopicMass/CSV — Used for the identical formula and mass of leucine and isoleucine, and of the L and D forms of an amino acid (phenylalanine as the example).