Mass Spectrometry and Peptide Identity Explained

HPLC tells you how pure a peptide is. It cannot tell you whether the main peak is the right peptide. That question is answered by mass spectrometry (MS), which measures the mass of the molecule itself. This article explains how peptide MS works, how to read the numbers on a certificate, and what "identity confirmed" does and does not establish.

The principle

A mass spectrometer does three things:

1. Ionises the sample, giving molecules an electric charge.
2. Separates the ions according to their mass-to-charge ratio (m/z) in an analyser.
3. Detects the ions and records their abundance.

The output, a mass spectrum, plots relative abundance against m/z. Because the instrument measures m/z rather than mass directly, interpreting a peptide spectrum means working out the charge on each ion.

Ionisation: ESI and MALDI

Large, fragile molecules such as peptides could not be ionised intact until the late 1980s. Two "soft" ionisation methods changed that, and their inventors shared part of the 2002 Nobel Prize in Chemistry (John Fenn for ESI, Koichi Tanaka for soft laser desorption; MALDI as used today was developed by Karas and Hillenkamp).

Feature	Electrospray ionisation (ESI)	Matrix-assisted laser desorption/ionisation (MALDI)
How ions form	Solution sprayed through a charged needle; droplets evaporate	Sample co-crystallised with a UV-absorbing matrix, then hit with a laser
Typical charge	Multiple charges ([M+2H]²⁺, [M+3H]³⁺ …)	Mostly singly charged ([M+H]⁺)
Common analyser	Quadrupole, ion trap, Orbitrap, Q-TOF	Time-of-flight (TOF)
Coupling to HPLC	Direct (LC-MS)	Offline
Strengths	Online with HPLC; high accuracy on modern instruments	Fast; simple spectra; tolerant of salts

Both are entirely adequate for identity testing of research peptides. LC-MS with ESI has the added benefit of giving a mass for every chromatographic peak, so impurities can be characterised as well.

Charge states and how to read them

In positive-mode ESI, a peptide picks up protons on its basic sites: the N-terminus and the side chains of lysine, arginine and histidine. A molecule of neutral mass M carrying z protons appears at:

m/z = (M + z × 1.007) ÷ z

Take BPC-157, with an average mass of about 1,419.5 Da:

Ion	Calculation	Observed m/z (approx.)
[M+H]⁺	(1,419.5 + 1.0) ÷ 1	1,420.5
[M+2H]²⁺	(1,419.5 + 2.0) ÷ 2	710.8
[M+3H]³⁺	(1,419.5 + 3.0) ÷ 3	474.2

A larger peptide shows a series, or "envelope", of charge states. Software combines them to calculate the single neutral mass, a step called deconvolution. A certificate should ideally show both the raw ions and the deconvoluted mass.

On a high-resolution instrument you can determine the charge directly from the isotope peaks: for a 2+ ion, neighbouring isotope peaks are 0.5 m/z apart; for a 3+ ion, about 0.33 apart.

Adducts

Ions do not always carry protons alone. Sodium ([M+Na]⁺, +22 Da relative to [M+H]⁺) and potassium ([M+K]⁺, +38 Da) adducts are common, especially in MALDI. They are not impurities in the peptide; they reflect salts in the sample or solvent. TFA adducts can also appear. A competent report labels them.

Monoisotopic vs average mass

Most elements have more than one stable isotope. Carbon is about 98.9% ¹²C and 1.1% ¹³C. A peptide containing 60 or more carbon atoms therefore exists as a family of isotopic forms differing by about 1 Da each.

• Monoisotopic mass is the mass of the molecule made only of the most abundant isotope of each element (¹²C, ¹H, ¹⁴N, ¹⁶O, ³²S). It corresponds to the first peak of the isotope pattern.
• Average mass is the abundance-weighted mean of all isotopic forms. It corresponds to the centre of the pattern and is the value usually quoted as "molecular weight".

For a small peptide the difference is under 1 Da; for semaglutide (about 4,114 Da average) it is around 2.5 Da. As molecules get larger, the monoisotopic peak also becomes smaller than its heavier neighbours and eventually hard to see.

The practical rule: compare like with like. A high-resolution measurement should be compared with the theoretical monoisotopic mass; a low-resolution or deconvoluted average should be compared with the theoretical average mass. Mixing them produces apparent errors of one to several daltons.

Mass accuracy and acceptance criteria

Mass accuracy is usually expressed either in daltons or in parts per million (ppm):

error (ppm) = (observed − theoretical) ÷ theoretical × 1,000,000

Typical expectations:

• Low-resolution instruments (single quadrupole, linear MALDI): agreement within about ±1 Da of the average mass.
• High-resolution instruments (Orbitrap, Q-TOF, reflectron MALDI-TOF): agreement within about 5–10 ppm of the monoisotopic mass.

A certificate should state the theoretical value, the observed value and the instrument or method, so the reader can check the agreement.

What "identity confirmed" means

When a certificate states that identity is confirmed by MS, it means the dominant species has a mass consistent with the expected molecular formula within the method's accuracy. That is strong evidence. It is not proof of every structural detail. Intact mass alone cannot:

• Distinguish isomers. A peptide with two residues swapped, or with one L-residue replaced by its D-form, has exactly the same mass.
• Separate isobaric residues. Leucine and isoleucine have identical masses, and glutamine and lysine differ by only 0.036 Da.
• Locate a modification. A +16 Da oxidation shows that something is oxidised, not where.

For these questions, tandem mass spectrometry (MS/MS) fragments the peptide along its backbone and reads the sequence from the fragment masses. Combining intact mass, MS/MS and an HPLC retention time matched to a reference standard gives very high confidence.

Using MS to explain impurities

Mass differences are diagnostic. A peak 16 Da heavier than the target suggests oxidation; 1 Da heavier on a high-resolution instrument can indicate deamidation; a loss equal to one residue mass indicates a deletion sequence; +56 Da suggests a surviving tert-butyl protecting group. LC-MS data of this kind is what allows a laboratory to name the small peaks on a chromatogram.

A quick checklist for MS data

1. Theoretical mass is stated, and whether it is monoisotopic or average.
2. Observed ions or deconvoluted mass are reported, ideally with the spectrum.
3. The difference is within the stated method accuracy.
4. Charge states and adducts are labelled.
5. For blends, each component has its own confirmed mass.

For the full picture of how identity, purity and content combine, see how to read a certificate of analysis.

References

• Fenn JB, Mann M, Meng CK, Wong SF, Whitehouse CM. Electrospray ionization for mass spectrometry of large biomolecules. Science. 1989;246(4926):64–71.
• Karas M, Hillenkamp F. Laser desorption ionization of proteins with molecular masses exceeding 10,000 daltons. Anal Chem. 1988;60(20):2299–2301.
• Tanaka K, Waki H, Ido Y, et al. Protein and polymer analyses up to m/z 100 000 by laser ionization time-of-flight mass spectrometry. Rapid Commun Mass Spectrom. 1988;2(8):151–153.
• Steen H, Mann M. The ABC's (and XYZ's) of peptide sequencing. Nat Rev Mol Cell Biol. 2004;5(9):699–711.
• de Hoffmann E, Stroobant V. Mass Spectrometry: Principles and Applications. 3rd ed. Wiley; 2007.
• Gross JH. Mass Spectrometry: A Textbook. 3rd ed. Springer; 2017.
• United States Pharmacopeia. General Chapter ⟨736⟩ Mass Spectrometry. USP–NF.