Peptide reference · Lipidated 31-residue GLP-1 analog

Semaglutide COA check: molecular weight, formula and what to look for

By the PeptideCOACheck team · Last updated November 2, 2026 · How we check

Use these reference values to check the identity result on a Semaglutide Certificate of Analysis. The theoretical average molecular weight of Semaglutide is 4,113.58 Da and the monoisotopic mass is 4,111.115 Da.

Check a Semaglutide COA now

FP · 4,111.1154 Fingerprint calculated from the formula: positions of the protonated ions, the natural isotope pattern of the neutral molecule (relative heights are calculated; ion heights are schematic) and the atom mix. It is a reference picture, not measured data.

Semaglutide reference data

Name	Semaglutide
Also known as	NN9535, GLP-1 analog
Type	Lipidated 31-residue GLP-1 analog
Molecular formula	C₁₈₇H₂₉₁N₄₅O₅₉
Average molecular weight	4,113.58 g/mol (Da)
Monoisotopic mass	4,111.1154 Da
Residues	31
Reference source	PubChem CID 56843331 (formula and mass cross-checked)

Expected mass spectrometry ions

A Semaglutide COA may list one of these m/z values instead of the neutral mass. That is still consistent with Semaglutide. How charge states work.

Ion	m/z (monoisotopic)	m/z (average)
[M+3H]³⁺	1,371.38	1,372.20
[M+4H]⁴⁺	1,028.79	1,029.40
[M+5H]⁵⁺	823.23	823.72
[M+6H]⁶⁺	686.19	686.60

What else can show up on a Semaglutide COA

Masses calculated from Semaglutide's formula for the adducts, degradation products and synthesis impurities a lab report can show. A match tells you what the extra peak probably is.

Signal	Value	What it means
Na adduct, deconvoluted (M + 21.98)	4,133.10 Da	Sodium adduct: an MS artifact, not a separate impurity. The [M+Na]+ ion itself is at M + 22.99
K adduct, deconvoluted (M + 37.96)	4,149.07 Da	Potassium adduct: an MS artifact. The [M+K]+ ion itself is at M + 38.96
TFA adduct (M + 113.99)	4,225.11 Da	Trifluoroacetic acid from purification seen as an adduct; residual TFA salt is real vial content
M + 16 (oxidation)	4,127.11 Da	Oxidation of Trp

Adduct rows are deconvoluted masses (the shift seen against [M+H]⁺); the singly charged [M+Na]⁺ ion itself sits at M + 22.99. Deletion masses alone can't say which copy of a repeated residue is missing.

At over 4 kDa, Semaglutide is harder to purify; 98% or higher with one sharp main peak is a strong result. The full checklist is in how to read a peptide COA.

Semaglutide-specific things to watch

Semaglutide is a GLP-1 analog with an Aib substitution and a C18 fatty diacid side chain attached through a linker. The side chain is a big part of the mass: about 4113.6 Da average, 4111.1 Da monoisotopic.

Because the average and monoisotopic masses differ by about 2.5 Da at this size, a COA should state which one it reports. This checker compares against both.

Look out for COAs that show a mass in the 3,300–3,400 Da range: that can indicate a missing side chain or a different GLP-1 sequence altogether. Charge states around 1029.3 m/z (4+) and 823.6 m/z (5+) are normal in ESI spectra.

At about 4113.6 Da, semaglutide appears as a ladder of charges: 1372.2 (3+), 1029.4 (4+) and 823.7 (5+) m/z, calculated from the average mass. A good COA reports the deconvoluted mass. On high-resolution spectra the most intense peaks can sit a few tenths of an m/z below these values, which is normal.

Semaglutide has one tryptophan and no methionine, so expect at most a single +16 Da oxidation peak (about 4129.6 Da average). Vials are sold as base, sodium or acetate salt; the salt changes net content, not the peptide mass. Some stores sell it under a house name such as "Sema (GLP-1)", so match the COA mass rather than the label.

Compounds with similar mass

If a COA's mass is off, compare it with these neighbors. A match to one of them can point to a mislabeled product.

Cagrilintide: 4,409.01 Da (+295.43 Da)
CJC-1295 (with DAC): 3,647.19 Da (-466.39 Da)
Retatrutide: 4,731.35 Da (+617.77 Da)

Sources

PubChem CID 56843331: Semaglutide formula and reference mass.
CIAAW: Standard atomic weights: Used to calculate average molecular weights.
NIST CODATA: proton mass: Used to convert m/z charge states to neutral mass.
PeptideCOACheck methodology: How the checker's thresholds work.

Last updated November 2, 2026. Written and maintained under our editorial policy. Found an error? See corrections.

Frequently asked questions

Why does my semaglutide COA show 1029?

It is the 4+ ion of the 4113.6 Da molecule.

Average or monoisotopic: which should a semaglutide COA report?

Either, as long as it says which. For semaglutide they differ by about 2.5 Da.

Can semaglutide be confused with tirzepatide by mass?

No. They are about 700 Da apart (4113.6 vs 4813.5 Da).

For laboratory research use only. This page describes analytical chemistry data. It is not medical advice and makes no claim about effects in humans or animals.