Semaglutide COA: What to look for in a certificate of analysis

A certificate of analysis (COA) is the primary document by which the identity and quality of a research peptide are established. For semaglutide, a legitimate COA will report a specific set of values that together confirm what the vial actually contains. This post walks through each of those values, what an acceptable range looks like, and what a legitimate lab report differs from.

This is a companion to our reading a tirzepatide COA post — same analytical framework, semaglutide-specific reference values.

What semaglutide is, analytically

Semaglutide is a 31-residue peptide with a fatty diacid side chain attached via a linker at Lys26. The full peptide has a molecular formula of C187H291N45O59, giving a monoisotopic mass of 4111.12 Da. Any legitimate mass spectrometry report should confirm a mass consistent with this value (within typical instrument precision, usually ±0.1-1 Da depending on the analyzer).

The four values that matter

A research-grade semaglutide COA should report, at minimum:

1. Mass verification (identity)

What it is: Confirmation by mass spectrometry (typically MALDI-TOF or ESI-MS) that the peptide in the vial matches the expected molecular mass of semaglutide.

What to look for: A reported mass at or near 4113.6 Da (average mass) or 4111.1 Da (monoisotopic mass). Reports may also show adduct masses ([M+H]⁺, [M+Na]⁺, etc.) which will be offset by predictable amounts. A legitimate MS report will include the spectrum image, not just a single number.

Red flag: A reported “mass” of a whole number without decimal precision (e.g., “4113 Da”), or an absence of the spectrum image itself. Mass reports without the underlying spectrum are unfalsifiable.

2. HPLC purity

What it is: High-performance liquid chromatography quantifies the percentage of the sample that is the target peptide versus impurities. For semaglutide, research-grade material typically reports ≥98% purity.

What to look for: A chromatogram (the actual trace, not just a number) showing a single dominant peak with retention time consistent with published semaglutide methods. Purity is calculated as the peak area of the main peptide divided by the total peak area.

Impurity peaks in a semaglutide sample most commonly reflect:

Deletion sequences (peptides missing one residue, typically from synthesis)
Oxidation products (methionine oxidation is common)
Deamidation products (asparagine/glutamine → aspartate/glutamate)
Truncation products

A well-characterized COA will identify the major impurities where possible.

Red flag: A reported purity above 99.9% without a chromatogram, or a chromatogram with unlabeled peaks representing more than 2% of total area.

3. Water content (Karl Fischer or LOD)

What it is: Lyophilized peptides absorb atmospheric moisture. Excess water content affects both the actual mass of peptide per vial and the storage stability of the material.

What to look for: For a lyophilized peptide, water content typically ≤5% by Karl Fischer titration or loss on drying (LOD). Higher values suggest either incomplete lyophilization or storage/handling problems.

Why it matters: A vial labeled “5 mg” that has 15% water content actually contains 4.25 mg of peptide. This matters both analytically (for concentration calculations) and for shelf life.

4. Endotoxin

What it is: Endotoxins are lipopolysaccharide components of gram-negative bacterial cell walls. In research use, endotoxin content is typically reported in EU (endotoxin units) per mg.

What to look for: Reported by LAL (Limulus amebocyte lysate) or recombinant Factor C assay. For research applications where downstream use may involve cellular or animal work, low endotoxin material is generally expected.

Red flag: Absence of endotoxin data on a COA intended to represent research-grade material.

Additional values often present

Beyond the four core parameters, legitimate research COAs may report:

Acetate content or TFA content. Peptides are often supplied as salt forms; TFA (trifluoroacetate) or acetate is the most common counterion. Salt content affects the actual peptide mass in a vial labeled “5 mg” — a peptide reported as a TFA salt at a certain percentage means less than 5 mg of the actual peptide is present. Some COAs report “net peptide content” separately.
Amino acid analysis. Confirms the amino acid composition matches the expected sequence.
Sterility. Not always reported for lyophilized research material; more common for injectable clinical material.
Appearance. Physical description of the lyophilized cake (color, form).

How to spot a fake or copied COA

Fraudulent COAs are unfortunately common in the peptide research space. Warning signs include:

The lab name doesn’t correspond to a real analytical laboratory. Legitimate labs have websites, addresses, contact information, and often accreditation (ISO 17025 or equivalent).
The lot number on the COA doesn’t match the vial label. Every lot should have its own COA; a lot number mismatch means either poor QC or an outright copy.
The date on the COA is old. COAs should reflect testing of the specific lot the material comes from, not a generic historical document.
The chromatogram is missing or clearly copy-pasted from another sample. Chromatograms should show the actual retention time and peak shape from the tested lot.
The mass spectrum is missing. A “mass verified” line without the underlying spectrum is not meaningful.
All values are suspiciously perfect. Real analytical work has expected variation. A COA reporting exactly 99.5% purity, 4113.6 Da mass, and 3.0% water on a round-number set is more likely to have been fabricated than measured.

What legitimate variation looks like

Real analytical variation on the same source material typically looks like:

Purity: 98.2-99.4% across lots is normal
Mass: within ±0.5 Da of theoretical is typical for a well-tuned instrument
Water content: 2-6% across lots is normal for well-lyophilized material
Retention time: shifts of a few seconds across lots are normal even on the same HPLC method

If two lots from the same source show exactly identical values across every parameter, that is more suspicious than lots that show small differences.

Storage considerations after receipt

Even with a clean COA, storage matters. Semaglutide’s stability profile follows the same general framework as other albumin-binding modified peptides: the lyophilized cake is stable at -20°C for extended periods, reconstituted stock has a shorter shelf life at 2-8°C, and repeated freeze-thaw cycles degrade both.

For the underlying chemistry of storage degradation pathways, see our lyophilized vs reconstituted post.