What the numbers on your peptide CoA actually mean

What a peptide CoA is

A peptide certificate of analysis (CoA) is a supplier-issued document that reports analytical test results for a research sample, and its two critical values are HPLC purity (target peak area ÷ all peaks × 100) and mass spectrometry identity (observed m/z versus theoretical mass).

In this article

1. What the HPLC Purity Number Actually Tells You
2. Reading the Mass Spectrometry Data
3. What a Complete CoA Should Always Include
4. Common Mistakes Researchers Make with CoAs
5. Frequently asked questions
6. What the community gets wrong

A peptide certificate of analysis (CoA) is a document a peptide supplier issues for one specific manufacturing batch. It reports the lab tests run against that batch and their results, and it typically arrives as a PDF - either emailed with an order or posted on the supplier's product page.

A CoA is not a claim about what a compound does; it is a record of identity and purity for that particular lot, issued by whoever manufactured or tested it. A CoA travels with the peptide it describes, not with hardware - it does not certify a pen, a cartridge, or a needle.

That distinction is why PreppinPeppers does not issue peptide CoAs: we manufacture and sell reconstitution hardware - pens, cartridges, needles, and kits - not peptides. Documentation for a peptide batch has to come from whichever supplier or lab actually tested that batch, not from a hardware manufacturer.

Key point: A CoA reports purity and identity for one manufacturing batch, and only the entity that tested that batch can issue it.

What the HPLC Purity Number Actually Tells You

How HPLC Separates Compounds

High-performance liquid chromatography (HPLC) works like a race track for molecules. The instrument forces a sample through a packed column, and each compound moves through at a different speed based on how strongly it interacts with the packing material. As compounds exit the column, a UV detector — typically set at 214 nm, a wavelength where peptide bonds absorb strongly — records each one as a peak on a chromatogram.

How the Percentage Is Calculated

Purity is a ratio, not a direct measurement of identity. The lab divides the target peak's area by the combined area of every detected peak, then multiplies by 100. A result of 98% purity means 98% of the UV-absorbing material leaving the column matches the target peak's retention time. The remaining 2% can include:

• Synthesis fragments
• Deletion sequences
• Residual solvents
• Other chemical impurities

What Counts as Acceptable Purity

Standard peptide research commonly uses ≥95% purity as a baseline threshold. Work requiring precise measurements at low concentrations often calls for ≥98% purity, since trace impurities make up a proportionally larger share of the added mass. A purity number is only as useful as the method behind it — check that the CoA lists the column type and gradient conditions used; without them, the percentage can't be evaluated properly.

What HPLC Cannot Tell You

HPLC separates by retention time, not molecular structure. It cannot confirm that the main peak is the intended sequence rather than an unrelated compound that happens to elute at the same time. Confirming actual molecular identity requires a separate technique, mass spectrometry.

Reading the Mass Spectrometry Data

A peptide CoA should include a mass spectrometry (MS) result that confirms the compound's identity by mass-to-charge ratio (m/z). This section walks through what that data should contain, how the numbers are generated, and which mismatches are worth flagging when you review a report.

Electrospray Ionization (ESI-MS)

Electrospray ionization (ESI-MS) is the standard technique for synthetic peptides. It ionizes the molecule by adding protons, producing multiply charged species the instrument detects:

• [M+H]⁺: One proton added
• [M+2H]²⁺: Two protons added
• [M+3H]³⁺: Three protons added

M is the neutral mass of the intact peptide sequence.

Comparing Theoretical and Observed Mass

Check that your CoA reports two figures:

• Theoretical molecular weight: calculated from the amino acid sequence.
• Observed m/z: the experimental reading from the spectrometer.

To verify them, multiply the observed m/z by its charge state, then subtract roughly 1.008 Da per added proton. The result should land within about 0.5 Da of the theoretical value.

Mass Offsets Worth Flagging

• +16 Da: possible methionine oxidation.
• -18 Da: possible dehydration, a lost water molecule during manufacturing.
• Excess mass near a protecting group's weight: possible incomplete deprotection during synthesis.
• Multiple unexplained peaks: possible synthesis failure, co-eluting impurities, or a mismatched sequence.

Some labs use MALDI-TOF instead of ESI-MS. It produces mostly singly charged ions and simpler spectra for larger peptides, while ESI typically gives higher resolution. Whichever technique your lab used, the report should state it clearly so you know what instrument tolerance to expect. Either method is a reasonable check as long as the observed value matches the theoretical one within that tolerance.

What a Complete CoA Should Always Include

A rigorous, complete CoA must supply sufficient analytical data to trace and confirm a vial's contents. When reading one, check for these five elements before treating the document as valid:

• Peptide identifiers: Full peptide name, complete amino acid sequence, and unique lot number.
• Physical properties: Net weight in milligrams and visual appearance (typically a lyophilized white powder).
• HPLC analytical data: Purity percentage alongside full method parameters (column specifications, UV detection wavelength, and solvent gradient).
• Mass spectrometry data: Both theoretical molecular weight and observed m/z values.
• Quality assurance metadata: Date of testing and analytical release.

A purity percentage on its own proves nothing - it is only checkable against the method parameters that produced it, so a figure listed without its column, wavelength, and gradient cannot be independently verified. Likewise, a CoA that lacks a matching lot number or omits mass spectrometry data is incomplete and cannot reliably confirm what is actually in the vial. Cross-check every field against the vial label before relying on the paperwork.

Common Mistakes Researchers Make with CoAs

1. Checking HPLC purity while ignoring mass spec: A 99% purity chromatogram is meaningless if the mass spec reveals a mass discrepancy of 50 Da. Always verify both purity and molecular identity.
2. Skipping the lot number comparison: Suppliers frequently update batches without changing website overviews. The lot number printed on the vial label must match the CoA document exactly.
3. Overlooking the testing date: The CoA reflects the peptide's quality at the time of quality control testing. A test performed 18 months prior provides no guarantee of current integrity if storage conditions fluctuated.
4. Accepting cropped summaries over raw analytical traces: Reputable manufacturers provide the full HPLC chromatogram and complete mass spectrum. A cropped percentage box hides co-eluting peaks and baseline irregularities.
5. Treating visual appearance as a substitute for data: Color or clarity changes in a vial are handling observations, not analytical results. Only the CoA's HPLC and mass spec data confirm identity and purity - appearance alone confirms nothing.
6. Assuming the CoA still applies after reconstitution: A CoA describes the sample as it was tested, before any diluent, such as bacteriostatic water, was introduced. It does not certify the peptide's condition once reconstituted.

Related from our lab: the pen · cartridges · bacteriostatic water · reconstitution calculators

Shared by PreppinPeppers for research, educational, and demonstration awareness only. We link to third-party coverage; we do not endorse it, and nothing here is medical advice or a recommendation to use any substance in humans or animals. Our products are sold for laboratory research use only.

What the research community gets wrong about reading a CoA

Most CoA discussions stop at two numbers: the HPLC purity percentage and the mass spec match. Both matter, but stopping there skips the factor that most often throws off bench calculations — how much of the powder in the vial is actual peptide.

Researchers frequently ask how to read a peptide CoA and what to check. These are the five points that get missed most often:

• Purity percent ≠ net peptide content. HPLC purity compares your target peak to other UV-absorbing impurities. It says nothing about water, counterion salts, or residual solvent. A vial listed at 99% HPLC purity commonly contains only 70% to 90% actual peptide by weight. Net peptide content, not HPLC purity, is the number molar calculations should use.
• Counterions carry real mass. Cleavage and purification leave salts — trifluoroacetate (TFA) or acetate — bound to basic residues, and TFA salts in particular add non-peptide mass. Treating total powder mass as pure peptide understates the true concentration.
• Lyophilized powder still holds moisture. Lyophilization reduces water content; it does not remove it. Karl Fischer titration is the standard method for quantifying what's left, and that residual moisture lowers net peptide content further below the HPLC purity figure.
• Purity numbers depend on the method used to get them. Column chemistry, mobile phase gradient, and detection wavelength (commonly ~214 nm) all affect what an HPLC trace can resolve. A weak separation method can hide impurities under the main peak.
• Purity and identity are two different tests. HPLC measures how clean a sample is; mass spectrometry confirms what the molecule actually is. A strong result on one does not stand in for the other.

From our bench: lining up CoAs from different suppliers for the same peptide, the HPLC purity lines often look nearly identical — it's the net peptide content line that actually separates them, on the CoAs that list it at all. A missing net peptide content value is, in our view, reason enough to pause and ask before running any weight-based reconstitution math on that lot.

Sources

Reminder: research and educational reference only. PreppinPeppers sells hardware and materials, not peptides. Not medical, dosing, or health advice, not evaluated by the FDA, and not intended for human or animal use.