Research peptide vials come in a familiar set of sizes: 2 mg, 5 mg, 10 mg, sometimes more. The number looks simple. In practice, "10 mg" can describe different things depending on the supplier, and the difference matters to anyone recording exactly how much material went into a lab workflow.
This guide explains what the strength on a peptide label usually refers to, why the amount of actual peptide in a vial is often less than the powder mass, how fill tolerance works, and what the other fields on a good label are for. It is about reading the label and the paperwork. It is not a handling or preparation guide.
The anatomy of a research peptide label
A well-made label carries, at minimum:
| Field | What it tells you |
|---|---|
| Compound name | Which peptide the vial is meant to contain |
| Strength / fill amount (e.g. "10 mg") | The nominal amount in the vial (see below for what it measures) |
| Form | Usually "lyophilized powder" (freeze-dried) |
| Lot or batch number | The link between this vial and its test records |
| Storage condition | For example "Store at 2–8 °C" or "Store at −20 °C" |
| Research-use statement | That the material is for laboratory research only |
| Supplier identity | Who is responsible for the product |
Some labels also show a salt form (for example "acetate" or "TFA salt"), a manufacture or retest date, and a sequence or molecular mass. More information is better, because each field is something you can check against the certificate of analysis (COA).
What "10 mg" usually measures
There are two common conventions, and suppliers don't always say which one they use:
- Gross mass. 10 mg of powder was put in the vial. That powder is peptide plus water plus counterions.
- Net peptide. The vial was filled so that it contains 10 mg of peptide itself, after accounting for water and counterions.
The first is easier and cheaper, so it's common. The second is more useful for quantitative work, but it needs a net peptide content measurement for each lot.
If a label or product page doesn't say which convention it uses, it's worth asking.
Why the powder isn't all peptide
Freeze-dried peptide powder is never 100% peptide. It also contains:
- Residual water, which freeze-drying reduces but never removes completely.
- Counterions. Peptides are salts. Their charged groups pair with counterions such as trifluoroacetate (TFA), acetate or chloride. Because trifluoroacetic acid is used to cleave peptides from the synthesis resin and during purification, peptides are commonly obtained as TFA salts (Erckes et al., Pharmaceuticals, 2025; DOI: 10.3390/ph18081163).
The counterion's share of the mass depends on the sequence. A peptide with many charged residues pairs with more counterions, so a larger share of its powder mass isn't peptide.
How large can the gap be for a well-made material? When China's National Institute of Metrology assigned an SI-traceable purity value to a synthetic peptide, measuring structurally related impurities, water, counterions, inorganic elements and volatile compounds separately, the assigned value was 796.5 mg of peptide per gram of material (Wang et al., Journal of Pharmaceutical and Biomedical Analysis, 2022; DOI: 10.1016/j.jpba.2021.114401). On that basis, a vial holding 10 mg of that powder by gross mass would contain roughly 8 mg of peptide. Nothing is wrong with that material. It's simply what a peptide salt weighs.
Purity is not the same as amount
A label or COA may say "99% purity." That figure normally comes from HPLC and describes the share of the UV signal that came from the main peak. It says nothing about how many milligrams of peptide are in the vial. Our guide to peptide purity testing explains the difference between area-percent purity and net peptide content. Related reading: Peptide Purity Testing Explained: Area-Percent, Net Peptide Content, and Why They Differ.
Fill tolerance: no vial holds exactly 10.000 mg
Filling small amounts of fluffy powder, or filling a solution and then freeze-drying it, always has some variation. Good manufacturers set a tolerance, for example "10 mg ± X%," and check that vials across a lot fall within it. This is called content uniformity.
Reference-standard makers take this seriously. The United States Pharmacopeial Convention (USP), writing with the UK medicines regulator, describes assigning a value to a bulk peptide by mass balance, then using that bulk as a standard to assign a final value to the lyophilized vials, with testing for content uniformity and identity across the vialed material (McCarthy et al., Pharmaceutical Research, 2023; DOI: 10.1007/s11095-023-03493-1). Research-grade suppliers don't work to that standard, but a COA that states a fill tolerance, or reports measured content, is telling you more than one that doesn't.
When the number on the label is simply wrong
Normal chemistry explains a gap of some percent between gross mass and peptide content. It doesn't explain a vial that holds half its stated amount.
Published testing of peptide vials bought online shows that does happen. In a Danish study, researchers bought vials of a peptide product from three online shops, each labeled as containing 10 mg. Using validated LC-UV and LC-MS/MS methods, they measured 4.32 mg to 8.84 mg of the peptide per vial (Breindahl et al., 2015; DOI: 10.1002/dta.1655). The correct molecule was present. The labeled amount wasn't.
Belgium's Scientific Institute of Public Health reported the same kind of problem more broadly: a "high variation in amount" of peptide per unit across the ten falsified peptide products it screened (Janvier et al., Talanta, 2018; DOI: 10.1016/j.talanta.2018.06.023).
That is why the lot number matters. A label strength is a claim. The lot record is the evidence for it.
Common vial sizes, and what to check for each
Whatever the size, the checks are the same:
- Does the strength on the vial match your order and the lot record?
- Does the supplier say whether the strength is gross powder or net peptide?
- Is a salt form stated?
- Does the COA report net peptide content or measured content, or only HPLC purity?
- Is there a fill tolerance?
Larger vials don't change the chemistry. A 10 mg vial of a heavily charged peptide still carries a larger share of counterion mass than a 10 mg vial of a lightly charged one.
Three questions to put to any supplier
If the label and product page leave you unsure, a short email clears most of it up:
- "Is the stated strength gross powder mass or net peptide?" A supplier that fills to net peptide will say so plainly. One that fills by gross mass should also be able to say so.
- "What is the salt form, and do your lot reports include counterion and water content?" This tells you how far gross mass and peptide content are likely to differ.
- "What is your fill tolerance, and is fill checked per lot?" A stated tolerance means someone is measuring.
Keep the replies with your lot records. They're part of the paperwork for the material.
Label red flags
- No lot number.
- Strength printed with no unit, or a unit that doesn't match the order.
- A handwritten or sticker-over-sticker label.
- Compound name that differs, even slightly, from the COA.
- No storage condition.
- Any claim about outcomes outside the lab. A research material's label describes the material, not what it might do.
How Seratide labels its vials
Every Seratide vial is labeled with the compound, strength, form, storage condition, research-use statement and a lot number in the format SRT-0000-XXX. That lot number links to a record on the Lot lookup page, laid out with HPLC purity, identity by LC-MS and endotoxin. The records there today are clearly labeled layout samples showing the format, with placeholder values; issued laboratory reports replace them as lots are analyzed. Browse strengths and forms on the research peptides shelf, or see our peptide basics page for how sequences and molecular masses are written.
Sources
All sources below were opened and their PubMed abstracts read on 2026-09-29. Claims are limited to what the abstracts state. (Source 3's title names the specific peptide studied; the article body deliberately does not.)
- Erckes V, et al. Towards a Consensus for the Analysis and Exchange of TFA as a Counterion in Synthetic Peptides and Its Influence on Membrane Permeation. Pharmaceuticals (Basel). 2025;18(8):1163. PubMed: https://pubmed.ncbi.nlm.nih.gov/40872554/ · DOI: https://doi.org/10.3390/ph18081163
- Wang S, et al. Mass balance method for SI-traceable purity assignment of synthetic oxytocin. J Pharm Biomed Anal. 2022;207:114401. PubMed: https://pubmed.ncbi.nlm.nih.gov/34656934/ · DOI: https://doi.org/10.1016/j.jpba.2021.114401
- Breindahl T, et al. Identification and characterization by LC-UV-MS/MS of melanotan II skin-tanning products sold illegally on the Internet. Drug Test Anal. 2015;7(2):164-72. PubMed: https://pubmed.ncbi.nlm.nih.gov/24771717/ · DOI: https://doi.org/10.1002/dta.1655
- McCarthy D, et al. Reference Standards to Support Quality of Synthetic Peptide Therapeutics. Pharm Res. 2023;40(6):1317-1328. PubMed: https://pubmed.ncbi.nlm.nih.gov/36949371/ · DOI: https://doi.org/10.1007/s11095-023-03493-1
- Janvier S, et al. Impurity profiling of the most frequently encountered falsified polypeptide drugs on the Belgian market. Talanta. 2018;188:795-807. PubMed: https://pubmed.ncbi.nlm.nih.gov/30029448/ · DOI: https://doi.org/10.1016/j.talanta.2018.06.023
For laboratory research use only.