Research peptides are priced by the vial and paid for by the experiment. A 5 mg vial listed at $40 and a chemically identical vial listed at $55 look like a $15 decision on a purchase order. However, if the cheaper vial holds 62% peptide by mass instead of 88%, or carries a sequence no one confirmed, the real cost includes the reagents, the instrument time, the technician hours, and the six weeks of work sitting downstream of a result you now have to repeat.
That distance between sticker price and total cost is where most lab procurement mistakes get made. The five questions work the same way as due diligence in any other spending decision: the number in the pitch is rarely the number that determines what you pay.
1. Can I see the Certificate of Analysis for this specific lot?
A Certificate of Analysis (COA) is the batch-level test report for the material in the vial you are being sold. A complete COA reports six independent measurements, each answering a different question:
- Analytical HPLC establishes purity. Detection is typically at 210 to 220 nm, and the purity figure is the area of the main peak as a share of the total area of all peaks.
- Mass spectrometry confirms identity by measuring molecular weight against the expected value for the sequence.
- Amino acid analysis breaks the peptide down with a strong acid and quantifies the amino acids released. It confirms composition but gives no sequence information, so it supports the mass spec result rather than replacing it.
- Karl Fischer titration measures water content.
- Gas chromatography identifies and quantifies residual solvents left over from synthesis and purification.
- Ion chromatography measures counterion content, usually trifluoroacetate or acetate.
If a supplier can produce a purity percentage but not the chromatogram behind it, you have a marketing claim rather than a measurement. Ask for the raw HPLC trace and the mass spec output. Both are routine attachments for any lab that ran the test.
2. Is the purity number the same as the amount of peptide I am paying for?
Purity tells you what share of the peptide material in your vial is the target sequence. Net peptide content tells you what share of the total powder mass is peptide at all. The rest of that mass is water, adsorbed solvents, counterions, and salts picked up during purification. A vial can be 98% pure and still be well under 70% peptide by weight.
For most lyophilized peptides, net peptide content runs between 60% and 90% of total mass. It is measured by elemental analysis, amino acid analysis, or UV spectrophotometry, and it should appear on the COA as its own line item. Run the arithmetic before you compare prices:
- Vial A: $40, 5 mg gross, 62% net peptide content = 3.1 mg of peptide, or $12.90 per mg
- Vial B: $55, 5 mg gross, 88% net peptide content = 4.4 mg of peptide, or $12.50 per mg
Vial B carries a 37.5% higher sticker price and is the cheaper of the 2 per milligram of actual peptide. It also gives you a more accurate starting concentration, which matters more than the money. If you weigh out gross mass and treat it as net peptide, every concentration in your study is wrong by the same hidden margin, and that error carries forward into every figure you publish.
3. Who ran the test, and when?
In-house testing is not worthless, but it is self-reported. Third-party testing by an independent laboratory is the check on it, and the COA should name the facility that ran the analysis rather than leaving it anonymous. There are three things to check on any COA you are handed:
- The date: A test run 18 months ago describes a lot as it was 18 months ago. Peptides degrade. Ask when the material was synthesized, when it was tested, and how long it has been in inventory.
- The lab: An independent facility named on the document can be looked up. “Third-party verified” with no name attached cannot.
- The internal consistency: Purity, net peptide content, water content, and counterion content should add up to something close to 100%. When they do not, ask why before you order.
Suppliers who publish lot-level COAs openly, like Penguin Peptides, are making a verifiable claim that you can check against the vial in your hand. Suppliers who send test documents only after you have paid are asking you to buy first and verify never.
4. How is it stored, and how will it be shipped?
A COA describes the material at the moment it was tested. What reaches your bench depends on what happened in between.
Lyophilized peptides should be held at -20 degrees C, and -80 degrees C is better for long-term storage. Once reconstituted, solutions hold for roughly a week at 4 degrees C; anything longer calls for -20 degrees C or colder. Repeated freeze-thaw cycles degrade peptides, so the standard practice is to split a stock solution into single-use working aliquots rather than returning to one vial 20 times.
Two handling details cost labs more material than they realize. First, let a cold vial come to room temperature before you take the lid off, because opening it cold pulls atmospheric moisture straight onto the powder, and moisture exposure is one of the fastest routes to degradation. Second, keep the vial tightly capped whenever it is not in active use.
Before you order, ask how the material ships, whether it ships cold, what the expected transit time is, and what the supplier does when a package is delayed in a warehouse over a weekend. A supplier who has an answer ready has thought about it. One who has not is quietly transferring that risk to you.
5. What happens when something is wrong?
Ask the following questions before you need the answer:
- If the mass spec on arrival does not match the COA, what is the return process and who pays for it?
- If a shipment arrives warm, is it replaced, and does the supplier ask for proof?
- Is there a named contact who can answer a technical question, or only a support ticket queue?
- Does the supplier state plainly that its products are for laboratory research use and not for human consumption, and is that language consistent across the site rather than buried in a footer?
That last point is a reliability signal as much as a compliance one. A supplier that is careless about how it describes what it sells is often careless about how it documents it.
A checklist before you approve the order
- Lot number on the COA matches the lot number shipping to you
- HPLC purity reported with the chromatogram attached
- Mass spectrometry result matching expected molecular weight
- Net peptide content stated as its own figure, not merged with purity
- Water content, residual solvents, and counterion content all reported
- Testing laboratory named, with a test date inside the last 12 months
- Cost recalculated per milligram of net peptide, not per vial
- Storage and shipping conditions confirmed in writing
- Return and replacement terms confirmed in writing
- Research-use-only language stated clearly by the supplier
Before you order
The discipline here is the same one that applies to any recurring purchase: compare total cost, not advertised price, and get the terms in writing before money moves. In a research setting the stakes are arranged differently. A bad vial wastes the reagents, the hours, and the credibility of every result built on top of it.
Research use only. All compounds referenced in this article are intended strictly for laboratory research. They are not approved by the FDA for human or veterinary use, are not intended to diagnose, treat, cure, or prevent any disease, and are not for human consumption. Researchers are responsible for compliance with all applicable institutional and regulatory requirements.
