What purity actually means in research peptides
The industry standard is 95%. Almost nobody asks the obvious follow-up question, and the answer is the difference between material you can use and material you can only hope about.
The short version. Ask a supplier two questions: what purity specification every lot is held to, and whether they confirm identity as well as purity. Vague answers to either are answers.
The other five percent
Peptide synthesis is a chain reaction — amino acids added one at a time, hundreds of steps for a long sequence. Every step has a yield below 100%, and what falls short does not disappear. It stays in the mixture as a truncated chain, a deletion sequence, or a fragment that is chemically similar to the target but is not the target.
Purification removes most of it. The specification tells you how much is left. At 95%, one part in twenty of what you are holding was never characterised by anyone — not identified, not quantified, not named on the label.
That is why the number matters, and why the gap between 95% and 99% is larger than four percentage points suggests: it is a fivefold difference in unknown material.
Purity and identity are different tests
This is where most evaluations go wrong. The two questions sound like one question and they are not.
How much is one thing?
Measured by HPLC. The sample is separated into components and the target is expressed as a percentage of the total. It answers “how clean” and nothing else.
Which thing is it?
Measured by mass spectrometry. The molecular weight of the dominant compound is compared against the expected value for the sequence on the label. It answers “is this what it says”.
A supplier running purity alone can honestly report 99% and still ship the wrong compound. The figure would be accurate. It would also be meaningless.
How to evaluate a supplier
Four questions. They take about thirty seconds to ask and the quality of the answers is the whole assessment.
- 01
What purity specification is every lot held to?
A number, not an adjective. "High purity" and "pharmaceutical grade" are marketing terms with no defined meaning. If a supplier cannot state a figure, there is no figure.
- 02
Do you confirm identity as well as purity?
Both should be run. If only one is, it will be purity — it is the cheaper test and it produces the more impressive number.
- 03
What analytical methods do you use?
HPLC for purity, mass spectrometry for identity, at minimum. A supplier who cannot name their methods is reporting figures they did not generate.
- 04
Where does material ship from?
Domestic shipping means no customs exposure, no seizure risk, and a real address if something needs resolving. It is the difference between a supplier and an intermediary.
Every lot to ≥99%, against an industry norm of 95%
Identity confirmed by HPLC and mass spectrometry. Purity measured by HPLC. Heavy metals, sterility and endotoxin screened as part of release. A lot that misses specification does not ship — it does not get discounted, relabelled, or quietly sold as something else.
The specification each lot is held to is published on every product page, before you buy, rather than produced afterwards if you think to ask.
Common questions
- What does 95% purity actually mean?
- That 95% of the material in the vial is the target compound. The remaining 5% is whatever else came out of the synthesis — truncated sequences, residual solvents, reaction byproducts. On a 100 mg vial that is 5 mg of uncharacterised material.
- Is purity the same as identity?
- No, and this is the distinction most buyers miss. Purity measures how much of a sample is a single compound. Identity confirms which compound it is. A high purity figure on the wrong molecule is worse than no figure at all, because it reads as proof.
- What is HPLC?
- High-performance liquid chromatography separates a sample into its components and measures the relative size of each. On the resulting trace, a single tall peak means one dominant compound. Additional peaks are everything else that came along with it.
- What should a certificate of analysis contain?
- A lot number, a date, the analytical method used, and who ran it. A certificate missing any of those describes some material at some point, not the vial in front of you.
ALL PRODUCTS ARE SUPPLIED FOR LABORATORY AND RESEARCH PURPOSES ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION, NOR FOR CLINICAL, DIAGNOSTIC OR THERAPEUTIC USE. Not evaluated by the U.S. Food and Drug Administration. This page describes analytical standards and does not describe the use, effect or application of any compound.