Testing Standards
The Purity Test Never Asks What The Compound Is. That Is A Different Instrument.
Pure and correct are two separate findings. One method measures how much of the contents is a single compound. Another asks which compound that is. A certificate reporting only the first has answered half the question and told you nothing about the other half.
Illustrative only. This is not a laboratory report and does not describe any tested material. It shows what a purity-only certificate can and cannot support.
Almost every buyer in this category treats purity as the quality score. Higher number, better vial. It is the figure on the listing, so it becomes the figure people compare.
But purity is not a score. It is the answer to one narrow question, and the question is narrower than most people assume.
Purity asks: of everything in this sample, what share is one single compound? That is it. It is a question about consistency.
It is not a question about correctness. Nothing in that measurement asks what the dominant compound actually is.
Pepsynth Labs runs both, on every lot.
Reverse-phase HPLC for purity and mass spectrometry for identity, on the same certificate, tied to your lot code.
Visit Pepsynth Labs →1A bag of coins, all the same size
Tip out a bag of coins and sort them by diameter. You find that 99% of them are the same size as each other.
That is a real finding and a useful one. The bag is consistent. Nobody mixed three currencies into it.
It does not tell you they are quarters.
Sorting by size never asked what the coins were. It asked how alike they are, got a clean answer, and stopped. To learn what they are you need to look at the face, which is a different operation entirely.
Consistency and identity are different questions. Sorting answers the first one only.
2What the purity method is actually doing
Reverse-phase HPLC pushes the sample through a column. Different molecules travel at different speeds, so they come out separated, and a detector counts what arrives and when.
The output is a chart of peaks. One large peak with very little else around it is a pure sample. That is where the percentage comes from.
The instrument records that a peak arrived at a certain time. It has no mechanism for recognising the molecule that made it.
The purity method separates. It does not recognise.
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3What the second instrument adds
Mass spectrometry measures the mass of the molecule itself. Every peptide has a molecular weight that follows from its sequence, so a measured mass either matches the expected one or it does not.
That is the identity check. Not how much of it there is, not how consistent the sample is. Simply whether the thing in the vial is the thing on the label.
It is a completely different question from purity, answered by a completely different machine, and it has to be run separately and reported separately.
Purity is a percentage. Identity is a yes or a no. You want both printed.
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4Why the cheaper test is the one you always see
Purity runs on standard equipment as part of a routine workflow. It is the least expensive analytical result a supplier can obtain, which is exactly why it is the one that ends up on every listing.
Identity confirmation is an additional method on additional equipment. Somebody has to commission it, and somebody has to pay for it.
A supplier who never orders it is not concealing a bad result. They do not have a result. Neither do you, and the listing looks identical either way.
The missing test is not a hidden failure. It is an absence of evidence, which is its own problem.
5Checking it takes one line
Open the certificate and look for a second method. That is the entire check.
- A purity line with a method beside it. Reverse-phase HPLC, not a bare percentage.
- An identity line. Mass spectrometry, reported separately from purity.
- Two results, not one result described twice. Purity and identity are different rows.
- The lot code on the document. Both results have to belong to your batch.
- A named laboratory and a date. Signed by whoever ran the methods.
If the document has purity and nothing else, you are looking at a consistency measurement being presented as a quality guarantee.
| The two questions | Pepsynth Labs | Typical supplier |
|---|---|---|
| How consistent is the sample | RP-HPLC, reported | Reported |
| Which compound is it | Mass spectrometry, reported | Usually not run |
| Methods named on the document | Both | Neither, or one |
| Results tied to your lot | Lot specific | Product level |
| Who ran the work | Independent U.S. laboratory | Unnamed |
Two questions deserve two answers.
Purity and identity are reported as separate lines on every lot, before you order rather than after.
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Comments
The coin analogy is the first explanation of this that actually stuck. Sorting by size is not the same as reading the face.
Same. I had been reading a purity number as an overall quality grade for years.
Checked four certificates in my email. Three had purity only. I genuinely thought identity confirmation was standard everywhere.
Absence of evidence rather than hidden failure is a fair way to put it. Most of them are not hiding anything, they simply never paid for the second test.
The illustration at the top is a good device. Shows exactly what the number supports and what it does not without overstating anything.
Look for a second method. That is a check I can actually remember and run every time.