Peptide purity: what +99.5 % means and what it does not say
· Novatheris
"+99.5 % purity" is the figure this market repeats the most and the one the fewest people know how to read.
It is not a percentage of the weight in the vial, it does not confirm which compound is inside, and it says nothing about endotoxins or metals. It is worth understanding exactly what it measures, because every other report gets easier to read from there.
Where the number comes from
Peptide purity is measured by HPLC, a technique that pushes the sample through a column where its components separate and come out one by one. An ultraviolet detector, between 210 and 220 nanometers, records each thing as it passes.
The result is a graph made of peaks. The peptide you are after is the big peak. The impurities are the small ones.
Purity is a simple division: area of the main peak over the total area of all the peaks. If the main peak takes 99.68 % of the area, that is the number on the report.
The key word is "relative"
Here is the most common misunderstanding. That percentage is relative to all the material that absorbs light at 220 nm, not to the weight of the powder in the vial.
Peptides are supplied as salts, usually acetate or trifluoroacetate, and they come with water attached. Neither the salt nor the water absorbs at that wavelength. So they simply never enter the count.
A vial can report 99.99 % purity and still have part of its weight be salt and water. There is no trick in that: purity answers "how clean is the peptide", not "how much peptide does the vial weigh".
So how much peptide is in there?
That is a different measurement: net peptide content. It is calculated by measuring nitrogen, because salts and water have no nitrogen and the peptide does.
Our two published reports show the difference with real numbers:
- Freedom Diagnostics Testing, certificate NOVA2609150054: HPLC-UV purity of 99.99 %, identity confirmed by LC-MS and net content of 23.28 mg, in a vial labeled 20 mg.
- Janoshik, report #123821: purity of 99.678 % and 35.48 mg of Tirzepatide, in a vial labeled 30 mg.
Two different measurements on the same page, and they are answering two different questions. Content not matching the label is not a lab error: the label is nominal and the analysis measures what is actually in the vial.
One useful technical note: Bachem recommends using the lowest purity value on the data sheet when calculating molar concentrations, because impurities that are themselves peptide count toward the weight.
Purity is not identity
A peak can be spotless and be the wrong compound.
HPLC purity measures separation, not identity. What confirms identity is mass spectrometry, which measures the molecular weight of the sample and compares it to the weight that compound should have. It can also be confirmed by coelution with a reference sample, or by amino acid analysis.
That is why serious reports carry both. On certificate NOVA2609150054 the stated method is "HPLC with UV detection coupled with mass spectrometry (LC-MS)", and the document prints the chromatogram and the mass confirmation.
A report that only says "99.9 % pure" and never confirms identity is telling you half the story.
Purity is not sterility, endotoxins or metals
Three more tests, three separate documents.
Endotoxins: residues from gram-negative bacteria. They are measured with the LAL test and the result is given in EU, endotoxin units. Our Janoshik report #123822 reports under 1 EU per vial. This is not the same as sterility either: sterility measures whether live microorganisms are present, and that is another test entirely.
Heavy metals: the international reference is the ICH Q3D(R2) guideline, whose Class 1 metals, the most toxic ones, are cadmium, lead, arsenic and mercury. Janoshik report #123823 reports those four as "not detected".
A vial with 99.9 % purity and no endotoxin report simply has no endotoxin report. Purity does not cover it.
And "not detected" is not zero
Every instrument has a detection floor. Below a certain amount it can no longer tell signal from noise.
"Not detected" means that if something is there, it sits under that floor. It is the answer anyone wants to see, but it is not a declaration of absolute absence.
Why 99.99 % and 99.68 % do not compare head to head
It is tempting to put two reports side by side and crown the higher decimal. It does not work that way, and the reasons are all in the method.
The method changes, the column changes, the wavelength changes, and so does the way the software integrates the peaks, meaning how it decides where each one starts and ends. Two different labs, measuring the same vial, have no reason to land on the same decimal.
What you can compare is something else: whether there is a report at all, whether it belongs to your batch or another one, and whether it confirms identity on top of purity.
So what does the "+99.5 %" seal mean?
It is the brand promise: the standard we hold ourselves to for every batch we bring in.
The real number for your vial is in the report for its batch, with its code printed on it and its verification key on the lab's page. When a vial does not yet have its own batch analysis, we show the seal and not another dose's report. Never the other way around.
What a purity number cannot tell you
Nothing about effects, safety or use in people. Purity is chemistry: it says how clean the material you bought as a research reagent is, and that is where it stops.
It also does not turn anything into "pharmaceutical grade" or into something approved by anyone. The legal footer on the Freedom certificate says it plainly: the peptides tested are for research use only and are not approved for human or veterinary use.
Four false statements you will hear
- "99.5 % purity means 99.5 % of the weight is peptide." No.
- "If it is pure, it is what the label says." No, that is what mass spectrometry confirms.
- "If it is pure, it has no endotoxins or metals." No, those are separate tests.
- "This lab's 99.99 % beats that lab's 99.68 %." They do not compare head to head.
If you want to see the real number for your vial, look up its batch code on the verification screen and the report for that batch comes up.