Peptide purity by HPLC: how each technique in the analysis works
· Novatheris
Peptide purity by HPLC is the most quoted number on any lab report. But behind that number there is an instrument, a column, a detector and software that makes decisions. Knowing how it works helps you understand why the result says what it says, and why it does not say other things.
This piece goes inside the techniques: HPLC, mass spectrometry, the endotoxin test and the metals analysis. If you are after what the "+99.5 %" seal means on its own, we cover that in what +99.5 % purity means.
What HPLC is
HPLC stands for high-performance liquid chromatography. The core idea is to split a mixture into its components, one at a time.
For peptides, the most common mode is reversed phase. The column is a steel tube packed with very fine particles coated with carbon chains, often C18. That surface is hydrophobic: it attracts the parts of a molecule that "avoid" water.
The dissolved sample enters the column pushed by a liquid, the mobile phase. Each molecule stays stuck to the surface for a different length of time, depending on how hydrophobic it is. The ones that stick less come out first, and the ones that stick more, later.
The gradient and retention time
The mobile phase is not fixed. It is typically a mix of water and acetonitrile, often with a little trifluoroacetic acid, and the share of acetonitrile rises during the run. That is called a gradient.
As the acetonitrile goes up, the liquid gets better at "unsticking" molecules from the column, and they come out in order. The moment each compound comes out is its retention time.
Under the same conditions, a compound always comes out at roughly the same minute. For example, on the Freedom Diagnostics Testing certificate NOVA2609150054, for Retatrutide 20 mg, the printed chromatogram shows a single peak near the 15-minute mark.
The detector and how the percentage is calculated
At the column outlet there is an ultraviolet light detector. For peptides it usually reads between 210 and 220 nanometers, because that is where the peptide bond absorbs, the bond linking one amino acid to the next.
Every time something passes the detector, the signal rises and forms a peak. The full plot, signal against time, is the chromatogram.
Purity is calculated like this: the area of the main peak divided by the area of all peaks, times one hundred. If the main peak takes 99.678 % of the area, as on our Janoshik report #123821 for Tirzepatide 30 mg, that is the number.
What +99.5 % means in this calculation
Put in HPLC terms, "+99.5 %" means the peptide peak takes at least 99.5 % of the area the detector sees. What is left, under half a point, is spread across the small impurity peaks.
For us it is the brand standard for every batch. The exact number for your vial is on its batch report, not on the seal.
What HPLC cannot see
Here are the limits, and they are worth knowing.
Whatever does not absorb at that wavelength does not count. The salt and water the powder carries do not show up on the chromatogram. That is why purity is a relative percentage, not the weight of the vial.
Two compounds can come out together. If an impurity has almost the same hydrophobicity as the peptide, it can come out stuck to the main peak and add to it. A clean peak does not guarantee there is nothing hiding underneath.
The software decides where each peak starts and ends. That integration depends on settings, and it changes from lab to lab. That is why two reports on the same compound do not give the same decimal.
It does not say which compound it is. HPLC separates and quantifies. It does not identify.
Mass spectrometry: identity
To know what the peak is, you use mass spectrometry, or MS. When it is connected to the HPLC it is called LC-MS.
The principle is weighing molecules. The sample is ionized, meaning the molecules pick up an electric charge. For peptides the usual method is electrospray ionization. A large molecule often carries several charges at once, so the instrument sees several signals for the same peptide.
What the instrument measures is the ratio of mass to charge, written m/z. From those signals, the software rebuilds the real molecular weight. That weight is compared with the theoretical one.
A reference example: according to PubChem, Tirzepatide weighs 4813.53 Da and Retatrutide 4730.477 Da. If the peak weighs what it should, identity is confirmed. If it weighs something else, it does not matter how clean it looks.
Certificate NOVA2609150054 states its method as "HPLC with UV detection coupled with mass spectrometry (LC-MS)" and prints the mass confirmation next to the chromatogram.
Endotoxins: the LAL test
Endotoxins are remnants of the cell wall of gram-negative bacteria. A peptide can be extremely pure by HPLC and still carry them, because HPLC is not looking for them.
They are measured with the LAL test, which uses an extract of blood cells from the horseshoe crab. That extract reacts to endotoxins with a clotting cascade. The reference chapter is USP 85.
There are three main ways to read that reaction:
- Gel clot: you check whether a firm gel forms or not. It is a yes-or-no test above a threshold.
- Turbidimetric: you measure how cloudy the mixture gets over time.
- Chromogenic: the reaction releases a color, and its intensity is measured.
The result is given in EU, endotoxin units. Our Janoshik report #123822 reports less than 1 EU per vial.
Heavy metals
The metals analysis looks for toxic elements that could have entered during manufacturing. The international reference guideline is ICH Q3D(R2), and its Class 1 metals are cadmium, lead, arsenic and mercury.
The usual industry technique for this is inductively coupled plasma mass spectrometry, ICP-MS. The sample is fed into a very hot plasma that breaks everything into atoms, and the instrument counts the atoms of each element by their mass. It detects very small amounts.
Our Janoshik report #123823 lists arsenic, cadmium and lead as "not detected", and mercury too, in the comments field. Not detected means below the method's limit, not zero.
Four techniques, four questions
- HPLC-UV: how clean is the peptide relative to what the detector sees?
- MS: is it the compound it claims to be?
- LAL: how much bacterial residue does it carry?
- ICP-MS or another metals method: are there toxic metals?
None of them answers another one's question. That is why a serious report does not stop at a single number.
You can see the purity, identity, endotoxin and metals reports for each analyzed batch, exactly as the lab delivers them, in the lab analysis section. Retatrutide, with its four reports from Janoshik and Freedom, is on the Retatrutide lab analysis page.