Two numbers dominate most peptide test reports: purity and content. Both come from analytical chemistry techniques that are standard in peptide laboratories. Understanding how they work makes it much easier to judge a report, and to spot claims that a report doesn’t support.
HPLC: separating what is in the vial
High-performance liquid chromatography (HPLC) dissolves a sample and pumps it through a column packed with a fine material. Different molecules travel through the column at different speeds, so they come out at different times. A detector, commonly a UV detector set to a wavelength that peptides absorb, records each one as a peak on a chart called a chromatogram.
In a pure sample, almost everything appears as one main peak. Impurities, such as incomplete or damaged peptide chains left over from synthesis, appear as smaller peaks either side.
How purity is calculated
Purity is usually reported as the area of the main peak as a percentage of the total area of all detected peaks. If the main peak accounts for 99.5% of the peak area, the purity is reported as 99.5%. Two points follow from this:
- Purity only counts what the detector sees. Material that doesn’t show up under the method, such as water, counter-ion salts or other non-peptide substances, is not part of the calculation. That is why some reports note that purity “includes peptide peaks only”.
- Purity is a ratio, not an amount. A sample can be very pure and still contain less, or more, of the peptide than its label states.
Mass spectrometry: confirming identity
Mass spectrometry (MS) measures the mass of molecules very precisely. Because every peptide sequence has a predictable molecular weight, finding the expected mass confirms that the material is the compound named, and not a different peptide that happens to produce a similar HPLC peak. Laboratories often combine the two techniques as LC-MS, using chromatography to separate the components and mass spectrometry to identify them.
Measuring content in milligrams
Content tells you how much peptide is actually in the vial. Laboratories typically measure it by comparing the sample’s response with a reference standard of known concentration, then calculating the total amount present. Results are reported in milligrams, for example 31.73 mg in a vial labelled 30 mg.
For blends, a laboratory may identify and measure each component separately and report the amount of each, rather than a single purity figure.
What these tests don’t cover
- Sterility and endotoxins require separate microbiological tests.
- Heavy metals and residual solvents need their own methods.
- Stability over time is not shown by a single test; a report reflects the sample on the day it was analysed.
Reading results with this in mind
A useful report states which tests were requested, gives both content and purity where both were measured, and can be checked against the laboratory’s own records. Every Apex BioLabs product page shows its full Janoshik Analytical report. You can compare them all in our test report library, or read our guide on how to read a peptide COA.
