Two Questions, Two Tests
When a supplier claims a peptide is "99% pure" or "third-party tested," that claim rests on two separate analytical techniques answering two different questions: what is this substance (identity), and how much of the sample is actually that substance versus everything else (purity). Understanding both makes it much easier to evaluate whether a Certificate of Analysis is actually meaningful.
HPLC: Measuring Purity
High Performance Liquid Chromatography works by forcing a liquid sample through a column packed with a specialised material under high pressure. Different molecules in the sample interact with that material differently, causing them to travel through the column at different speeds — a property called retention time.
As each component exits the column, a detector measures it and plots the result on a graph called a chromatogram. Each distinct molecule in the sample shows up as a peak at its own retention time. The target peptide should produce one large, dominant peak; any impurities, breakdown products, or leftover synthesis byproducts show up as smaller peaks elsewhere on the same graph.
How Purity Percentage Is Calculated
The purity percentage quoted on a CoA is typically calculated as the area under the target peptide's peak, divided by the total area under all peaks on the chromatogram. A result of "99% purity" means the target peptide accounted for 99% of everything detected in that sample — with the remaining 1% being impurities of some kind.
Mass Spectrometry: Confirming Identity
Mass spectrometry (MS) works differently. Rather than separating a mixture, it ionises a sample and measures the mass-to-charge ratio of the resulting ions. Because every molecule has a specific, predictable molecular weight, this measurement can confirm — or disprove — that a sample actually contains the compound it claims to.
For a peptide like Melanotan 2, a correct MS result will show a peak at the expected molecular weight for that specific amino acid sequence. If the observed molecular weight doesn't match, it indicates the sample isn't the compound it's labelled as — regardless of how "pure" that (wrong) substance might otherwise be.
Why Both Tests Are Needed
HPLC and MS are complementary, not interchangeable. A sample could be:
- High purity, wrong identity — 99% of the vial is a single substance, but MS reveals it isn't the correct peptide
- Correct identity, low purity — MS confirms the right molecular weight is present, but HPLC shows it only makes up a small fraction of the total sample
- Correct identity, high purity — the result researchers actually want, and the only combination that both tests need to independently confirm
A CoA that only reports one of these tests is only answering half the question. This is why independent, third-party testing that includes both HPLC and MS results is the standard researchers should look for, rather than relying on a single purity percentage in isolation.
Third-Party vs. In-House Testing
Any supplier can run their own internal quality checks, but a testing result carries more weight when it comes from an independent laboratory with no financial interest in the outcome. Third-party testing removes the inherent conflict of interest in a company grading its own product, and is generally considered the standard for credible research-grade material.
Other Analytical Methods Worth Knowing
HPLC and MS are the two most commonly cited tests, but they're not the only analytical tools used in peptide quality control. Depending on the supplier and the compound, a few other techniques may appear on more detailed testing reports:
Nuclear Magnetic Resonance (NMR)
NMR provides detailed structural information about a molecule by measuring how atomic nuclei respond to a magnetic field. It's a more resource-intensive test than routine HPLC/MS and is less commonly included in standard commercial CoAs, but it can offer additional structural confirmation beyond molecular weight alone.
Amino Acid Analysis
This method breaks a peptide down into its individual amino acid components and quantifies each one, confirming that the correct amino acids are present in the correct ratios. It's a useful cross-check against MS results, particularly for catching subtle sequence errors that might not significantly change the overall molecular weight.
Most research peptide suppliers rely on HPLC and MS as their core testing pair, since together they answer the two questions — identity and purity — that matter most for everyday research use. The additional methods above are more commonly seen in academic or pharmaceutical-grade characterisation work rather than routine commercial CoAs.
Frequently Asked Questions
Is a higher purity percentage always better?
Generally yes for research consistency, though the relevant threshold depends on the specific protocol. Most research applications look for ≥98% purity, since lower purity introduces more unaccounted-for variability between batches, which can confound results that depend on precise, repeatable concentrations.
Can HPLC results alone confirm a peptide is genuine?
No. HPLC shows how much of a sample is a single consistent substance, but it doesn't independently prove that substance is the correct peptide — a highly pure sample of the wrong compound would still produce a clean HPLC chromatogram. This is exactly why mass spectrometry identity confirmation is the necessary second half of a proper testing pair.
How often should testing be repeated for stored stock?
There's no universal rule, but for research where precise concentration matters, periodically re-testing stock that's been stored for many months — particularly after any temperature excursions or repeated freeze-thaw cycling — is reasonable practice rather than assuming original testing results remain valid indefinitely.
Why do different suppliers sometimes report slightly different purity figures for what should be the same compound?
Small variations between labs are normal and can come from differences in equipment calibration, column type, or exact methodology — this is one reason a single percentage figure shouldn't be treated as an absolute, universal truth. Larger discrepancies, however, are worth investigating, and are part of why checking which specific lab performed a given test matters as much as the number it produced.