Two questions, not one
When people ask whether a peptide is “good,” they are usually collapsing two separate questions into one:
- Is it the right molecule? — a question of identity.
- How pure is it? — a question of purity.
These are answered by two different techniques. Identity is confirmed by mass spectrometry, usually as part of LC-MS. Purity is measured by HPLC, usually reversed-phase HPLC (RP-HPLC). Neither one answers the other’s question, which is why a serious COA reports both. Below is what each actually measures, in plain terms, and why passing one while skipping the other leaves a real gap.
How HPLC measures purity
HPLC (high-performance liquid chromatography) separates the components of a mixture. The sample is dissolved and pushed through a column packed with a stationary material; in reversed-phase HPLC, that material is nonpolar, and a gradient of solvents gradually changes the mobile phase’s strength. Different molecules stick to the column and release at different times, so they exit — elute — separately. A detector, typically UV at a wavelength like 214 or 220 nm where the peptide bond absorbs, records a peak each time something comes off the column.
The result is a chromatogram: a series of peaks over time. Purity is reported as percent area (% area) — the area under the main peak divided by the total area of all peaks. A 98% area result means the target peak accounts for 98% of everything the detector registered under those conditions.
Two limits are built into this method:
- It is relative to what the detector sees. Components that don’t absorb at the chosen wavelength — water, many salts, counterions — may be invisible. So % area is not the same as % by weight.
- It is method-dependent. Change the column, gradient, or wavelength and the numbers can shift. This is why the method belongs on the COA.
Crucially, HPLC tells you how many things are present and in what proportion — but a single clean peak does not tell you what that peak is. It could be your peptide. It could be a different molecule that happens to elute at a similar time and absorb UV.
How mass spectrometry confirms identity
Mass spectrometry (MS) answers the “what is it” question. The instrument ionizes the molecule, then measures the mass-to-charge ratio (m/z) of the resulting ions with high precision. From that, the analyst determines the molecule’s mass and compares the observed mass against the theoretical mass calculated from the stated amino-acid sequence.
That theoretical value is usually given as a monoisotopic mass (built from the most abundant isotope of each atom) or an average mass (using average atomic weights). A close match between observed and theoretical mass is strong evidence that the molecule present is the one claimed.
Two practical notes:
- Peptides commonly show up as multiply charged ions (for example [M+2H]²⁺), so the raw m/z reading needs to be interpreted or deconvoluted into the true neutral mass before comparison.
- Coupling MS to a chromatographic separation (LC-MS) means the mass can be tied to a specific eluting peak — so you learn not just that the right mass is somewhere in the sample, but which peak it corresponds to.
What MS does not do on its own is quantify how pure the sample is. A correct mass confirms the target is present; it doesn’t establish that little else is.
Why you need both
Put the two together and the reason for pairing them becomes obvious.
Purity passes, identity unconfirmed. Suppose a sample shows a single sharp HPLC peak at 99% area, but no mass spectrum was run. You know the sample is homogeneous — mostly one thing — but you have not confirmed that the one thing is the target peptide. A wrong sequence, a truncated analog, or an unrelated compound could produce a beautiful, high-purity chromatogram. High purity of the wrong molecule is still the wrong molecule.
Identity confirms, purity unknown. Now suppose LC-MS confirms the target mass is present, but no purity number was reported. You know the right molecule is in there — but not whether it makes up 95% of the sample or 55%, with the balance being related impurities, deletion sequences, or degradation products. The correct peptide can be present and still be a minority of what’s in the vial.
Neither result alone supports a confident conclusion. Together, they answer the full question: the right molecule (MS identity), present at a stated level of purity (HPLC % area).
Where net content fits in
There is a third, separate measurement worth naming so it isn’t confused with the first two: net peptide content. Because lyophilized peptides carry water, counterions, and residual salts, the actual peptide can be well under 100% of the powder’s weight — even when purity is high. Net content, measured by quantitative HPLC against a standard or by amino acid analysis, estimates that weight fraction. It is not purity and not identity; it’s how much peptide by mass. All three questions are distinct.
The takeaway
- HPLC purity tells you how pure — the proportion of the main peak, under stated conditions, by % area.
- Mass-spec identity tells you whether it’s the right molecule — observed mass versus theoretical.
- Net content tells you how much is peptide by weight — a separate measurement again.
A COA that reports only one of these is answering only part of the question. Purity without identity, or identity without purity, is an incomplete picture — and for something you’re evaluating, the gap is exactly where problems hide.