HPLC vs mass spec: purity vs identity
HPLC asks how clean the sample is. Mass spec asks what the molecule is. A real COA answers both.
HPLC (high-performance liquid chromatography) measures purity: the share of detected material that is the main compound. Mass spectrometry measures molecular weight, which confirms identity: whether that compound is the peptide on the label. A high purity score alone cannot prove identity.
How pure?
Reports a percentage. Does not tell you which compound.
Which compound?
Reports molecular weight. Does not tell you how pure.
What does HPLC purity actually measure?
HPLC pushes the sample through a packed column. Different molecules travel at different speeds, so they leave the column at different times. A UV detector records each one as a peak. Purity is the area of the main peak divided by the total area of all detected peaks.
So 99.82% means 99.82% of the detectable material at that wavelength is the main peak. That is a strong result. It is also all HPLC can say: it does not name the compound in the main peak.
How do you read an HPLC chromatogram?
The horizontal axis is retention time
How long each component took to pass through the column, in minutes. The target peptide comes out at a characteristic time for that method.
The vertical axis is detector response
How strongly each component absorbs UV light. Taller and wider peaks mean more material.
One tall peak is the target
Its area, as a share of all peak areas, is the purity figure on the COA.
Small peaks are everything else
Together they make up the remainder. At 99.82% purity, they add up to 0.18%.
A flat baseline matters
A noisy or drifting baseline makes small peaks harder to measure and the purity figure less certain.
What do peptide purity grades mean?
“Research grade” is not a fixed standard. These bands are a practical way to read the number on a COA.
| HPLC purity | How to read it |
|---|---|
| Below 95% | Low. More than 1 part in 20 is something other than the main compound. |
| 95 to 98% | Common in lower-cost research material. Below Astramide’s release floor. |
| 98 to 99.5% | Meets our floor. Lots below 98.0% do not ship. |
| 99.5% and up | Very high purity by HPLC. Still says nothing about identity on its own. |
How does mass spectrometry confirm identity?
Every peptide has a theoretical molecular weight, fixed by its amino acid sequence. Mass spec measures the real one.
Ionize
The peptide is given an electric charge so it can be steered.
Separate
Ions are sorted by mass-to-charge ratio.
Detect
The instrument records which masses arrive and how many.
Compare
Observed weight is checked against theoretical, commonly within about 1 to 2 Da.
In LC-MS, liquid chromatography separates the sample first and the mass spectrometer reads each part. Some reports confirm identity a different way, by matching retention time to a reference standard. The COA should say which method was used.
HPLC vs mass spec, side by side
| HPLC | Mass spectrometry | |
|---|---|---|
| Question it answers | How pure is the sample? | Is it the right molecule? |
| What it reports | Purity as a percentage of peak area | Observed molecular weight, or “Confirmed” |
| What it misses | Which compound the main peak is | How much of the sample is that compound |
| Typical COA line | HPLC-UV purity 99.82%, spec at or above 98.0% | Identity LC-MS, Confirmed |
Why high purity is not enough
Lot CP10 scored 99.85% on HPLC. By purity alone it looked like a top lot. LC-MS then showed the molecule was not the one on the label. It did not ship, and its report stays published.
Without the identity test, nothing on the report would have flagged it.
Now try it on a real report
Every lot we have tested is public, including the ones that failed. Pick one and run the checks.
Questions about HPLC and mass spec
What is the difference between HPLC and mass spectrometry?
HPLC separates a sample and measures how much of the detected material is the main compound, which gives a purity percentage. Mass spectrometry measures molecular weight, which shows whether that compound is the peptide on the label. Purity and identity are different questions, so a complete COA answers both.
Does 99% HPLC purity mean the peptide is correct?
No. HPLC purity says 99% of the detected material is one compound. It does not say which compound. Our lot CP10 scored 99.85% purity and failed identity by LC-MS, so it was not released. Identity needs its own test.
What does LC-MS mean on a COA?
LC-MS is liquid chromatography coupled to mass spectrometry. The sample is first separated, then the mass spectrometer measures molecular weight. On a COA it is the usual method behind an identity result such as "Confirmed".
How close should the observed molecular weight be to the theoretical weight?
For intact peptides, labs commonly accept a match within about 1 to 2 Da of the theoretical molecular weight, depending on the instrument and method. The COA should state the result or the acceptance criterion it used.
Can HPLC detect every impurity?
No. A UV detector only sees material that absorbs at the chosen wavelength. Water, some salts and residual counter-ions can be missed, which is one reason net content is measured separately from purity.
Next: how to read a peptide COA in 5 checks, the peptide glossary, or how every Astramide lot is tested before release.