Where the number comes from
On most research-material reports the purity figure is a chromatographic purity, produced by high-performance liquid chromatography (HPLC). The sample is dissolved, passed through a column, and its components leave the column at different times. A detector records a response over time, and each component appears as a peak.
The laboratory integrates the peaks and divides the area of the main peak by the total area of every peak it detected. A result of 99.2% means the main component accounted for 99.2% of the detected signal under that method. Nothing more, nothing less.
What the figure does not measure
- Quantity. Purity says how much of the detected material is the main component. It says nothing about how many milligrams are in the vial. Quantity is a separate measurement and is not on most purity reports.
- Identity. A tall, clean main peak does not prove which substance it is. Identity is supported by mass spectrometry, which compares the observed mass with the theoretical mass calculated from the molecular formula. See HPLC purity vs. mass spectrometry identity.
- Anything the detector cannot see. Water, residual salts, and counter-ions from synthesis do not absorb at the detection wavelength and are invisible to a UV detector. A chromatographic purity of 99% can therefore coexist with a net content of the active material that is noticeably lower once water and salts are accounted for. Laboratories report that separately as net peptide content or by amino-acid analysis when it has been requested.
- Sterility, endotoxin, or biological activity. None of these are part of a purity method. If a report claims them, look for the separate method and result rather than inferring them from a purity figure.
Comparing two purity results
Two figures are only comparable when the method is comparable. Before treating 99.6% as better than 99.3%, check that:
- Both reports name the same method family, column type, and detection wavelength. A different wavelength can change which impurities are visible and therefore the denominator of the calculation.
- Both reports state a reporting limit or integration threshold. A method that ignores peaks under 0.1% will produce a higher purity than one that integrates everything down to 0.05%.
- Both describe the same material at the same labeled strength. A report for one strength does not describe another.
- Each report can be confirmed on the issuing laboratory's verification page. See how to verify a certificate of analysis.
With those points aligned, a difference of a few tenths of a percent is within what different runs of the same method routinely produce. A difference of several percent, or additional named impurity peaks, is worth reading closely.
Reading the impurity side of the report
The remainder after the main peak is the sum of everything else the detector saw. Better reports list the largest secondary peaks individually with their retention times and areas. A single unnamed impurity of 0.7% tells a different story from seven impurities of 0.1% each, even though both add up to the same purity figure. Truncated sequences and oxidation products typically elute close to the main peak; a well-resolved chromatogram shows them as separate, smaller peaks rather than a shoulder on the main one.
| Statement on a report | What it establishes | What it does not establish |
|---|---|---|
| Purity 99.4% (HPLC, area %) | Main-peak share of detected signal under the stated method | Identity, quantity, water or salt content |
| Observed mass 4113.5, theoretical 4113.6 | Identity of the main component | Purity or quantity |
| Net content 88% | Share of vial mass that is the active material | Chromatographic purity of that material |