ANALYTICAL METHODS · PRACTICAL GUIDE
HPLC Peptide Purity: Work Through a 98% Result
Learn what the percentage measures, then check how the peaks were counted.
Two peptide reports both say 98%. One includes a complete graph and area table; the other shows only a cropped main peak. The headline number is the same, but the information you can check is different.
We will calculate a result, change one calculation choice and then separate chromatographic purity from the amount of peptide in a vial. You do not need to operate an HPLC instrument to follow the lesson.
1. What does HPLC separate?
HPLC passes a sample through a column and records a detector signal over time. Components that behave differently in the method can appear as separate peaks. Imagine several runners reaching a finish line at different times: the instrument records their arrival rather than photographing the whole starting group.
The horizontal axis is time. The vertical axis is detector response. In a UV method, the selected wavelength matters. A component that responds weakly or does not appear as a separate peak may not be represented fairly by its apparent area.
2. Calculate 98% from the area table
Assume the target peak has already been assigned, every listed peak is included and no response-factor correction is applied. Read the area table rather than estimating from the picture.
| Peak | Area | Share |
|---|---|---|
| Target | 980 | 98% |
| Other peak A | 12 | 1.2% |
| Other peak B | 8 | 0.8% |
| Total | 1,000 | 100% |
Add the included areas: 980 + 12 + 8 = 1,000. Divide the target area by that total.
980 ÷ 1,000 × 100% = 98%
This result means 98% of the included detector area belongs to the assigned target peak. It is not a direct weighing of the powder.
3. Why does the number change if a peak is excluded?
Suppose the peak with area 12 is left out. The new total is 988, so 980 ÷ 988 × 100% is about 99.19%. The sample has not changed. The denominator has.
An exclusion can be justified when the analytical procedure identifies a solvent or system signal that should not be counted. It should have an explanation. Agilent describes how integration settings affect peak handling; this is why the processing method deserves attention alongside the graph.
4. A larger number is not the whole comparison
Before comparing 98% with 99%, ask whether both tests used an equivalent method and reporting basis. Check detection wavelength, separation, target assignment and integration rules. A shoulder may hide an unresolved component; a smooth-looking peak alone cannot rule that out.
HPLC also does not answer every other quality question. Identity, water, residual solvents and endotoxin need evidence appropriate to those questions. Do not substitute the purity figure for a test that was not performed.
5. Do not turn area purity into milligrams
A vial labeled 10 mg with 98% HPLC area purity does not establish 9.8 mg of target peptide. The weighed powder may contain non-peptide material, and area response is a different measurement. If your assay needs a known target concentration, request a suitable content or assay result and its basis.
Check your answer
475 ÷ (475 + 25) × 100% = 95%. This is an area percentage, not a mass percentage.
6. Read an HPLC report in this order
First match the sample and lot. Then find the method, full graph, target assignment and area table. Recalculate the percentage. Finally ask about shoulders, omitted signals and unavailable tests. If an item is missing, describe the missing item rather than deciding from a cropped image that the material is good or bad.
Use this guide in a product enquiry
For materials such as GHK-Cu、LL-37, identify the exact form, quantity and research question first. Then ask what records are available for the relevant lot. Matching product names do not make different specifications interchangeable.
Discuss products and document requirements with NHD. Continue learning: Reading a chromatogram / Purity versus content.