909 AMINOSPurity vs. Measured Content: Two Different Questions on a Certificate
HPLC purity tells you what share of the material the method detected is the target compound; measured content tells you how many milligrams of that compound are actually in the vial.
- Purity (HPLC area %): the target peak's share of the detector response, not the percentage of the vial's weight.
- Content: how much of the compound is actually in the vial, in mg.
- A sample can be very pure and still hold slightly more or less than the label amount.
- Lot TESA-2609A: 99.22% purity; 10.28 mg measured in a 10 mg vial.
A certificate of analysis often reports two numbers that look alike but answer different questions:
- Purity: of the material the method detected, how much is the target compound?
- Measured content: how much of the target compound is actually in the vial?
A sample can score high on one and differently on the other. Reading them together, along with each specification, gives a more complete picture than either number alone.
Purity (HPLC, area %)
High-performance liquid chromatography (HPLC) separates the components of a sample as they pass through a column. A detector records each component as a peak.
- Purity by area % is the target compound's integrated peak area as a share of the total integrated peak area under the method's conditions (column, mobile phase, detection wavelength).
- Peak area is detector response, not mass. Different substances can respond differently at the same wavelength, so area % is not a mass fraction (% by weight) of the sample.
- It describes the relative composition of what was detected, and it does not measure the weight of material in the vial.
- Anything the detector doesn't respond to under those conditions isn't part of the calculation. That includes water, counter-ions (salts) and some substances that don't absorb UV light.
Measured content
A content result reports an amount: mg of the compound per vial, often also given as a percentage of the quantity on the label ("label claim"). A content specification can set a lower limit, an upper limit or both (for example, a range around 100% of label claim). The certificate states which applies.
Why the two can differ
- Purity looks at proportions; content looks at quantity. A sample can be highly pure and still contain somewhat more or less than the label quantity. Fill amount, and the counter-ions and water present in lyophilized materials, affect content but not purity.
- Methods differ by laboratory. Some laboratories report content from the same chromatographic method. Others report a "net peptide content" determined by a different technique, for example nitrogen or amino-acid analysis. Always read the method next to the number.
Example: lot TESA-2609A
From the KMD Analytical certificate for Tesamorelin, lot TESA-2609A (certificate 9AL-20260921-C7PG-01, issued 29 Sep 2026):
| Method | Result | Specification |
|---|---|---|
| HPLC-DAD: purity (area %) | 99.22% | ≥ 95.0% |
| HPLC-DAD: content per vial | 10.28 mg (102.8% of label claim) | ≥ 90% of label claim (10 mg) |
Read together:
- The purity result for this lot (99.22%): the target compound's peak accounted for 99.22% of the total integrated peak area this HPLC method detected. This is a share of detector response, not of mass.
- Content 10.28 mg: the laboratory reports 10.28 mg of net peptide in the vial, which is 102.8% of the 10 mg on the label.
- Both results are reported alongside their specifications, and both meet them. For content, this certificate states only a lower limit (≥ 90% of label claim). The certificate states that the specification set is client / method-defined.
The certificate makes the same distinction this page does. It notes that purity is not a measure of net peptide mass content (peptide versus counter-ion, water or residual solvent), and that this is reported separately as content per vial. It also reports identity ("Identity Conforms (RT + UV vs. reference)"): the sample's retention time and UV spectrum matched the reference.
Checklist for any certificate
- Are both purity and content reported, and is the method named for each?
- Does each result have a specification next to it, and does the certificate say who set the specifications?
- Is content expressed as an amount (mg) and as a percentage of label claim, and does its specification set a lower limit, an upper limit or both?
- Does the certificate name the issuing laboratory, its certificate number and the lot, and does that lot match the vial?
- Is the original laboratory document available, so the figures can be checked at the source?
A certificate reports only the tests it lists, for the sample tested from the named lot. It doesn't describe other lots or tests that weren't performed.
Frequently asked questions
What does HPLC purity (area %) measure?
The target peak's share of the total integrated peak area that the method detected. It is a proportion of detector response, not the percentage of the vial's weight.
Why can a highly pure sample contain more or less than the label amount?
Purity is a proportion and content is an amount. The fill quantity, and the water and counter-ions present in lyophilized material, change the content without changing the area-% purity.
What is net peptide content?
The measured amount of the peptide itself, reported in milligrams per vial, as distinct from counter-ions and water. Laboratories measure it with different techniques, so read the method next to the figure.
What did the TESA-2609A certificate report?
99.22% purity by HPLC area % (specification ≥ 95.0%) and 10.28 mg per vial, 102.8% of the 10 mg label claim (specification ≥ 90%). Both results met their specifications.
Sources
- KMD Analytical certificate 9AL-20260921-C7PG-01 (lot TESA-2609A), original PDF
- ICH Q2(R2) Validation of Analytical Procedures (U.S. FDA guidance page)
Sources are cited for reference. Their publishers have not reviewed or endorsed this guide.
For laboratory research use only. Not for human or veterinary use. Not FDA approved.
Source certificate: lot TESA-2609A · Documentation Center · Ver en español