Why two labs can test the same peptide and get different results

Two certificates for the same product disagree. That does not automatically mean one laboratory is wrong. Most differences come from comparing different things. Here are the seven common causes, roughly from most to least frequent.

1. One measured purity, the other measured content

Purity is a percentage: the main peak's share of everything the detector saw. Content is milligrams in the vial. A sample can be 99% pure and hold 8 mg in a 10 mg vial. If one certificate reports 99% and another reports 80% of label, they may both be right. Purity is not content explains the difference.

2. They tested different vials

Filling varies from vial to vial, and contamination varies even more. Two vials from the same batch can give different content and endotoxin results. Two vials from different batches can differ completely. Check that both certificates name the same batch.

3. The methods were not the same

HPLC conditions vary between laboratories: the column, the solvents, how fast their mix changes, and the detector setting. These change how well impurities separate from the main peak. Under one method an impurity comes out on its own and is counted. Under another it hides inside the main peak and the purity looks higher. Neither laboratory made a mistake. They answered the question under different conditions.

4. Net weight versus total powder

Peptide powder is never pure peptide by weight. It also holds water and salts left from manufacturing, often called counter ions. A label may state the total powder weight, while a content test measures the peptide itself. A vial labelled 10 mg of powder can correctly measure less than 10 mg of peptide.

5. Different reference standards

Content is measured against a reference standard of known amount. If two laboratories use standards of different quality, or one corrects for the standard's own purity and water content and the other does not, their milligram figures will differ.

6. Handling between the tests

Freeze-dried powder is stable, but it picks up moisture once opened, and peptides mixed into liquid break down with time. A vial tested months after another, or after being opened, may genuinely contain something different.

7. Measurement uncertainty

Every measurement has a margin. Two careful measurements of the same vial rarely give exactly the same number. Small differences, within a few percent, are normal and not a sign that either result is wrong. Large differences are worth explaining using the six causes above.

How to compare two certificates fairly

1. Same quantity. Compare purity with purity and content with content.
2. Same batch. Check both certificates name the batch on your vial.
3. Dates. Note when each laboratory received its sample.
4. Method. Look for the method beside each result. A purity figure with a chromatogram and peak table can be checked. A bare percentage cannot. See How to read a peptide chromatogram.
5. Verification. Confirm each certificate with whoever issued it. See How to tell if a peptide COA is real.

If the two still disagree after all five checks, a third test on a sealed vial from the same batch settles it. Identity, content and purity starts at €169 on the price list.