Laboratory quality
Identity testing
How a laboratory answers the question "is the vial actually what the label says?", and where the answer stops.
- Last updated:
- 2026-07-22
Overview
Identity testing is the set of analytical steps that verify a sample is the molecule its label claims. For peptides, the primary identity technique is mass spectrometry (usually LC-MS): the observed molecular mass is compared to the theoretical mass computed from the sequence.
Secondary methods include amino-acid analysis, sequencing (Edman degradation or MS/MS fragmentation), and comparison of an HPLC retention time to a reference standard.
Why it matters
Identity is the most fundamental question a CoA can answer. Purity, potency, and stability numbers are meaningful only if you know what molecule you are measuring.
Key concepts
Mass confirmation
Mass agreement between observed and theoretical is necessary but not sufficient. It rules out many kinds of mistake but does not distinguish sequence isomers with the same composition.
Reference-standard comparison
Where a reference standard exists, comparing HPLC retention time and mass spectrum against the standard is stronger evidence than either alone.
Sequencing
Full sequencing (Edman or MS/MS) confirms the amino-acid order, not just the composition. It is more work and correspondingly less common on routine CoAs.
Common misconceptions
- "Identity confirmed" on a CoA is not a guarantee for every vial in the batch. It is evidence about the sample that was tested.
- "Same molecule" and "same product" are not synonymous. Two identical peptides in different formulations behave differently.
- Identity testing does not detect microbial contamination or endotoxin.
Practical interpretation
When a CoA states identity was confirmed by LC-MS, look for the observed mass value and confirm it matches the theoretical mass for the peptide you expected.
Limitations
Identity tests only cover what the analyst thought to look for. A vial containing the correct peptide plus an undisclosed second molecule may still pass identity testing on the first.