Understanding Peptide Testing
Peptide testing is not a single measurement. Identity, quantity, purity, endotoxin and sterility are distinct analyses answering distinct questions. Understanding what each test establishes — and what it does not — is the difference between reading a report and trusting a number.
What is identity testing?
Identity testing asks whether the compound present is the compound represented on the label. Analytical techniques such as mass spectrometry compare the observed molecular mass with the theoretical mass of the target peptide, and chromatographic retention behaviour can provide supporting evidence.
What is peptide quantity?
Labelled quantity is what a vial claims to contain. Measured quantity is what analysis finds. Because lyophilized material also contains counterions, residual water and salts, gross weight and net peptide content are not the same figure, and quantitative analysis is the only way to distinguish them.
What does HPLC purity establish?
HPLC purity expresses the target peak as a percentage of the total detected peak area under a specified method. It is a relative measure of peptide-related species. It does not establish identity, it does not describe total vial contents, and it does not establish sterility or endotoxin content.
How does mass spectrometry help?
Mass spectrometry measures the mass-to-charge ratio of ionised molecules, supporting molecular identification. Coupled to chromatography as LC-MS, it can also reveal species that a UV-based purity assay would not usefully characterise.
What is endotoxin testing?
Bacterial endotoxin testing measures lipopolysaccharide from Gram-negative bacteria, typically by LAL or recombinant Factor C methods, reported in endotoxin units per milligram or per vial. It is independent of chemical purity.
What is sterility testing?
Sterility testing is a microbiological assay that looks for viable organisms under defined incubation conditions. Sterility and chemical purity are separate questions answered by separate methods; neither implies the other.
What is TFA analysis?
Trifluoroacetic acid is used in peptide cleavage and purification and can remain as a counterion in finished material. Residual TFA or counterion analysis quantifies how much remains, which matters when interpreting sensitive experimental systems.
What is contaminant screening?
LC-MS contaminant screening looks for unexpected species in a sample — for example related peptides, process residues or substitutions — that a routine purity assay may not identify by name.
What is heavy metal screening?
Elemental analysis, commonly by ICP-MS, screens for metals that can be introduced through raw materials or processing. Results are reported per element against the method's detection limits.
| Test | What it measures | What it doesn't establish |
|---|---|---|
| HPLC | Relative chemical purity | Sterility, identity, total content |
| LC-MS | Molecular identification | Sterility, endotoxin |
| Endotoxin | Bacterial endotoxin | Chemical purity, sterility |
| Sterility | Microbial contamination | Peptide identity or purity |
| TFA | Residual TFA content | Peptide purity or identity |
Continue reading: How peptide purity is measured · Understanding COAs · Lab results