Written and reviewed by Peptide.Express Editorial Team||
Peptide Third-Party Testing, Explained
A peptide Certificate of Analysis is a batch record. HPLC area percent, LC-MS identity, net peptide content, and endotoxin each answer a different question. Peptide.Express does not run in-house purity testing. Certificates this catalog links name Mistral Diagnostics or Freedom Diagnostics. For laboratory research use only. Not for human or veterinary use.
Four measurements, four questions
A single percentage cannot stand in for the other three. Area percent says how much of the UV trace sits in the main peak. Mass spectrometry says whether that peak matches the named peptide. Net peptide content says what peptide mass the laboratory recorded. Endotoxin, when a line is present, says how much lipopolysaccharide the LAL assay reported. None of those results is implied by the others.
| Measurement | Method | It answers | It does not answer |
|---|---|---|---|
| Purity | HPLC area percent | What share of UV-absorbing material is in the main peak | Whether that peak is the named peptide |
| Identity | LC-MS or LC-MS/MS | Whether the observed mass matches the named peptide | How pure the sample is |
| Net peptide content | Assay reported on the certificate | Measured peptide mass for that sample | The labeled fill of the vial |
| Endotoxin | LAL, when the laboratory reports it | Lipopolysaccharide in that batch | Sterility, and a missing line is not a result |
HPLC area percent
Reverse-phase HPLC drives the sample through a nonpolar column, commonly C18, on a water and acetonitrile gradient. Peptides leave the column in order of hydrophobicity. The detector records absorbance. The chromatogram is that trace.
Area percent is the area of the main peak divided by the summed area of the peaks in the trace. It is a proportion of UV-absorbing material. It does not name the peak, and it does not describe material that the detector does not see.
On Peptide.Express product pages, ≥99% by HPLC is a purchasing specification. The measured area percent for a lot is the number on that lot's certificate, and it can differ from the specification. A clean chromatogram can still be the wrong molecule. Read the mass result before treating the percentage as identity.
LC-MS identity
Liquid chromatography coupled to mass spectrometry measures the mass-to-charge ratio of what elutes. Tandem mass spectrometry (LC-MS/MS) fragments that ion and reads the pieces. Either way, identity is the comparison of the observed mass with the theoretical mass of the named peptide.
Purity and identity stay separate on a usable certificate. A dominant peak without a matching mass is an unnamed peak. Print only the mass fields the laboratory reported for that lot. A missing mass line is not confirmation.
Net peptide content
Labeled fill is the presentation named on the vial. Assayed net peptide content is the peptide mass the laboratory recorded for the sample it tested. They are different fields.
Counter-ions, residual water, and the way the laboratory defines the analyte can put those two numbers apart. This page does not quote a milligram result. The figure that belongs to a vial is the one on the certificate whose lot matches the label. A product-level document is not a substitute for that match.
Endotoxin testing
LAL means Limulus amebocyte lysate, a reagent that reacts with lipopolysaccharide from Gram-negative bacteria. Endotoxin is heat-stable and passes through sterilizing filters, so a sterility result is not an endotoxin result. In cell culture, endotoxin can activate TLR4 and shift cytokine readouts that have nothing to do with the peptide under study.
When the laboratory reports an LAL value, that value is the result for the batch. A blank line is not a pass, and it is not proof the assay was never run. This page states no endotoxin-unit limit. A limit that is not on the certificate for the lot in hand would be an invented number.
Laboratories named on Peptide.Express certificates
Mistral Diagnostics or Freedom Diagnostics are the laboratory names this catalog treats as the letterhead on the certificates it links. Batch records are sometimes typed as Mistral, as Mistral Diagnostics, or as Mistral Diagnostics followed by a city. Those spellings are one laboratory. Freedom Diagnostics is the other name. A lot names one laboratory, not both.
The name on the certificate is the laboratory that reported that lot. Peptide.Express does not conduct in-house purity testing. Verify a document against the issuing laboratory's own records when that laboratory offers a lookup. The public list of certificates is the lab-results library.
How to read one lot
Match the lot on the vial to the lot on the certificate before reading any number. Then read compound name, HPLC area percent together with the chromatogram, observed mass against the theoretical mass, appearance, endotoxin if a line is present, the test date, and the laboratory name. If the lots do not match, the rest of the document does not describe that vial.
Questions
What does HPLC area percent measure?
The share of UV-absorbing material that sits in the main chromatographic peak. It does not identify that peak. On Peptide.Express pages, ≥99% by HPLC is a purchasing specification. The measured area percent is the figure on that lot's certificate. Research use only.
What does LC-MS confirm that HPLC does not?
Identity. LC-MS compares the observed mass with the theoretical mass of the named peptide. HPLC area percent says how much of the trace is the main peak. A dominant peak without a matching mass is an unnamed peak. Research use only.
Is net peptide content the same as the labeled fill?
No. Labeled fill is the presentation on the vial. Net peptide content is the peptide mass the laboratory recorded. They are separate fields, and this page does not quote a milligram figure for either. Use the certificate whose lot matches the vial. Research use only.
Which laboratories does Peptide.Express name on certificates?
Mistral Diagnostics or Freedom Diagnostics. A shortened "Mistral" spelling and a city suffix refer to Mistral Diagnostics. A lot names one laboratory. Peptide.Express does not run in-house purity testing. Research use only.
Does a sterility result replace an endotoxin result?
No. Endotoxin is heat-stable and survives sterile filtration. An LAL result, when the laboratory reports one, is the endotoxin figure for that batch. A missing line is not a pass. Research use only.