When a Certificate of Analysis reports "≥99.0% HPLC" and "Mass Spec confirmed," what is actually being measured? These are two distinct analytical methods that answer two different questions. Both matter. Understanding what each one tells you — and what each one does not — is one of the most useful skills a peptide researcher can develop.
HPLC: How Pure Is It?
High-Performance Liquid Chromatography (HPLC) is a separation technique. A liquid sample is pushed under high pressure through a packed column. Different molecules in the sample travel through the column at different rates depending on their size, polarity, and how they interact with the column's stationary phase. The result is a chromatogram — a graph showing peaks as each molecular species exits the column at its characteristic retention time.
For peptide purity, the value reported as "% HPLC" is the area under the main peak divided by the total area under all detected peaks. A 99% HPLC purity result means 99% of the detectable material in the sample is your target peptide; the remaining 1% is everything else combined — synthesis byproducts, degradation products, residual reagents, etc.
What HPLC Cannot Tell You
HPLC purity has limits. It only measures what the detector can see (typically UV-absorbing species at 214 nm for peptide bonds), and it does not tell you the identity of the dominant peak — only that it represents most of the material. A 99% pure sample of the wrong peptide will still read as 99% pure.
This is why HPLC purity alone is insufficient quality documentation. You need a complementary identity test.
Mass Spectrometry: What Is It?
Mass spectrometry (MS) measures the mass-to-charge ratio of ionized molecules. For peptide research, MS is typically used as an identity confirmation step — comparing the observed molecular weight to the expected weight of the target peptide.
If the expected molecular weight of your peptide is 1419.5 g/mol and the MS spectrum shows a peak at 1419.5 (or its expected charged state like [M+H]+ at 1420.5), you have positive confirmation that the dominant species is your target compound.
Why MS Alone Is Not Enough
MS confirms identity but does not quantify purity. A sample that contains the target peptide plus contamination from other peptides could still produce a clear MS signal for the target. MS tells you "yes, this molecule is in there" — not "this molecule is 99% of what is in there."
The Combined Picture
HPLC and MS are complementary. Together they answer the two essential questions:
- HPLC: How pure is the sample? (Quantitative — gives a percentage)
- MS: Is the dominant compound the right one? (Qualitative — gives identity confirmation)
A complete COA reports both. Anything less should be treated as incomplete documentation.
Other Tests You May See on a COA
USP <85> Endotoxin (LAL)
Bacterial endotoxins — pyrogenic lipopolysaccharides from gram-negative bacterial cell walls — are tested via the Limulus Amebocyte Lysate (LAL) assay. The result is reported in endotoxin units per milliliter (EU/mL). USP <85> requires research peptide samples to meet specified limits.
USP <71> Sterility / Microbial Limits
Microbial sterility testing confirms the absence of viable bacterial, fungal, or yeast contamination in the lyophilized product. The result is reported as Pass/Fail.
Identity Verification Beyond MS
Some labs supplement MS with UV-Vis spectrophotometry to confirm the lambda max absorption profile of the peptide, or with amino acid analysis to verify sequence composition. These add additional layers of identity confidence.
How to Read a Purity Number
- 99.0% HPLC — typical specification floor for serious research-grade peptides
- 95-98% HPLC — entry-level grade, may be acceptable for some screening work but not for rigorous research
- <95% HPLC — generally unsuitable for reproducible research; major contamination present
- 99.5%+ HPLC — premium research-grade, often the spec for clinically-relevant compounds
Axiom Peptides batches are tested to ≥98% HPLC minimum, with most batches testing above 99.0%. Every COA includes HPLC purity, USP <85> endotoxin, and USP <71> microbial results. View any current batch COA at axiompeptides.net/coa.
The Bottom Line
A reputable research peptide COA includes at minimum: HPLC purity (quantitative), an identity test such as Mass Spec (qualitative), bacterial endotoxin testing, and batch traceability (lot number, test date). If a supplier offers fewer than these — or makes you ask before sending one — that should be a red flag, not a minor inconvenience.
For Research Use Only. The information in this article describes published preclinical research and animal-model studies. No clinical claims are made about therapeutic effects in humans. Products sold by Axiom Peptides are intended for laboratory research and are not for human consumption.