The Confusion
A COA says a peptide is “98% pure by HPLC.” Most people read that as: “98% of what's in this vial is the peptide I ordered.” That is not what it means. HPLC purity and net peptide content are two different measurements that answer two different questions — and net peptide content is always lower than HPLC purity for the same product.
What Each Number Measures
HPLC Purity
“Of all the peptide-related compounds in this sample, what percentage is the target peptide?”
Measures the proportion of UV-absorbing compounds that are the desired peptide versus synthesis impurities (truncated sequences, oxidized forms, etc.). It only looks at peptide-related material.
Typical value: 95–99%
Net Peptide Content
“Of the total weight of powder in this vial, what percentage is actually peptide?”
Measures the proportion of the total sample weight that is peptide versus non-peptide components (water, counterions, salts). Accounts for everything in the vial, not just peptide-related material.
Typical value: 60–85%
Why Net Peptide Content Is Always Lower
Lyophilized (freeze-dried) peptide powder is not 100% peptide. It contains non-peptide components that are a normal part of the manufacturing process:
Counterions (acetate or TFA salts)
During peptide synthesis, trifluoroacetic acid (TFA) is used in the cleavage step. TFA binds to positively charged amino acid residues (lysine, arginine, histidine, and the N-terminus) as a counterion. These TFA or acetate salts remain in the final product and can account for 10–30% of the total weight, depending on how many basic residues the peptide has.
Residual moisture
Even after lyophilization, peptide powders retain some water content — typically 3–8% by weight. This is normal and does not indicate a quality problem.
Residual salts
Small amounts of buffer salts from the purification process may be present in the final product.
HPLC does not detect any of these components — they don't absorb UV light at the detection wavelength and don't appear on the chromatogram. So a peptide can be 98% pure by HPLC (meaning 98% of the peptide-related compounds are the target peptide) while having a net peptide content of only 70% (meaning 30% of the total vial weight is water, salts, and counterions).
A Concrete Example
A vial labeled “BPC-157, 5mg, 98% purity”
• Total powder weight: ~7.1 mg
• Net peptide content (70%): ~5.0 mg of actual peptide
• HPLC purity (98%): of that 5.0 mg of peptide, 4.9 mg is BPC-157 and 0.1 mg is synthesis impurities
• The other ~2.1 mg: counterions (TFA/acetate salts), residual moisture, trace buffer salts
The “5mg” on the label refers to the net peptide weight — the actual amount of peptide in the vial, accounting for the non-peptide fraction. A responsible manufacturer calculates the fill weight so you get 5mg of peptide despite the total powder weighing more.
Why This Matters
What to Look For on a COA
A COA that reports only HPLC purity is not telling you the full picture — it's missing the weight fraction.
Net peptide content is measured separately, typically by amino acid analysis (AAA) or nitrogen content analysis.
A net peptide content of 60–85% is normal for lyophilized peptides. Lower than 50% would warrant questions.
If the COA doesn't report net peptide content, ask the supplier how they calculate their fill weights — are you getting 5mg of peptide or 5mg of powder?
Quick Reference
| HPLC Purity | Net Peptide Content | |
|---|---|---|
| Measures | Target peptide vs. synthesis impurities | Peptide vs. total powder weight |
| Includes salts/water? | No — invisible to HPLC | Yes — accounts for everything |
| Typical range | 95–99% | 60–85% |
| Method | HPLC with UV detection | Amino acid analysis, nitrogen analysis, or gravimetric |
| Answers | How clean is the peptide fraction? | How much of the vial is actually peptide? |
The Bottom Line
HPLC purity tells you how clean the peptide fraction is. Net peptide content tells you how much of the total weight is peptide. Both are important, and they answer different questions. A product can be 98% pure by HPLC and still contain only 70% peptide by weight — that's normal, not a defect. What matters is whether the labeled amount reflects net peptide weight, and whether the supplier adjusted their fill weights accordingly. For a deeper look at HPLC purity specifically, see our purity percentages guide.