When a peptide vial is labelled 5 mg, that label describes the total content. It does not mean there are 5 mg of pure, active compound ready to use. The purity percentage on the certificate of analysis tells you what fraction of that 5 mg is actually your target peptide — and that distinction has direct consequences for every dose calculation you make.
The Arithmetic of Purity
If your vial contains 5 mg of peptide at 95% purity, the effective content is 4.75 mg of active compound. The remaining 0.25 mg consists of impurities — synthesis byproducts, related fragments, residual reagents, and so on. If you reconstitute on the assumption that the full 5 mg is available as active peptide and dose based on that calculation, every dose you draw will be approximately 5% less than you intended.
For a single administration, 5% is probably not meaningful. But across a 12-week protocol with daily dosing, that systematic under-dosing accumulates. And if you are comparing results across experiments using peptides from different batches with different purities — say 95% versus 88% — the difference in effective dose becomes substantial: roughly 8% per administration, consistently.
How to Correct for Purity
The correction is straightforward. Divide your target dose by the purity fraction to get the mass you need to draw. If you want 200 mcg of active peptide and your compound is 95% pure, you need to draw the equivalent of 200 ÷ 0.95 = 210.5 mcg of the total peptide content. A peptide purity calculator does this arithmetic automatically — you enter the target dose and the purity percentage and it returns the corrected draw amount.
What Counts as Acceptable Purity?
For research-grade peptides, 98%+ is excellent and represents careful synthesis with thorough purification. Most well-regarded suppliers consistently hit 95-98%. Below 90% is considered poor quality for research purposes, and below 85% raises serious questions about manufacturing process and quality control.
It is worth noting that some peptides are genuinely harder to synthesise at high purity — longer sequences and those containing cysteine or other reactive residues present more synthesis challenges. A slightly lower purity for a complex 30-amino-acid peptide is more understandable than the same purity for a simple 5-mer. Context matters.
Purity vs Identity: Two Different Questions
High purity does not guarantee correct identity. A peptide can be 99% pure and still be the wrong sequence if something went wrong during synthesis. This is why mass spectrometry confirmation matters alongside HPLC purity — purity tells you how much of the dominant compound there is, while MS tells you whether that dominant compound is actually what it should be. Both tests serve different purposes and both belong on a credible CoA.
When Purity Drops After Reconstitution
Peptide degradation after reconstitution is a real issue. Even starting from a high-purity vial, poor storage after reconstitution can degrade the active compound over time, effectively reducing the purity of what you are actually dosing. This is one reason why correct refrigerator storage and respecting the 28-day guideline for BAC-water-reconstituted solutions matters — it preserves the purity you paid for.
If you notice that a reconstituted solution is becoming cloudy or is behaving differently over time, degradation may be occurring. When in doubt, prepare a fresh vial rather than continuing to use a solution of uncertain quality.