Peptide content: the number purity hides.
Peptide content is the percentage of the vial's total mass that is actually peptide — the rest being residual water, counterions (TFA, acetate), and salts. A peptide can be 99% pure and only ~70% content. Content is the number that makes reconstitution math accurate; purity alone can mislead it.
The definition
Content (often "net peptide content") answers: of everything in the vial, what fraction by weight is peptide? A lyophilized research peptide is never 100% peptide by mass:
- Residual water — lyophilization leaves a few percent moisture, sometimes more.
- Counterions — peptides purified by HPLC are usually isolated as salts (commonly TFA or acetate); the counterion is a real fraction of the mass.
- Residual salts and solvents — from purification and handling.
Typical content values for research peptides run from about 60% to 90%.
Purity vs content, in one picture
- Purity: within the peptide fraction, how much is the right sequence? (say, 99%)
- Content: of the vial's total mass, how much is peptide fraction at all? (say, 75%)
Multiply them and you get the true amount of target peptide in the vial: a "5 mg" vial at 99% purity and 75% content holds roughly 3.7 mg of actual peptide. That difference propagates directly into concentration and dose calculations.
How content is measured
- Amino acid analysis (AAA) — hydrolyze the peptide, quantify each amino acid, back-calculate peptide mass. The classical method.
- Elemental analysis — nitrogen or sulfur content against theoretical values.
- UV absorbance — where the sequence has a suitable chromophore.
- Gravimetric/TGA — measures water loss, which corrects the moisture component only.
Why it matters for research math
If you reconstitute a "5 mg / 2 mL" vial assuming 100% peptide, your true concentration may be 25–40% lower than the label math suggests. Using content-corrected amounts is one of the differences between naive and careful concentration work.
Where to find it
Content should appear on the COA — sometimes as "net content," "peptide content," or via an amino acid analysis page. If it's absent entirely, the vendor may not measure it, which is itself information.
Questions. answered.
What is the difference between purity and content?
Purity is the composition of the peptide fraction; content is the peptide fraction's share of total mass. Purity is a chemistry claim, content is a mass accounting.
What is typical peptide content?
Roughly 60–90% for lyophilized research peptides, depending on water retention and the counterion. TFA salts and hygroscopic sequences run lower.