Analytical testing
Net peptide content: why purity is only half the story
Two vials can both be '99% pure' and contain very different amounts of peptide. Water, counter-ions and the measurement of net content explained.
By Positive Peptides editorial team 2 min read
When a laboratory reports that a peptide is "99% pure", the figure refers to the chromatographic purity of the peptide fraction — the proportion of UV-absorbing material that is the intended molecule. It does not describe how much of the powder in the vial is peptide at all. That quantity is net peptide content, and it is routinely overlooked.
What else is in the vial
A lyophilised synthetic peptide typically contains:
- Water. Peptides are hygroscopic; bound water commonly accounts for 5–10% of the mass and can be higher for highly charged sequences.
- Counter-ions. Peptides purified by reverse-phase HPLC with trifluoroacetic acid (TFA) carry TFA as a counter-ion to each basic residue (lysine, arginine, histidine, N-terminus). Many suppliers exchange this for acetate. Either way, counter-ion mass is significant — for a lysine-rich peptide it can approach 10–20%.
- Residual solvents and inorganic salts in smaller amounts.
The result is that a vial of 99%-pure peptide may be only 70–85% peptide by weight. For a 5 mg vial, that is the difference between roughly 3.5 mg and 4.3 mg of actual peptide.
Why it matters
Any experiment that depends on concentration — receptor binding, dose–response, cell assays — is affected. Two batches with identical purity but different net content give different molar concentrations when weighed out identically. Quantitative work should be based on net content, or on a direct concentration measurement (for example, UV absorbance at 280 nm for peptides containing tryptophan or tyrosine).
How net content is measured
- Amino acid analysis (AAA) hydrolyses the peptide and quantifies each amino acid against standards. It gives both composition (supporting identity) and total peptide mass.
- Nitrogen determination (for example by combustion/Dumas or Kjeldahl) estimates peptide from nitrogen content, assuming the sequence is known.
- Quantitative NMR is used by some laboratories.
- UV absorbance gives a quick estimate for peptides with chromophores, but depends on the accuracy of the extinction coefficient.
A COA that reports net peptide content will typically also report water content (Karl Fischer) and counter-ion content (ion chromatography), because the three should add up sensibly.
What to look for on a COA
If net content is not reported, it is unknown. Treat purity as a statement about the quality of the peptide fraction, not about the quantity of peptide. Wherever Positive Peptides states a purity figure, the method behind it (HPLC) is stated with it, so that readers know exactly which measurement was made.
This article is for scientific information only.
This article is provided for scientific information and education. It does not constitute medical advice and does not recommend any compound for human use.