Guide
Net peptide content and purity are not the same number
A certificate says 99% purity. The label says 10 mg. It is easy to read those together as 'about 10 mg of peptide, and it is 99% good'. They do not combine that way, and the gap between them is larger than most people expect.
Two different questions
Purity is a ratio. It asks: of the material the analysis detected, what proportion was the target compound? It is reported as area percent from a chromatogram.
Net peptide content is a quantity. It asks: of the total mass in this vial, how much is actually peptide? It is reported as a percentage of weight, and it is a completely separate measurement.
A sample can be highly pure and still be a minority of the powder by weight, because the things making up the rest of the weight are not impurities in the chromatographic sense. They are simply not visible to the method that produced the purity number.
What else is in the powder
None of these appear as peaks in a UV chromatogram, so none of them reduce the purity figure. All of them occupy mass in the vial.
- Counterions. Synthetic peptides are typically purified by reversed-phase HPLC using trifluoroacetic acid, and are isolated as the TFA salt. Peptides with several basic residues carry several TFA molecules. Depending on the peptide's size and charge, the salt can account for a substantial share of the powder's weight. Acetate salts are also common.
- Residual water. Lyophilised peptides are hygroscopic and retain bound water. A few percent by weight is normal and it varies with handling and storage.
- Residual solvent. Traces from synthesis and purification, generally small.
- Salts from buffers, where the process leaves them.
How the gap arises
The two numbers measure different denominators. Purity divides target peak area by total detected peak area. Content divides peptide mass by total powder mass.
Because counterions and water sit outside the purity calculation entirely but inside the vial, a material can be extremely pure by chromatography and still contain meaningfully less peptide by weight than the gross figure on the label suggests. The effect is larger for small, highly charged peptides, where salt is a bigger fraction of a smaller molecule.
This is not a defect and it is not deception. It is a consequence of how peptides are made and how the two measurements are defined. It only becomes a problem when a number answering one question is presented as if it answered the other.
How content is actually measured
Net peptide content requires a different method from the one that produces a purity figure.
Amino acid analysis hydrolyses the peptide into its constituent amino acids and quantifies them against standards, giving peptide mass directly. It is regarded as the reference method and it is destructive, slow and comparatively expensive.
Quantitative NMR compares signal against an internal standard of known concentration.
Nitrogen determination measures total nitrogen and infers peptide content from the sequence's known nitrogen composition.
Ultraviolet absorbance at 280 nm can be used where the sequence contains tryptophan or tyrosine, using the calculated extinction coefficient. It is quick, and it does not work at all for sequences lacking those residues.
Each costs more than running a purity gradient, which is a large part of why the number is rarely offered.
Why almost nobody publishes it
Three reasons, in descending order of charity.
It costs more. Amino acid analysis is a separate submission to a laboratory with the capability, and the price per sample is not comparable to a purity run.
It is not what buyers ask for. Purity has become the number the market compares on, so that is the number suppliers produce. A supplier publishing content alongside purity is answering a question nobody asked, using a figure that looks lower than a competitor's purity percentage to anyone reading quickly.
And it can be commercially inconvenient. A gross fill weight is a larger number than a net peptide weight, and it is the one on the label.
What to do with this
Read the two figures as answers to separate questions, and notice which one you have been given. A purity result is genuinely useful - it establishes that the material is overwhelmingly one substance, and combined with an identity result it establishes which substance.
If quantity matters for the work in hand, purity does not answer it, and no amount of confidence in a 99% figure will make it answer it. That requires content, measured by a method that reports mass. Ask whether it exists; the answer tells you something either way.
Common questions
What is net peptide content?
The proportion of a vial's total powder weight that is actually peptide, expressed as a percentage of mass. It is a separate measurement from purity, which is the proportion of detected material that is the target compound - a ratio rather than a quantity.
Can a peptide be 99% pure and contain less peptide than the label says?
Yes. Purity is measured by chromatography and does not count counterions, residual water or residual solvent, because those are largely invisible to the detection method. They still occupy weight in the vial, so gross fill weight and net peptide mass are different figures.
What is TFA and why does it matter for peptide weight?
Trifluoroacetic acid is commonly used in reversed-phase purification, and peptides are frequently isolated as the TFA salt. Peptides with several basic residues bind several counterions, and that salt is a real fraction of the powder's weight while contributing nothing to the purity percentage.
How is net peptide content measured?
Most reliably by amino acid analysis, which hydrolyses the peptide and quantifies the resulting amino acids against standards. Quantitative NMR, nitrogen determination and UV absorbance at 280 nm are also used, though UV only works for sequences containing tryptophan or tyrosine.
Why do most peptide suppliers not publish net peptide content?
It requires a separate and more expensive analysis than a purity run, buyers have learned to compare on purity so that is the number suppliers produce, and the net figure is lower than the gross fill weight printed on the label.
Check the evidence for a Causa lot
Our lot pages publish the available results for assayed batches. Request the original laboratory Certificate of Analysis using the lot number, and compare it with the material you are considering. Materials without a qualifying result are marked assay pending. A summary is not a substitute for the original laboratory report.
Related research documentation guides
Research use only. Not for human or veterinary use. This guide explains how to read an analytical document. It is not medical advice and not guidance on use.