Net peptide content versus gross vial weight
A 10 mg vial does not hold 10 mg of peptide. The difference between gross weight, purity and net content, and how to handle it in concentration-critical work.
A vial labelled "10 mg" does not contain 10 mg of peptide. It contains roughly 10 mg of lyophilised solid, of which the peptide itself is typically 70-90%; the rest is counter-ion and bound water. For most bench work this is a detail; for any assay where the absolute concentration matters, it is the difference between a reproducible result and a systematic error of up to a third. This note explains the difference between gross weight and net peptide content, how each is measured, and how to handle it in practice. All Alphex products are for laboratory and in-vitro research only.
Three numbers that are often confused
- Gross weight (nominal vial content). The mass of solid in the vial: peptide plus counter-ions plus residual water. This is what "10 mg" on a label refers to across the research peptide market.
- Purity. The proportion of the peptide fraction that is the target sequence, measured by HPLC peak area. 99+% purity says nothing about counter-ions or water, because they do not absorb at 220 nm and are not in the calculation. See what HPLC purity actually tells you.
- Net peptide content. The proportion of the gross solid that is peptide of any sequence, determined by amino acid analysis or elemental (nitrogen) analysis. Multiply gross weight by net peptide content to get the mass of peptide; multiply that by purity to get the mass of target sequence.
Worked example: a 10 mg vial at 80% net peptide content and 99.2% purity contains 8.0 mg of peptide, of which 7.94 mg is the target sequence. Reconstituted in 1 mL, the true concentration is 7.9 mg/mL, not 10 mg/mL.
Where the non-peptide mass comes from
- Counter-ions. After purification the peptide is isolated as a salt. Basic residues (lysine, arginine, histidine, the N-terminus) each carry a counter-ion, usually acetate or trifluoroacetate. TFA is heavier (113 g/mol) than acetate (59 g/mol), so TFA salts have lower net peptide content for the same peptide. Alphex supplies acetate salts. See acetate versus TFA.
- Water. Lyophilised peptides retain a few percent moisture and pick up more if exposed to humid air.
- Sequence dependence. A short, basic peptide (many lysines relative to length) carries proportionally more counter-ion than a longer, more neutral one. TB-500 (two lysines in seven residues) sits at the lower end of net content; a 39-residue peptide such as GLP-3 (RT) sits higher.
How to handle it in practice
- Decide whether it matters. For comparative or qualitative work where every arm uses the same stock, it cancels. For dose-response curves, kinetics, receptor binding, or anything reported in molar terms, it does not.
- Use the COA if it reports net content. Some certificates include amino acid analysis or a peptide content figure. Alphex COAs report chromatographic purity; net peptide content is not part of the standard certificate, so treat nominal mass as gross.
- Measure it yourself where it matters. UV absorbance at 280 nm works if the sequence contains tryptophan or tyrosine (of the catalogue, only Ipamorelin's naphthylalanine gives useful absorbance there, and BPC-157, TB-500 and GHK-Cu have neither residue). Otherwise, quantitative amino acid analysis or a BCA-type assay calibrated for peptides.
- Or apply a documented assumption. Many laboratories use a fixed correction (for example 80% for acetate salts of short peptides) and state it in the methods. It is less accurate than measurement but far better than assuming 100%.
Frequently asked questions
Is a supplier who says "10 mg" being misleading?
No, it is the universal convention: label mass is gross lyophilised content. The misleading version is a supplier who lets you believe purity and net content are the same thing.
Does higher purity mean higher net content?
No. They are independent. A 99% pure peptide can have 75% net content and a 95% pure one 85%.
Can I request net content for a batch?
Ask via the COA request form; where the testing laboratory has run amino acid analysis for a batch we will send it, but it is not part of the standard certificate.
Alphex research peptides are supplied for laboratory and in-vitro research by qualified researchers only. They are not authorised for human or veterinary use.
Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.
Published 18 June 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.