The number almost nobody reports
Peptides are not supplied as pure molecules. What goes into the vial is a lyophilized solid containing the peptide plus whatever came with it through synthesis and purification: counterions from the final purification step, usually acetate or trifluoroacetate, and residual water, which a hygroscopic solid holds onto.
So a vial labelled 5mg contains 5mg of that solid. The peptide fraction is smaller, and how much smaller is a real, measurable quantity called net peptide content.
How large is the difference
Typically the peptide is 70 to 90 per cent of the solid mass, with the remainder split between counterion and water. For a strongly basic peptide with several arginine or lysine residues, more counterion is needed to balance the charge and the peptide fraction is lower. For an acidic sequence it is higher.
Two figures from published Arctic Lab Supply reports show the scale in practice:
| Product and lot | Reported net peptide | Line on the report |
|---|---|---|
| BPC-157 5mg, lot BCP-2502-B | 5.27 mg | Net peptide content, reported |
| GHK-Cu 50mg, lot GHK-2501-B | 52.49 mg | Net peptide content, reported |
In both cases the reported net peptide figure sits slightly above the labelled amount, which is how a supplier accounts for the salt and water fraction: fill enough solid that the peptide itself meets the label. The reason you can see that at all is that the report states it. On a certificate that omits the line, the same question has no answer.
Why purity and net peptide content are different questions
This is the distinction that makes the topic worth a page. They are frequently conflated and they measure unrelated things.
- Purity by HPLC asks: of the peptide-like material present, what fraction is the intended compound? Salt and water are invisible to it — they do not absorb at the detection wavelength and do not appear as peaks.
- Net peptide content asks: of the total mass in the vial, what fraction is peptide at all?
A preparation can be 99% pure and 70% peptide simultaneously, and both numbers are honest. The first says the peptide present is the right one; the second says how much peptide is present. Neither substitutes for the other, and a certificate reporting only the first has told you nothing about the second.
Why the counterion is worth knowing
The two common counterions are not equivalent. Acetate is the more benign and generally preferred; trifluoroacetate is a residue of the purification step and is the one worth knowing about, because it can interfere with certain downstream assays — a laboratory result affected by residual purification salt is a real phenomenon rather than a theoretical one.
A report that states the salt form is telling you something a report that omits it cannot. Where it is absent, it is a fair question to ask.
What to look for
- Is net peptide content stated at all, or only purity?
- Is the salt form named — acetate, trifluoroacetate, or other?
- Is water content reported, particularly for a hygroscopic sequence?
- Does the stated mass on the identity line describe the free peptide or a salt? For a salt, the arithmetic differs.
None of these is exotic. They are ordinary lines on a thorough certificate, and their absence is the most common gap between a report that looks complete and one that is. See how to read a certificate of analysis and what a purity percentage means.
Storage of the lyophilized solid
Residual water is part of what this page is about, and it is not static — a hygroscopic solid gains water from the air whenever a vial is opened warm or left unsealed. A net peptide figure describes the material as analysed; the longer a vial is handled carelessly, the less that figure describes what is in it now.
Stability is a documented property rather than an assumed one: a laboratory establishes it by holding material under defined conditions and re-analysing at intervals. Where a retest date is stated, the question worth asking is what data supports it. See storing lyophilized research materials.
What the published literature covers
This is an analytical topic rather than a compound, and the relevant literature is method literature: peptide content determination by amino acid analysis, nitrogen determination, and quantitative absorbance, along with published work on counterion effects in downstream assays. These are established techniques rather than contested ones.
The compound has not completed the regulatory process that would establish safety or efficacy for any medical indication, and findings in animal models and cell culture do not transfer automatically to other species. Arctic Lab Supply does not publish research conclusions, recommend applications, or provide guidance on experimental design.