A 10mg Vial Does Not Hold 10mg of Peptide

Reconstitute a vial labeled 10 mg in 1 mL, write 10 mg/mL on the tube, and if the net peptide content of that material is 78 percent you have just overstated your concentration by 28 percent. What the vial held was 7.8 mg of peptide. Everything computed downstream inherits the discrepancy.

Worse than an ordinary slip, this one is systematic instead of random. Replicates will not average it away. It propagates into every value derived from the stock, it puts the results out of comparison with any group that did apply a correction, and in concentration-response work it displaces the entire curve sideways. Repairing it costs nothing once the number is in hand: multiply the assumed concentration by the reported fraction.

Why the label was never the peptide mass

Three things share the vial and only one of them is what you ordered. Peptides carrying basic residues come off purification as salts, and because reversed-phase purification runs on trifluoroacetic acid, such material ordinarily arrives as a trifluoroacetate salt. Every basic site takes a counter-ion, and no TFA equivalent is light: each one adds 114 Da. Water accounts for the third component. Lyophilized peptide is hygroscopic, bound water persists even in carefully handled material, and every opening in ambient humidity contributes a little more.

Net peptide content is the name for the peptide fraction expressed against total vial weight. Values generally land somewhere between 70 and 90 percent, with a sequence carrying several lysine or arginine residues sitting toward the bottom of that band because it has more counter-ions to carry. None of this is a supplier shortcut. It falls out of how peptides get purified and dried.

Purity answers a different question

Here is where certificates are misread most often. HPLC purity is a ratio taken among peptide species: of the peptide present, what proportion corresponds to the target sequence rather than to a deletion or an oxidation product. Counter-ion and water are invisible to it, since neither throws a UV-absorbing peak in the peptide region. Which is how 99 percent purity and 75 percent peptide by mass end up honestly printed on the same page. One figure reports how much of the peptide here is the right one; the other reports how much peptide is here to begin with.

Three ways the number gets determined

If nobody will hand you the figure

Ask first, because the data sometimes exists without appearing on the standard document. Failing that, amino acid analysis can be bought in as a service, and it earns its cost wherever the work is quantitative. Where neither route is open and nominal weight has to stand in, say so in your methods. An assumption placed on the record is defensible; the same assumption left silent is not.

What it does to price comparison

A commercial edge comes with all this. Two vendors quoting 10 mg at matching prices are not selling equivalent things if one runs at 85 percent peptide and the other at 70 — a gap of 21 percent in material actually delivered. Purity percentages alone will never surface that difference, which is part of why the figure tends to go unmentioned.

Related reading: how to read a certificate of analysis and how purity and identity are verified.

All products referenced are supplied for laboratory research use only and are not approved for human consumption, clinical, or veterinary use.

Related listings. Every listing is subject to the figure, and it is worth keeping in view when weighing one size against another, such as GHK-Cu 50mg beside 100mg.

Any mg/mL or molar figure for a stock or HPLC standard should be worked from the net content on the certificate, not from the label weight.