Does a 5 mg Vial Contain 5 mg of Peptide?

Every reconstitution calculation starts with a number taken on faith. The vial says 5 mg, so you type 5 mg into the arithmetic, add your diluent, and work from the concentration that comes out. The maths itself is simple and hard to get wrong.

The number you started with is the part worth examining. A lyophilized vial is not pure peptide, and depending on what the label is describing, “5 mg” can mean three different things.

Three numbers, all called “the amount”

Gross powder mass. What the vial would weigh if you tipped the contents onto a balance. Peptide, plus everything that came with it.

Net peptide content. The fraction of that mass which is actually peptide. For a lyophilized peptide this is typically somewhere between 50% and 90%, and commonly 60–90%, depending on the sequence, the salt form and how it was purified and dried.

Mass of the target peptide. Net peptide content narrowed further to the specific sequence you ordered, rather than peptide-like material in general. This is where HPLC purity comes in.

These are not variations on one measurement. They are three separate quantities, and a certificate can report one, two or all three.

What the rest of the mass is

Three things, mostly.

Counter-ions. Peptides come off reverse-phase purification as a salt, usually trifluoroacetate, sometimes acetate. The acid binds to the N-terminus and to basic side chains, and it does not simply wash off — the salt form is chemically bound, which is why suppliers offer a salt exchange as a separate service rather than a rinse. The load scales with how many basic sites the sequence has. A peptide of molecular weight around 1,000 carrying two positive charges works out to roughly 81% peptide by theoretical calculation, with the remaining fifth being counter-ion and associated mass.

Water. Lyophilized powder is hygroscopic. It picks up moisture during handling, during storage, and every time a vial is opened in a humid room. Residual moisture is measured by Karl Fischer titration, and it is one of the reasons the same product can weigh differently at two points in its life.

Residual solvent and salts. Traces of the organic solvents used in synthesis and purification, plus buffer salts such as sodium.

The arithmetic, worked

Take a vial labelled 5 mg, where the label describes gross powder mass. Suppose net peptide content is 80% and HPLC purity is 99%.

• Gross mass: 5.00 mg
• Peptide: 5.00 × 0.80 = 4.00 mg
• Target sequence: 4.00 × 0.99 = 3.96 mg

Reconstitute that vial with 2 mL of bacteriostatic water and the concentration you believe you have is 2.5 mg/mL. The concentration you actually have is 1.98 mg/mL. Your working figure is about 21% high, and the ≥99% purity on the certificate did nothing to warn you, because purity was never the number doing the damage.

Why this does not average out

Worth being clear about, because it is the difference between an annoyance and a problem.

Ordinary pipetting error is random. Run enough replicates and it distributes around the true value. A net-peptide-content error does not behave that way. It is systematic and it runs in one direction: the concentration is always lower than assumed, by the same proportion, in every aliquot from that vial, on every day you use it. Replicates do not cancel it. They reproduce it faithfully.

If two vials from different suppliers carry different net peptide content and you compare results on label mass, the difference between them is baked into your data as if it were an effect.

Purity and content are independent

This is the point most often collapsed. HPLC purity, measured by ultraviolet absorbance in the 210–220 nm region, tells you what proportion of the peptide-related material is the target sequence. It is blind to water, counter-ions, salts and residual solvent, because those are largely invisible to the detector at that wavelength.

So a sample can be 99% pure and 60% peptide at the same time, with no contradiction whatsoever. One number describes the composition of the peptide fraction; the other describes how much of the vial that fraction represents. Reading a certificate properly means treating them as two questions.

The figure that settles it

A certificate reporting assay on an anhydrous, ion-free basis has already done this arithmetic. That figure states how much target peptide is present with water and counter-ions mathematically stripped out, which is precisely the number your reconstitution calculation needs. Where it appears alongside net peptide content, a Karl Fischer water figure and a purity figure, the vial is fully described and nothing is left to assumption.

Kinotype’s certificates report peptide content, water content by Karl Fischer, residual solvents, sodium, and assay calculated on an anhydrous, ion-free basis. They are published on the COA page before purchase.

What to ask when the figure is missing

• Does the label mass describe gross powder, or peptide content?
• Is net peptide content reported, and by what method — amino acid analysis, elemental analysis, or UV?
• What salt form is it, and how many basic sites does the sequence carry?
• Is there a water content figure, and how recent is it?

A supplier who can answer those has measured them. A supplier who treats the questions as unusual has told you the label is an estimate.

If you cannot get the number

Work from the label, but write down that you did. An assumption you have recorded is a limitation; an assumption you have forgotten is an error. Keep one supplier and one lot for any set of results you intend to compare, since a consistent unknown offset is survivable in a way that a changing one is not. And do not compare potency across suppliers using label mass, because you would be comparing two different quantities that happen to share a unit.

The short version

Purity tells you what fraction of the peptide is the right peptide. Net peptide content tells you what fraction of the powder is peptide at all. You need both, and only one of them is usually advertised. Ask for the assay on an anhydrous, ion-free basis, and if it is published before you buy, you never had to ask.