Purity is not content

The question almost everyone is actually asking, underneath whatever they type into a search bar, is this: my vial says 5 mg — is there 5 mg in it?

It's a reasonable question, and a purity figure does not answer it. A certificate reporting 99.1% purity feels like it should settle the matter. It doesn't, and the gap between what that number means and what people assume it means is the single most common misunderstanding in peptide analysis.

This isn't a story about fraud. Most of the difference between a label figure and the peptide actually in a vial is not deception at all — it's chemistry that nobody explained. Understanding it will tell you more about what you bought than any single number on a certificate.

Two different ways purity gets measured

You will see both on certificates, so it's worth knowing which one you're reading.

The older approach is HPLC with ultraviolet detection, usually at 214 nm — the wavelength where the peptide bond absorbs light. Everything in the sample is separated on a column, and the detector reports whatever absorbs as it comes off. Because it responds to peptide bonds generally, it sees peptide impurities you weren't expecting, which is its real strength.

The approach we use is LC‑MS/MS on a triple quadrupole, reading purity at the MRM transition. The first quadrupole selects the intact peptide by mass. A collision cell fragments it. The third quadrupole watches for specific fragments. That precursor-to-fragment pair is a transition, and monitoring more than one is about as close to certainty on identity as analytical chemistry gets — it's why retatrutide and tirzepatide can be told apart despite sitting only 82 daltons apart.

A targeted method finds what you point it at. That precision is the whole reason a second measurement has to exist.

The trade-off is inherent to the design. MRM reports on the transitions being monitored, so purity measured this way is purity with respect to species detectable at those transitions. It is exceptionally good at confirming that what's in the vial is the molecule you were sold. It was never built to tell you how much of the vial is peptide.

What neither method can see

The part that matters is this, and it is true of both approaches equally.

Every chromatographic purity measurement — UV or mass spectrometric — is a comparison between things it can detect. Water is not one of them. Neither is salt. Neither is residual solvent. None of it absorbs at 214 nm, none of it produces a peptide fragmentation transition, and none of it appears in a purity percentage.

So a purity figure of 99% means: of the material this method could see, 99% was the target. It says nothing whatsoever about material the method cannot see — and on a lyophilised peptide, that invisible fraction is substantial.

What net peptide content measures

Net peptide content asks the other question: of everything in this vial, what proportion is actually peptide?

We quantify it against an external calibration curve — running known concentrations of a reference standard to build a curve, then measuring the sample against it. That returns an actual quantity of peptide, in milligrams, rather than a ratio. It's a separate determination from the purity run, which is part of why it appears on fewer certificates: it costs an extra measurement, and no regulator is requiring it for research-grade material.

So a sample can be 99% pure and 78% peptide simultaneously, and both figures are correct. Ninety-nine percent of the detectable material is the right peptide. Seventy-eight percent of the vial's contents are peptide at all. The remaining 22% is real material you paid for that is not peptide.

Where the missing mass goes

Three places, and the largest is the one almost nobody knows about.

When a synthetic peptide is purified, it comes off the column as a salt — most commonly a trifluoroacetate (TFA) salt, because TFA is standard in the mobile phase. Every basic site on the molecule — the free N-terminus, every lysine, every arginine, histidines in part — picks up a counterion and holds onto it through lyophilisation.

Those counterions are heavy. Trifluoroacetate weighs about 114 daltons each. On a small peptide carrying two or three of them, that is not a rounding error — it's a substantial share of what ends up weighed into the vial.

The other two are simpler: residual water, since lyophilised peptides are hygroscopic and hold moisture, and residual solvent left over from synthesis and purification.

What that looks like in practice

Theoretical net content for five common compounds, assuming complete purity. Every one of these is a vial that could honestly report 99%+ on an HPLC certificate.

Theoretical values from the equation above, assuming 100% purity and no residual water or solvent. Real vials will fall below these figures, not above them.

An ipamorelin vial labelled 5 mg, containing a perfectly pure peptide with no water and no solvent, carrying three TFA counterions, holds about 3.4 mg of actual peptide. Nothing has gone wrong. Nobody has cheated. That is simply what a 5 mg TFA salt of a 712-dalton peptide is.

Why small peptides are hit hardest

Look down the table and the pattern is unmistakable. Tirzepatide loses under 9% to counterions. Ipamorelin loses closer to a third.

The reason is arithmetic. Counterion mass is roughly fixed per basic site, but peptide mass varies enormously — ipamorelin is 712 daltons, tirzepatide is 4,814. The same counterion weight is a much larger fraction of a much smaller molecule.

The smaller the peptide, the bigger the gap between what the label weighs and what the label means.

This is why the shorter compounds — ipamorelin, and the growth-hormone secretagogues generally — draw the most complaints about being underdosed, while the GLP‑1 family draws the most complaints about being counterfeit. They're different problems with different causes, and only one of them is anyone's fault.

Salt form changes the answer

Compare the TFA and acetate columns. Acetate weighs about 59 daltons against trifluoroacetate's 114 — so material supplied as an acetate salt carries materially more peptide for the same stated milligram figure.

Pharmaceutical peptide manufacturing routinely exchanges TFA for acetate, partly for this reason. Research-grade material often skips that step, because it costs money and no regulator is asking.

This is why salt form belongs on a certificate, and why its absence is worth noticing. Two vials of the same peptide, same stated weight, same purity, different salt — and one contains meaningfully more peptide than the other. Nothing on the label tells you which.

What to look for on a certificate

Four things, in order of how much they tell you:

Net peptide content, stated as its own figure. If it isn't there, the certificate has answered the easier question and left yours open.

Salt form. TFA or acetate. It changes the arithmetic by several percentage points and costs nothing to disclose.

Purity, with the method stated. "99%" alone means little. "99.1% by LC‑MS/MS at the quantifying MRM transition" or "99.1% by RP‑HPLC at 214 nm" both mean something, because each tells you what was measured and how. A bare percentage tells you neither.

Identity confirmed by mass, ideally on more than one transition. Purity tells you the sample is consistent. Only mass confirms it's the molecule you were sold — and a very pure sample of the wrong compound will still report excellent purity.

All testing described here is for research use only. It is not for human or veterinary administration, and is not a clinical or diagnostic service. Figures in the table are theoretical calculations provided to illustrate an analytical principle; they are not measurements of any specific product.