How much of the powder is the peptide — and what is the rest?

How it works

1. 01
   Dissolve to a known concentration

   The sample is weighed and dissolved so the injection is comparable lot to lot.
2. 02
   Separate on a gradient

   A reversed-phase C18 column with a water/acetonitrile gradient spreads the peptide and its relatives out by hydrophobicity over about 30 minutes.
3. 03
   Detect at 214 nm

   The peptide backbone absorbs at 214 nm, so every peptide-like impurity is seen — not just the ones with aromatic residues that show at 280 nm.
4. 04
   Integrate

   Peak areas are integrated with a stated threshold. Purity is the main peak area divided by total peak area. Impurity peaks above 0.5% are listed.

What the result means — and what it doesn't

A purity of 98.6% means 98.6% of the UV-absorbing material is the main peak. It is not the fraction of the vial's weight that is peptide — counter-ions like TFA and residual water don't absorb at 214 nm and are invisible here. Purity greater than 98% is one of the seal's four checks.

Why another lab might report a different number

• Wavelength: 280 nm misses impurities with no aromatic residues and usually reads higher. We use 214.
• Gradient and column: a steeper gradient merges close peaks into the main one and inflates purity.
• Integration threshold: ignoring peaks under 1% versus under 0.1% changes the answer by a point or more. Ours is printed.
• Some labs report "net peptide content" (peptide by weight) as purity. Different quantity, different number.

That's why every result here prints the method and the limit next to the number. The full science article on why tests vary.