The arithmetic connecting a vial label to a molar concentration is simple. The corrections applied along the way are where the errors live.
The basic calculation
A vial labelled 10 mg, reconstituted in 2 mL:
10 mg ÷ 2 mL = 5 mg/mL
To convert to molarity, divide by molecular weight. For a peptide of 1,419.5 g/mol:
5 mg/mL ÷ 1,419.5 g/mol = 3.52 × 10⁻³ mol/L = 3.52 mM
That is the nominal concentration. Three corrections separate it from the real one.
Correction 1: purity
If the certificate reports 98% purity by HPLC, 2% of the peptide present is something else. The corrected figure:
3.52 mM × 0.98 = 3.45 mM
For most work at 98-99% purity this is a small adjustment. It matters when comparing across batches with different purities, where an uncorrected comparison bakes in a systematic offset.
Correction 2: net peptide content
This is the large one and the one usually missing.
The 10 mg on the label is total vial mass, including counterion, residual water and salts. If a net content assay reports 78% peptide:
10 mg × 0.78 = 7.8 mg of actual peptide
7.8 mg ÷ 2 mL ÷ 1,419.5 g/mol = 2.75 mM
The nominal figure of 3.52 mM overstates the truth by 28%. For an arginine-rich peptide as a TFA salt, corrections of this size are routine rather than exceptional.
If net content is not reported, either request it or state clearly in your records that concentrations are nominal.
Correction 3: salt form
Where net content is unavailable, the salt form allows an estimate. Count the basic sites — lysine, arginine, histidine, free N-terminus — and assume roughly one counterion each. Trifluoroacetate adds 113 g/mol per site; acetate adds 59.
For a peptide of 1,419.5 g/mol with three basic sites as the TFA salt:
3 × 113 = 339 g/mol of counterion
peptide fraction ≈ 1,419.5 ÷ (1,419.5 + 339) = 81%
This is an approximation — actual stoichiometry varies — but it is far closer than assuming 100%.
Molar mass and the size trap
Equal masses of different peptides are not equal molar quantities, and the discrepancy grows with the size difference. One milligram of a 275 g/mol dipeptide is 3.6 µmol. One milligram of a 4,800 g/mol incretin analogue is 0.21 µmol — seventeen times fewer molecules.
Comparisons framed in mass rather than moles between peptides of different sizes are not comparisons of molecular quantity.
Correct for net content if you have it, estimate it from the salt form if you do not, and record which you did. A concentration quoted without saying whether it is nominal or corrected is ambiguous by a margin that is often larger than the effect being measured.
Concentration units
mg/mL is convenient for handling and useless for comparing across compounds.
Molar is the unit that means something physically and the one to use in any quantitative context.
Percent solution appears occasionally and should be defined explicitly — it is ambiguous between w/v and w/w.