Quantification
Quantification: 7 articles from Peptides Info. Handling, storage and laboratory reference.
qNMR for Peptide Content: Internal Standards and Limits
Quantitative proton NMR assigns the absolute content of a weighed peptide by comparison with a certified internal standard, without a peptide reference material and without destroying the sample. The equation, a worked example, the acquisition settings that decide accuracy, and how the result compares with amino acid analysis and HPLC assay in an inter-laboratory study.
A205 and A214: Quantifying Peptides With No Trp or Tyr
When the sequence has no tryptophan or tyrosine, A280 returns nothing. The far-UV alternative reads the peptide bond itself. Coefficients at 205 and 214 nm, a worked calculation for a generic decapeptide, and the buffer, counter-ion and stray-light constraints that make this harder than A280.
Amino Acid Analysis as a Reference Method
The only routine assay that answers how much peptide is in a weighed solid. Hydrolysis and what it destroys, derivatisation and separation, what it costs, and what its absence from a certificate leaves undetermined.
Counter-Ion Content: Trifluoroacetate and Acetate
Synthesised peptides arrive as salts, and the salt is not a trace. How much mass the counter-ion carries, how it is measured, when exchange is worth requesting, and where it sits in the accounting that reconciles gross weight to peptide.
Gravimetric and Volumetric Preparation Error
Balance resolution against pipette tolerance, the volume below which weighing wins, the density and temperature terms that convert between them, and how error behaves as it passes down a dilution series.
Karl Fischer Titration and Residual Moisture
Water is the largest non-peptide term nobody measures. How the titration works, what volumetric and coulometric modes are each good for, where the sub-three-per-cent convention came from, and what a moisture figure on a certificate does and does not describe.
UV Absorbance at 280 nm: Concentration Without a Balance
An optical measurement of peptide concentration that is blind to counterion, water and salt — and that returns nothing at all for a sequence carrying no aromatic residues. Coefficients, path length, interference and the failure cases.