A peptide labeled "99% purity" can contain as little as 70% actual peptide by weight. This confusion costs researchers time and leads to false experimental results. This article explains the critical difference between HPLC purity and net peptide content, what a COA actually measures, and how to calculate the true amount of peptide you have.
HPLC purity is a chromatographic measure, not a mass measure. When a peptide sample is injected into an HPLC (High-Performance Liquid Chromatography) instrument, it separates into peaks — the main peptide peak and any impurity peaks. The instrument measures the area under each peak and calculates purity as:
Purity (%) = (Area of main peak / Total area of all peaks) × 100
This number tells you what percentage of the chromatographic profile is your desired peptide, not what percentage of the vial's total weight is actual peptide. Contaminants like salt, solvent, and moisture do not separate from the peptide during HPLC — they run straight through the instrument or don't elute at all — so they are invisible to this measurement.
Net Peptide Content (NPC), expressed as a percentage, is determined by amino acid analysis (AAA) or UV spectrophotometry. It measures the actual mass of peptide in your sample as a percentage of the gross weight. The formula is:
NPC (%) = (Mass of peptide / Gross mass of vial) × 100
The "non-peptide" mass is typically: water (residual moisture), salt counter-ions (TFA, chloride, acetate), and residual solvents. Synthetic peptides made by solid-phase peptide synthesis (SPPS) are often isolated as TFA salts, which can comprise 10–45% of the gross weight. This is why a "99% pure" peptide might only be 65% NPC — the 34% gap is mostly TFA salt and water.
You receive a vial labeled:
The 99% purity says the HPLC trace is clean. The 70% NPC says that only 7 mg of the 10 mg vial is actual peptide; the remaining 3 mg is TFA salt, water, and trace impurities. When you reconstitute that vial in 1 mL of water, your actual peptide concentration is 7 mg/mL, not 10 mg/mL. If your assay requires precise molar quantities, you must account for this difference — otherwise your doses will be off by 30%.
To calculate the actual amount of peptide in your sample:
True peptide mass (mg) = Gross mass × NPC (%) × HPLC purity (%)
For the example above:
True peptide = 10 mg × 0.70 × 0.99 = 6.93 mg
For molar calculations, you also need the molecular weight (MW). If your peptide has MW 1500 Da:
Moles = (True peptide mass / MW) = (6.93 mg / 1500 Da) = 4.62 μmol
Only after this calculation can you accurately prepare a solution of known molar concentration.
If you assume 10 mg of 99%-pure peptide contains 10 mg of usable peptide, and you're off by 30%, your dose per well or per sample is wrong. Cell assays are sensitive to dose — an underestimated concentration can lead to a false negative; an overestimated concentration can mask toxicity. For concentration-response studies, this error skews your EC50 and IC50 estimates. For metabolic work, molar accuracy is non-negotiable.