Physicochemical
A280 concentration
Turn a UV absorbance reading at 280 nm into a concentration with the Beer–Lambert law. Enter the measured absorbance, the molar extinction coefficient (ε₂₈₀), and path length. Add the molecular weight to also get mg/mL.
Who this is for
For researchers measuring concentration on a UV spectrophotometer or nanodrop. It applies the Beer–Lambert law, the standard relationship between how much UV light a sample absorbs and how concentrated it is.
How to use it
- Absorbance (A₂₈₀)The reading from the instrument at 280 nm.
- Extinction coefficient (ε)How strongly this peptide absorbs at 280 nm — compute it on the property calculator, or read it from a datasheet.
- Path lengthThe distance light travels through the sample, in cm. A standard cuvette is 1 cm.
- Molecular weightOptional — add it to also get the answer in mg/mL.
Reading your results
- Molar / µMConcentration as a count of molecules per litre (moles, and micromoles, per litre).
- Mass conc.Concentration by weight (mg/mL), shown when you provide a molecular weight.
A worked example
A reading of 0.85, an ε of 5500, in a 1 cm cuvette:
0.85 ÷ (5500 × 1) = 1.545×10⁻⁴ M (≈ 154.5 µM)
Common questions
What is A280?
The amount of UV light at 280 nm a sample absorbs. Peptides absorb there mainly through tryptophan and tyrosine, so the reading tracks concentration.
Where does the extinction coefficient come from?
It's set by the peptide's Trp, Tyr, and cystine content. Calculate it from the sequence on the property calculator.
Why does mg/mL need a molecular weight?
The reading gives a molar concentration (molecules per litre). Converting that to a weight per volume requires knowing how much one molecule weighs — the molecular weight.
The math, for the curious
Beer–Lambert: A = ε × c × l, so c (M) = A ÷ (ε × l). With molecular weight: mg/mL = c (M) × MW.