Protein Concentration Calculator

This calculator relies on the Beer-Lambert law, which states that absorbance is directly proportional to the concentration of the solution and the path length of the light through it. The core formula implemented is concentration = (absorbance / (extinction_coef * pathlength)) * molecular_weight * dilution_factor. Here, absorbance represents the optical density read by a spectrophotometer, extinction coefficient is the molar attenuation coefficient of the protein, pathlength is typically 1 cm for standard cuvettes, molecular weight is measured in Daltons or kDa, and the dilution factor accounts for any sample pre-dilution.

Suppose you are analyzing a purified protein sample in the laboratory. You record an absorbance at 280 nm of 0.45. The known molar extinction coefficient for your protein is 30,000 M⁻¹cm⁻¹, and the cuvette pathlength is 1 cm. Furthermore, your protein has a molecular weight of 50,000 g/mol, and you diluted the original sample by a factor of 5 before taking the measurement. First, divide the absorbance by the product of the extinction coefficient and pathlength (0.45 / 30,000 = 0.000015). Next, multiply this result by the molecular weight (0.000015 * 50,000 = 0.75 g/L). Finally, multiply by your dilution factor of 5 to get the final concentration of 3.75 g/L.

To ensure maximum accuracy in your protein assays, always verify that your spectrophotometer readings fall within the linear range of the instrument, typically between 0.1 and 1.0 absorbance units. If your absorbance is higher, dilute the sample appropriately and include that dilution factor in your calculation. Additionally, ensure your blank sample uses the exact same buffer matrix as your protein solution to eliminate background interference from reagents or salts.