= Protein mass purity
<Protein mass purity> is the fraction of the total protein mass in a preparation contributed by the intended <protein>. With target mass $m_T$ and total protein mass $m_P$, the percentage is $100m_T/m_P$. The denominator includes other proteins, rather than the solvent or the entire mass of the solution.<Validating Identity, Mass Purity and Enzymatic Purity of Enzyme Preparations by John E. Scott and Kevin P. Williams>[Definitions, Mass Purity]{ref}
For a constructed preparation containing $0.84\,\mathrm{mg}$ target protein and $0.16\,\mathrm{mg}$ other proteins, mass purity is $84\%$. This does not say that $84\%$ of the target molecules are active, nor that $84\%$ of measured catalytic activity comes from them. Those are different quantities from a mass fraction.
An image-based estimate also requires a response model. Suppose <densitometry> gives target-band intensity 8 and contaminant intensity 2. If both stain equally per unit mass, the inferred purity is $8/(8+2)=80\%$. If, instead, the target signal per mass is twice the contaminant's, the inferred masses are proportional to 4 and 2, giving $4/6\approx67\%$. These invented response factors illustrate the assumption rather than a measured staining difference.
Thus an <SDS-PAGE> band pattern supports a mass-purity estimate when identification, detection and calibration are appropriate. <Enzymatic purity> and <specific activity (enzyme)> require additional assay information.
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