. Thus, a plot of 𝑒𝑟𝑓 -1 𝜑 𝑣𝑠 1/√𝑡 should be a straight line from the slope of which, the diffusivity can be estimated. Since there is a lag time (𝑡 0 ) for the diffusion front to reach the region where the concentration is measured, iy makes sense that it the diffusivity be estimated from the slope the plot of 𝑒𝑟𝑓 -1 𝜑 𝑣𝑠 1 √(𝑡-𝑡0) in the long-time limit, i.e. when 𝑡 ≫ 𝑡 0 .