\frac{\partial M}{\partial t}(t)= \mathop{\sum}{i=1,,2}\frac{\partial {M}{i}}{\partial t}=\mathop{\sum}{i=1,,2}\left{4{C}{0}\frac{{D}{i}}{{x}{0}}\right}\left[\exp \left(-\frac{{D}{i}}{{x}{0}^{2}}\frac{{\pi }^{2}}{4}t\right) \right. \ \left.+\exp \left(-\frac{{D}{i}}{{x}{0}^{2}}\frac{9{\pi }^{2}}{4}t\right)+\exp \left(-\frac{{D}{i}}{{x}{0}^{2}}\frac{25{\pi }^{2}}{4}t\right)\right]