Unless you have an equation describing these data points, you can only estimate the slope of tangent (derivative) using gradient() function and then interpolating the result.
For example
t = 0:10:100;
Ca = [50,26.1,13.6266,7.11,3.7137,1.9387,1.012, .5284, .2758, .1440, .0752];
dCa_dt = gradient(Ca)./gradient(t);
df = @(t_) interp1(t, dCa_dt, t_); % derivarive of Ca at any point t=t_
Result
>> df(2)
ans =
-2.2757
>> df(45)
ans =
-0.1968
