Can you use a changing boundary condition in pdepe?

I'm trying to set up a pdepe plot that for lower values of t, the graphs cross the x-axis and for higher values of t, the graphs go to zero. Does anyone know how I can achieve this? This is the code I have at the moment but all of the graphs are going to zero:

function pdex1

m = 0;
x = linspace(0,1,20);
t = linspace(0,20,50);
hold on

sol = pdepe(m,@pdex1pde,@pdex1ic,@pdex1bc,x,t);
C = sol(:,:,1);

hold on
plot(x,C(1,:));
plot(x,C(4,:));
plot(x,C(10,:));
plot(x,C(15,:));
plot(x,C(20,:));
plot(x,C(25,:));
plot(x,C(30,:));
axis([0 1 0 1])
% --------------------------------------------------------------
function [c,f,s] = pdex1pde(x,t,u,DCDx)
L = 0.1;
Dg = 5.2e-6;
A = L^2/Dg;
c = A;
f = DCDx;
s = 0;
end
% --------------------------------------------------------------
function C0 = pdex1ic(z)
C0 = 0;
end
% --------------------------------------------------------------
function [pl,ql,pr,qr] = pdex1bc(xl,Cl,xr,Cr,t)
pl = 0;
ql = 1;
pr = Cr - 1;
qr = 0;
end
end

[Tom]

Can you explain more about the graphs crossing the x axis? Since the left hand boundary is set to 0 flux, the right hand boundary is set to 1, and the initial condition is 0, there is no reason for any results to be negative, unless I have misunderstood?