Calculate total flow from flow rate

I am looking to calculate the total flow from a mass flow rate - or simply integrate mass flow rate. I am new to matlab so I am wondering if there is a function to do this or if I will need to write my own function to complete this. I can do this easily in excel (multiply flow rate by sample time, sum results), but am not sure how to do this in matlab. I have tried to use the 'trapz' function, but because my flow goes slightly negative at times, this function does not seem to be working. Can any one point me in the right direction?

[dpb]

What makes you think trapz isn't working? Certainly it's simple enough to do a cumulative summation approximation directly as well...

fl=dt*cumsum(f);

if dt is constant, otherwise

    fl=cumsum(f.*dt);

Not as an accurate an approximation as a trapezoidal approx, though...