Reflecting and absorbing boundaries

Hi,

I am new to Comsol. I would like to model some instantly released molecules inside a reservoir. The diffusion coff of these molecules is 6e-7 cm2/s. I have some questions about it.
1. How can i define the molecules in my model? can i define them an impulse in the middle of reservoir?
2. How can I define reflecting or absorbing boundaries inside the reservoir in Transport of diluted materials?
3. and also how can I find the flux rate in the absorbing boundaries?

Thanks much,