> I kind of figured that but there isn't a lever to open the cartridge, and
> when I pry on the plastic cover It seems like its going to break rather
> than come off does anyone know the trick? the cartridge is for organic
> vapors from 3m.

I think that you should go back to the supplier and have a look at another respirator. It sounds like the prefilter retainers were not in the box with the respirator when you bought it. The retainers have a little plastic lever so you can take them off.

Ian