Abstract: An analytical chemistry apparatus and method for introducing a reagent into a flowing stream of liquid carrier in order to quantitatively analyze one or more components of a sample added into the carrier. The reagent is permeated across a short section of membrane having relatively large pores. The use of such a membrane: (a) allows the membrane to be protected from physical damage by covering it with a perforate structure; (b) significantly reduces bandspreading across the membrane reagent addition device; (c) reduces the pressure drop across the membrane reagent addition device; and (d) still allows for the permeation of an effective amount of the reagent into the carrier. The reagent is preferably pressurized to minimize leakage of carrier across the membrane and the reagent can be self-pressurized by essentially completely filling the reagent reservoir of the invention with reagent and then hermetically sealing the reservoir.