1. A sample preparation method for combining reagents within a fluid sample, comprising: a. adding at least one reagent to a chamber wherein the reagent is bound to a substance capable of slowly releasing the reagent; b. filling the chamber with a fluid; c. distributing the substance with bound reagent such that the closest distance between any point inside the chamber and the substance is smaller than the diffusion length of the reagent alone in the fluid; and d. releasing the reagent from said substance within the chamber.