A method of modifying the concentration of reactants and carrying out a chemical reaction on a microfluidic device in which first and second reactants are delivered into a reaction channel combined, the second reactant different from the first reactant and capable of reacting with the first reactant. The first reactant is subjected to a stacking process, thereby producing a first stacked reactant. The second reactant is subjected to the stacking process, thereby producing a second stacked reactant. The first stacked reactant is exposed to the second stacked reactant so that the first stacked reactant and the second stacked reactant undergo a chemical reaction.