A method of testing a sample to determine a concentration of a target material included in the sample and a microfluidic device in which a reaction of the sample and a reagent occurs are provided. The method includes mixing a sample with a reagent that changes optical characteristics in accordance with a concentration of chlorine ions in the sample, and a capturing material that captures some of the chlorine ions in the sample; measuring the optical characteristics after mixing the sample with the reagent and the capturing material; and determining a concentration of the chlorine ions in the sample based on the measured optical characteristics.