A composition for reducing the inhibitory effects of contaminants on nucleic acid amplification is provided. The composition includes effective amounts of ferric iron, an organic iron-chelating reagent, and a non-ionic surfactant. Optionally, the composition includes polyvinylpyrrolidone. The composition has a pH of about 8.45 to 8.85. The organic iron-chelating reagent has a first affinity constant greater than or equal to 104.2 with respect to ferric iron and a second affinity constant less than 103.8 with respect to magnesium. The first affinity constant and the second affinity constant are determined in deionized water at pH 8.45 and 20° C. Methods of using the composition to prepare a sample for nucleic acid amplification are also provided.