text: * Contamination by phenol, which is commonly used in nucleic acid purification, can significantly throw off quantification estimates. Phenol absorbs with a peak at 270 nm and a A of 1.2. Nucleic acid preparations uncontaminated by phenol should have a A of around 2. Contamination by phenol can significantly contribute to overestimation of DNA concentration.
  * Absorption at 230 nm can be caused by contamination by phenolate ion, thiocyanates, and other organic compounds. For a pure RNA sample, the A should be around 1:2:1, and for a pure DNA sample, the A should be around 1:1.8:1.
  * Absorption at 330 nm and higher indicates particulates contaminating the solution, causing scattering of light in the visible range. The value in a pure nucleic acid sample should be zero.
  * Negative values could result if an incorrect solution was used as blank. Alternatively, these values could arise due to fluorescence of a dye in the solution.