Why does my isolated RNA have a low OD 260/280 ratio?
FAQ ID -97

The A260/ A280 ratio is influenced considerably by pH. Since water is not buffered, the pH and the resulting A260/A280 ratio can vary greatly. Lower pH results in a lower A260/ A280 ratio and a reduced sensitivity to protein contamination*.

For accurate values, we recommend measuring absorbance in 10 mM Tris·Cl, pH 7.5. Always be sure to calibrate the spectrophotometer with the same solution. Please see the Appendix sections in the RNeasy handbooks for additional information.

* Wilfinger, W.W., Mackey, M., and Chomczynski, P. (1997) Effect of pH and ionic strength on the spectrophotometric assessment of nucleic acid purity. BioTechniques 22, 474.