A260/A280

The ratio of absorbance values measured at 260 nm and 280 nm gives a rough estimation of the purity of the sample and which nucleic acid it contains. If there is a protein contamination, the ratio is lower, if RNA, the ratio is higher. The ratio for pure double-stranded DNA is about 1,85-1,88 and pure RNA around 2,1. There is a rule of thumb: if the ration A260/A280 is >1,8, the sample is pure nucleic acid.

A260/A230

Various contaminants may affect in the ratio A260/A230: chaotropic salts used in the extraction, the detergents like Triton X-100 and Tween20 used in the lysis buffers, proteins and phenol, often used in RNA extraction. A260/A230 ratio for pure dsDNA is 2,3-2,4 and for pure RNA 2,1-2,3.

Low Sample concentration        

The pathlenght used for measuring the absorbance in the NanoDrop is shorter than 10 mm. According to the instrument specifications it is possible to measure dilute nucleic acid samples reliably down to 2 ng/µl.If you are measuring dilute nucleic acids, <20 ng/µl, consider the purity ratio only directional: the ratio is calculated by dividing a very small numerical value with another very small value and there could be some variation between replicates.     

The effect of the solvent

The pH of the solvent may have an effect on the results. If your DNA or RNA samples are dissolved in water, there could be more variation than with buffered solvents.