Contaminants in Your Nucleic Acid Sample? 

Science Hub

Measuring absorbance on UV range has long been an established method for quantifying nucleic acids and determining the purity of the sample. According to the Lambert-Beer law the amount of light absorbed at 260 nm wavelenght is directly related to the concentration of nucleic acid in the sample. There are generally accepted factors for nucleic acids used to calculate sample concentrations when measuring with a standard cuvette with pathlenght of 10 mm. For example, a double-stranded DNA sample, which gives an absorbance value of 1 at 260 nm, has a concentration value of 50 ng/µl so the factor is 50. Additionally the absorbance ratios measured at different wavelenghts are used to evaluate the purity of the sample.


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.


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. 

Acclaro algorithm and NanoDrop One

It is not that easy for a researcher to exploit the results measured with different wavelenghts. The Acclaro Sample Intelligence software developed first for the NanoDrop One instrument can find and correct contamination in your samples and even differentiate DNA from RNA. The software can also advise on how to continue purifying the sample if needed. This feature has been the one most appreciated within the users. Would you be interested in testing this with your own samples? Reach out to Pia!

Yes, I would like a NanoDrop One demo

Pia Ojala

Product Specialist

+358 20 7410 274

Pia Ojala