Enter your A260, A280, and optional A230 and A320 background values into the OD260/OD280 Ratio Calculator, then select your nucleic acid type and dilution factor to get your sample's purity ratio, corrected A260, and concentration — plus a purity assessment for your DNA or RNA sample.
Purity Ratio
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Concentration
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Purity Assessment
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Corrected A260
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A pure DNA sample should have an OD260/OD280 ratio between 1.7-2.0. Ratios significantly lower than 1.7 may indicate protein contamination, while ratios higher than 2.0 may suggest RNA contamination or alkaline conditions.
The OD260/OD230 ratio should be between 2.0-2.2 for pure nucleic acids. Lower ratios may indicate contamination with organic compounds like phenol, guanidine, or other chemicals that absorb at 230nm.
A320 is used for background correction as nucleic acids don't absorb significantly at this wavelength. Subtracting A320 from other readings helps account for light scattering and turbidity in the sample.
Concentration is calculated using the formula: (A260 × dilution factor × conversion factor) / path length. The conversion factor depends on the nucleic acid type: 50 μg/ml for dsDNA, 40 μg/ml for ssDNA/RNA, and 20 μg/ml for oligonucleotides.
Low ratios (below 1.7) typically indicate protein contamination in your sample. Proteins absorb strongly at 280nm due to aromatic amino acids like tryptophan and tyrosine, increasing the denominator and lowering the ratio.
Yes, if you diluted your sample before measurement, you must multiply by the dilution factor to get the actual concentration of your original sample. For example, if you diluted 1:10, use a dilution factor of 10.
Different nucleic acids have different extinction coefficients: dsDNA (50 μg/ml per OD unit), ssDNA and RNA (40 μg/ml per OD unit), and oligonucleotides (20 μg/ml per OD unit). Using the correct factor ensures accurate concentration calculations.