A260 concentration, purity assessment, and mass-to-moles conversions for nucleic acids.
Beer-Lambert: Concentration = A260 x extinction coefficient x dilution factor
Enter your Nanodrop or spectrophotometer readings. We'll interpret the purity ratios and flag contamination.
UV spectrophotometry at 260 nm is the most common method for nucleic acid quantification. The extinction coefficients are: dsDNA = 50 ng/uL per A260, ssDNA/oligo = 33 ng/uL per A260, and RNA = 40 ng/uL per A260. These assume a 10 mm path length; Nanodrop uses a 1 mm path and auto-corrects internally.
The 260/280 ratio assesses protein contamination (target: 1.8 for DNA, 2.0 for RNA). The 260/230 ratio detects organic contaminant carryover from extraction — guanidinium salts, phenol, EDTA, carbohydrates (target: 2.0-2.2). Both ratios should be measured in a consistent buffer (TE pH 8.0) as they are pH-sensitive.
A 260/280 ratio below 1.7 for DNA or below 1.9 for RNA indicates protein contamination (proteins absorb strongly at 280 nm). Consider re-extracting with phenol-chloroform cleanup or a column-based kit with additional wash steps. Note: very dilute samples (<10 ng/uL) give unreliable ratios.
A 260/230 ratio below 1.8 indicates contamination with organic compounds that absorb at 230 nm: guanidinium thiocyanate (from lysis buffer), phenol, TRIzol, EDTA, or carbohydrates. This is the most common issue with column-based RNA extractions. Additional ethanol washes or ethanol precipitation usually resolves it.
Use the formula: nM = (ng/uL x 106) / MW. For an oligo, MW = bases x 330 Da (average for ssDNA). Example: 25-mer primer at 100 ng/uL → MW = 25 x 330 = 8,250 Da → nM = (100 x 106) / 8,250 = 12,121 nM = 12.1 uM. Use the Mass ↔ Moles tab in this calculator for instant conversion.