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.
The 260/280 ratio flags protein and phenol contamination, while the 260/230 ratio flags organic and chaotrope contamination. A260 measures the nucleic acid itself, A280 measures protein and other aromatics, and A230 measures organic contaminants such as guanidine, phenol, EDTA, and carbohydrates.
For the 260/280 ratio, ~1.8 indicates pure DNA and ~2.0 indicates pure RNA. Values meaningfully below these targets flag protein or phenol carryover, since proteins absorb strongly around 280 nm.
For the 260/230 ratio, 2.0-2.2 is clean, and values below ~1.8 flag guanidine, phenol, or EDTA carryover from the extraction chemistry. Use the Purity Check tab above to enter your A230, A260, and A280 readings and interpret both ratios automatically.
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.
Apply the Beer-Lambert law, A = ε x c x l, and rearrange: c (mol/L) = A260 / (ε x pathlength). For oligonucleotides, use the sequence-specific molar extinction coefficient; for dsDNA, the 50 ug/mL-per-A260 mass convention is common. To convert a mass concentration to molarity, use nM = (ng/uL x 106) / MW (in Da).