DNA Concentration Calculator (ng/µL from A260)

Nucleic acid quantification by absorbance rests on the Beer-Lambert law, which makes absorbance proportional to concentration and to the optical path length. The purine and pyrimidine bases absorb ultraviolet light with a maximum near 260 nm, so A260 scales with the amount of nucleic acid present. The proportionality constants used in practice are empirical: an A260 of 1.0 in a 1 cm path corresponds to about 50 ng per microlitre of double-stranded DNA, 33 for single-stranded DNA and oligonucleotides, and 40 for RNA. Single-stranded species give a higher reading per unit mass because base stacking in a duplex suppresses absorbance by roughly 20 to 30 percent, an effect known as hypochromicity. Concentration is therefore A260 multiplied by the type-specific coefficient and by any dilution factor, divided by path length in centimetres. Two ratios accompany the number. A260 divided by A280 detects protein, whose aromatic residues tryptophan and tyrosine absorb near 280 nm; clean DNA sits near 1.8 and clean RNA near 2.0, and values below about 1.7 indicate protein carryover. A260 divided by A230 detects organics and chaotropes such as phenol, guanidine salts, EDTA, and carbohydrates, with 2.0 to 2.2 regarded as clean and values under 1.8 as contaminated, most often from incomplete column washing. The method's central limitation is that it is not specific: DNA, RNA, free nucleotides, and degraded fragments all absorb at 260 nm, so a sample of sheared DNA reads the same as an intact one. Readings below roughly 2 to 5 ng per microlitre are unreliable and turbidity inflates results, which is why fluorometric dye assays are preferred when species specificity or low-level accuracy matters.

The optical principle long predates molecular biology, with the absorption relationship developed through work by Pierre Bouguer in the eighteenth century, Johann Lambert, and August Beer in 1852. Its application to biological macromolecules dates to the 1940s, when Otto Warburg and Walter Christian described in 1942 a method for correcting protein estimates for nucleic acid contamination using the ratio of absorbance at 280 and 260 nm, effectively inverting the logic used today. Practical measurement became routine with the Beckman DU spectrophotometer, introduced in 1941 by Arnold Beckman's company, which for the first time gave laboratories reliable quantitative readings in the ultraviolet. The specific conversion factors and purity thresholds entered general use through molecular cloning practice in the 1970s and 1980s, and were codified for a generation of researchers by the Molecular Cloning laboratory manual first published by Tom Maniatis, Ed Fritsch, and Joe Sambrook at Cold Spring Harbor Laboratory in 1982 and revised in later editions with Sambrook and David Russell. Those texts fixed the 50, 40, and 33 coefficients and the 1.8 and 2.0 ratio targets still quoted today. The workflow changed again in 2001 with the introduction of the NanoDrop microvolume spectrophotometer by NanoDrop Technologies of Wilmington, Delaware, later acquired by Thermo Fisher Scientific. By holding 1 to 2 microlitres between two fibre-optic pedestals with path lengths as short as 0.05 mm, it eliminated cuvettes and dilution steps and made measuring a precious extract a non-destructive routine step rather than a deliberate sacrifice of sample.