dsDNA: 1 OD260 = 50 µg/mL | ssDNA: 33 µg/mL | RNA: 40 µg/mL
UV spectrophotometry at 260 nm is the standard method for nucleic acid quantification. Nucleic acids absorb UV light due to their aromatic bases. Beer-Lambert law: A = εlc, where ε for dsDNA gives the conversion 1 OD260 = 50 µg/mL. The 260/280 ratio assesses purity: pure DNA = 1.8, pure RNA = 2.0. Low ratios (<1.6) indicate protein or phenol contamination. The 260/230 ratio (ideal 2.0-2.2) detects organic solvent contamination. Modern NanoDrop spectrophotometers require only 1-2 µL. For very dilute samples, fluorometric methods (PicoGreen, Qubit) are more sensitive and specific for dsDNA. Accurate concentration measurement is critical for: cloning, PCR setup, sequencing library prep, and transfection.