Nucleic Acid Quantification and the A260/A280 Ratio

Before any molecular biology experiment can proceed, the researcher needs to know two things about a nucleic acid sample: how much is there, and how clean is it. UV absorbance spectroscopy answers both questions simultaneously, without reagents, in under 10 seconds. Understanding the physics behind the 260 nm and 280 nm absorption peaks turns a pair of numbers into a powerful quality-control tool.

Why Nucleic Acids Absorb at 260 nm

DNA and RNA consist of purine and pyrimidine bases stacked along a phosphate-sugar backbone. The aromatic ring systems of these bases contain delocalized pi-electrons that absorb ultraviolet light strongly at approximately 260 nm. The extinction coefficient at 260 nm is well established: a 50 ug/mL solution of double-stranded DNA (dsDNA) produces an absorbance of 1.0 at a 10 mm path length. Single-stranded DNA (ssDNA) has a slightly different coefficient (33 ug/mL per A unit) because unstacked bases absorb more efficiently, while RNA sits at about 40 ug/mL per A unit. These factors, combined with Beer-Lambert law and a measured path length, convert the raw absorbance reading directly into concentration.

The A260/A280 Purity Ratio

Proteins absorb at approximately 280 nm due to the aromatic side chains of tyrosine and tryptophan residues. Protein contamination is one of the most common nucleic acid impurities, arising from incomplete cell lysis cleanup or residual bovine serum albumin from buffers. Dividing the absorbance at 260 nm by the absorbance at 280 nm gives a ratio that reflects how much of the UV signal is from nucleic acid versus protein.

Accepted purity thresholds are: pure dsDNA approximately 1.8, pure RNA approximately 2.0. A ratio significantly below these values indicates protein or phenol contamination. A ratio above 2.0 for DNA often indicates RNA carry-over. Values are buffer-dependent: samples in slightly acidic buffers read lower ratios than the same sample in Tris-EDTA (pH 8.0), so buffer choice should be consistent.

The A260/A230 Ratio: Solvent and Salt Contamination

Guanidinium salts, EDTA, phenol, and carbohydrates all absorb near 230 nm. A healthy A260/A230 ratio should fall between 2.0 and 2.2; values below 1.8 suggest residual chaotropic salt from a column-based extraction or carryover from a phenol-chloroform step. The NanoQ series acquires the full spectrum from 220 nm upward in a single flash, so both ratios are reported automatically without a second measurement.

Microvolume Measurement in Practice

Using 1-2 uL on the K LAB NanoQ, a typical genomic DNA prep at 50-500 ng/uL falls comfortably within the instrument dynamic range without dilution. The 4-point auto path length correction accounts for the exact film thickness formed by each sample, so concentration accuracy is maintained even when sample viscosity varies between high-salt and low-salt elution buffers. Results — concentration, A260/A280, A260/A230, and a baseline-corrected spectrum — are displayed within seconds of lowering the arm.

Limitations to Keep in Mind

• Free nucleotides (dNTPs) absorb at 260 nm and inflate concentration readings if not removed post-PCR
• Highly degraded RNA reads a correct A260/A280 ratio but still fails downstream assays — gel or chip electrophoresis is needed to verify integrity
• Very dilute samples (below about 2 ng/uL for DNA) approach the noise floor of even high-performance microvolume instruments and may benefit from fluorometric quantification

Used correctly, the A260/A280 ratio remains the fastest and most widely accepted first-pass purity check, making a microvolume spectrophotometer an essential tool at every bench that handles nucleic acids.