What a 260/280 ratio does not prove

A 260/280 ratio is a comparison of two absorbance readings, and the comparison is about protein more than it is about anything else. Nucleic acids absorb strongly near 260 nm. Proteins absorb relatively more near 280 nm, largely from aromatic amino acids. Divide one by the other and you get a hint: if 280 is unusually large beside 260, protein is likely contributing. That is the decision this number can support. It cannot tell you that the strands are long, that they belong to a named species, or that a polymerase will copy them this afternoon.

Nucleic acid extraction, including the sample lysis that came before the bind, decides which other molecules share the cuvette or the pedestal. Those method classes are laid out in how DNA extraction methods differ. The ratio is what you read afterwards. It is not a second extraction.

What the two wavelengths are actually seeing

The bases in DNA and RNA have a peak near 260 nm. Tryptophan and tyrosine in proteins pull relatively more absorbance toward 280 nm. In a clean extract, measured in a slightly alkaline buffer, laboratories often see DNA near 1.8 and RNA near 2.0. Those figures are neighbourhoods, not statutes. Acidic water can drop a DNA ratio even when the tube is the same tube you measured yesterday in Tris. Phenol, which absorbs near 270 nm, can shove the spectrum sideways so that both the concentration and the ratio lie. Free nucleotides absorb at 260 nm and inflate the mass without looking like protein, so the ratio can stay flattering while a dye that prefers long double-stranded DNA disagrees.

Because 260/280 is a quotient, a pathlength shared by both wavelengths cancels out of the ratio. It does not cancel out of the concentration. The usual 1 cm planning factors are about 50 ng per microlitre of double-stranded DNA per absorbance unit at 260 nm, about 40 for RNA, and about 33 for single-stranded DNA, and only after the instrument has expressed the reading on a 1 cm basis. A tidy ratio can sit on a mass that is wrong by a large factor.

A260/A230 is the partner clue, and it answers a different contaminant. Chaotropic salts such as guanidinium, leftover phenol, and carbohydrates absorb strongly toward 230 nm. When they remain, 260/230 falls. People often hope to see this second ratio near 2 or higher, but the useful observation is a depression relative to a clean extract in the same buffer, not a universal cutoff copied from a different kit. EDTA also absorbs in that region, so an elution buffer that is heavy in EDTA can look "dirty" at 230 nm while doing exactly what a storage buffer is supposed to do. Know what you blanked against.

Dirty blanks move both numbers

The blank is supposed to be the elution buffer with no sample, read on the same surface. Everything after that is extra absorbance. If the blank is dirty, you subtract the wrong baseline from every later tube, and both ratios move.

A pedestal filmed with the previous sample's guanidine adds absorbance around 230 nm to the blank. The instrument then over-subtracts at 230 nm and the next sample's 260/230 can look better than the liquid deserves. A blank that missed a protein smear, or a cuvette still wet with a Bradford-type reagent from the bench next door, distorts 280 nm and the 260/280 ratio follows. A water blank of a Tris-EDTA eluate does the opposite kind of harm: the buffer's own absorbance, especially near 230 nm, is counted as if it were sample. The ratios then describe the buffer plus the nucleic acid, and two honest samples in two different buffers stop being comparable.

The practical test is dull and decisive. Read the blank itself across the spectrum before any sample. It should sit near zero from the low 200s through 320 nm. If it does not, clean the surface or the cell, load fresh elution buffer, and blank again. A 260/280 you obtained on a wandering blank is not a property of the DNA. It is a property of the arithmetic. Re-read after the blank is flat. Only then decide whether protein, chaotrope, or carbohydrate is even a fair suspicion.

Turbidity joins the same family of lies. Magnetic beads, glycogen crumbs, or cell debris scatter light across wavelengths. Scatter often lifts the whole baseline, and a quick look at 320 nm, where nucleic acids should be quiet, tells you the reading is not a clean peak. A ratio from a cloudy eluate is not evidence of purity. Clear the particles, or accept that absorbance is the wrong tool until you do.

What has to be proved some other way

Integrity is a shape. Genomic DNA that was sheared to a few hundred bases can show 1.8 and still be useless for a long-read library or a large clone. Degraded RNA can show a ratio near 2.0 while the ribosomal bands are gone. Look at a gel, or at an electrophoretic trace, when length is the claim. The ratio has no axis for length.

Species is a sequence question. Nothing in Beer's law encodes taxonomy. A mixed extract from tissue, saliva, soil, or a culture can be chemically "clean" and biologically mixed. If you need to know what genome you have, use primers you have checked, or compare a read to a public collection such as NCBI BLAST and a taxonomic browser. Do not write the organism's name next to a ratio and call that an identification.

PCR success is an enzyme question. Inhibitors that barely absorb at 260, 280, or even 230 can still stop a reaction. Ethanol carried from a wash is the everyday case. Heme, heparin, humic acid and polysaccharides are the matrix cases. Too much DNA is another: the ratio looks excellent because the tube is full of DNA, and the polymerase is overwhelmed. Dilute tenfold and repeat, or spike a known target into an aliquot. Recovery after dilution means the original tube was inhibitory or too concentrated. A blank extraction in the same PCR tells you whether the nucleic acid extraction itself planted a product. None of that information is inside 260/280.

How to use the number without promoting it

Blank with the real elution buffer. Confirm the blank trace is flat. Read the sample. Record A260, both ratios, the buffer, and the pathlength factor. Then branch.

If 260/280 is low and the blank was clean, suspect protein, a phenol-distorted spectrum, or a very acidic diluent. A cleanup aimed at protein is a rational next step when the assay cares. Remeasuring the same dirty tube in a different buffer can move the ratio without removing anything. Say which of those you did.

If 260/280 looks fine and 260/230 is low, suspect guanidine, phenol, carbohydrate, or EDTA from the buffer itself. A skipped wash after a chaotropic lysis is the common laboratory cause. A second wash class, done the way that protocol describes, is the test. Do not "correct" a low 260/230 by changing the concentration factor.

If both ratios look fine and the downstream enzyme is silent, leave the spectrophotometer. You are in inhibition, overload, or a dead enzyme. If both ratios look fine and a gel shows a low smear, the molecules are short. Sample lysis can manufacture the leftover: an unfinished proteinase step pulls 260/280 down, and a chaotrope that was not washed out pulls 260/230 down. Fix that step. Re-reading the eluate is not a purification.

Limits, safety, and a shared reader

This is a research explanation of an optical hint. It does not validate a diagnostic quantification, a forensic identity, or a biosafety level. The eluate may still be chemically harsh or biologically active. Chaotropes and phenol belong to the extraction, not to the lamp. Follow the safety data for what you opened, and follow institutional rules for the specimen. The WHO laboratory biosafety manual is background for that local decision, not an approval of the measurement.

Ultraviolet light inside a closed spectrophotometer is an instrument hazard only if someone defeats the housing. Do not. The more ordinary hazard is believing a ratio far enough to skip the gel and the control.

A bench where several people share one pedestal

The failure mode that matters in a crowded instrument corner is not a new law of optics. It is the previous user's liquid. Someone blanks with water, someone else blanks with TE, someone leaves a salty ring, and the next three ratios are no longer about those samples. Write the blank buffer on the worksheet before you export a spreadsheet. If the reader reboots and returns with a default factor, the concentrations change and the ratios may not, which is exactly how a tidy ratio conceals a bad mass. Clean until a fresh blank is flat, then read the batch. Dust and a dried fingerprint move 230 nm as effectively as guanidine does, and they move 280 nm as well once the film is broad. Both ratios are then properties of the glass, not of the extract.

What an enquiry should say if ratios are part of the acceptance check

If you are asking for columns, beads, elution buffer, or a reader, state which ratio you will record and which decision it is allowed to support. A specification that says "A260/A280 of 1.8" without a blank, a buffer, and an orthogonal check will accept sheared, mixed, or inhibited DNA. Ask instead for a method class whose leftovers you know, and state the assay that must still work.

The materials sit in the sample preparation catalogue, and the surrounding workflow is the sample preparation pathway. Send the specimen and the assay with the quote request. The nucleic acid isolation enquiry reference is a prompt for that conversation. It is an enquiry reference, not a measurement someone else has already performed on your tube. Ask whether a quotation is possible. Keep the acceptance language limited to what absorbance can see.