A low 260/230 ratio usually points to carryover from extraction chemistry or the sample matrix. The important question is not whether the ratio looks ideal in isolation, but whether the preparation is fit for its downstream use.
Guanidinium and other binding salts can remain when wash volume, membrane drying, or elution conditions are inadequate. These contaminants may inhibit enzymes used in PCR, reverse transcription, ligation, or sequencing-library preparation.
Organic extraction can produce strong absorbance near 230 nm when phase separation is incomplete. Repeated extraction is not always the best answer because it may reduce yield or integrity.
Residual wash ethanol, carbohydrates, peptides, EDTA, and other matrix-derived compounds can depress the ratio. Overloading a column or bead-binding reaction increases carryover risk.
| Check | Interpretation | Next step |
|---|---|---|
| Blank and measurement range | An incorrect blank or very dilute sample can distort absorbance ratios. | Reblank with the exact elution buffer and confirm concentration fluorometrically. |
| Extraction history | Identify phenol, guanidinium, ethanol, high salt, or overloaded binding capacity. | Choose a cleanup that targets the likely contaminant. |
| Integrity | A clean ratio does not prove intact DNA or RNA. | Use a gel or electrophoretic integrity method appropriate to the application. |
| Downstream function | Some preparations perform adequately despite a nonideal ratio. | Run a small PCR, RT-qPCR, digestion, or library-prep suitability check before reprocessing everything. |
Depending on the contaminant and required fragment size, options may include an additional membrane wash, magnetic-bead cleanup, precipitation, buffer exchange, or a fresh sample-specific extraction. Avoid repeated cleanup without a defined contaminant hypothesis because every additional manipulation can reduce recovery.
If the material is limited, the contaminant source is unclear, or the preparation must support sensitive qPCR, sequencing, cloning, or transfection, preserve an aliquot and review the workflow before repeating extraction.
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