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After my most recent RNA extraction, the RNA samples had very high 260/230 absorbance ratios, (ranging from 5 to 25).

I've never gotten numbers like this and I know the ratio is supposed to be ~2 in general.

What could be the reason for such a high 260/230 absorbance?

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scientistsolutions.com/t11627-high+260_230+ratio+for+dna.html suggests that high ratios are caused by high salt concentrations in your sample? –  Jack Aidley Feb 19 '13 at 22:28
    
Thanks for the link, but in my experience salt contamination would cause a low 260/230 ratio, as salts absorb at 230 nm. –  Drosophila Feb 20 '13 at 16:32
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2 Answers

The 260/230 ratio gives you idea about the contaminants in your sample. Guanidine isothiocyanate, which is usually used for RNA isolation, absorbs at 230nm, so that might be what you see. But another explanation might be that your sample is very dilute (how much RNA did you get?), so that your A260 is also very low. Then it would be difficult to accurately measure A230, which will result in overestimated 260/230 ratios.

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The RNA concentration was not lower than usual (1000 ng/µL), but in the future I'll keep this possibility in mind. –  Drosophila Feb 20 '13 at 16:29
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Gergana is right about the effects of the Guanidine isothiocyanate, but not the details there of. Guanidine isothiocyanate absorbs strongly at 260 and NOT 230 (guanidine hcl is the one that absorbs strongly at 230). As such if your RNA is highly contaminated with guanidine isothiocyanate, which is used in at least one buffer in just about every RNA prep kit, your 260/230 is going to be much larger then 2 (also the "concentration" will be artificially high. So if you are getting a 260/230 of 25 and have a "concentration" of "1000 ng/µL" you may actually have less then 200 ng/µL of RNA and a ton of Guanidine isothiocyanate throwing everything off.)

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