What does it mean [A260/A280] ? - (Sep/11/2007 )

My measure RNA concentration by using nanodrop 1000.

The value of 260/280 absorbance is around 2.4

I know that if the RNA is pure, the value should nearly 2.0

Anybody can tell me how about my RNA purity, is it pure ?

And, why my RNA value is higher than 2.0 ?

Ps. I use BPC grade Water, Sigma, for making Blank measurement, and also for RNA elution from MinElute spin column, Qiagen.

Thank you very much.

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Hi there,

The 260/280 reading gives a loose indication of the purity of a RNA or DNA sample. Absorbance at 260nm measures the RNA concentration and 280nm measures the protein concentration in the sample (see this article for more detail- it describes analysis of DNA solutions, but its pretty much the same for RNA). A260/280 does not have to be exactly 2 for a pure sample - the value you have obtained (2.4) indicates that the sample is pretty good.

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well RNA 260/280 ratio may be higher than 2. But 2.4 starts to be little to much for me. I would not recommend to use samples with higher ratio than 2.3, although 2.4 may be ok too.

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If the RNA concentration were low I wouldn’t trust in this value as a purity indicator.
I would use it.

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Thank you very much, everybody.

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I was teached to consider a good sample if ranging between 1.8-2.2.
OBviuosly you msut make considerations if youo go out of that range but in some cases you could also decide to use the sample anyway.

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Hi!
I used to use Qiagen kits but in my new lab they don't use them. I obtained a bad ratio for 280/260 of about 0.9.
I think there are still some proteins in my sample. What can I do with this?

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phenol:chloroform(isoamyl alcohol) extraction.

reprecipitation with na acetate and ethanol.

then resuspend with te, tris or dih2o (depending on what you will do next).

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