Typically researchers are interested in the 260/280 and the 260/230 ratio.  Protein has a high absorbance at 280 while nucleic acids have a high absorbance at 260.   On the other hand contaminants like phenol have a high absorbance at 230.  High A230 or A280 indicates that your nucleic acid prep has a high amount of contamination which may or may not affect downstream assays.  Typically your 260/280 ratios should be around 2 (ranges 1.8-2.1) and 260/230 ratios are optimally  2.  260/280 is almost always around 2 but I have observed a wider range of variation for 260/230 depending upon the person performing the purification process, quality of reagents, etc... Keep in mind that these ratios can be greaty skewed if your concentration of nucleic acids is fairly low, especially for the 260/230 ratio so if it looks suboptimal it might still be okay for downstream assays assuming they are not ultrasensitive to the presence of contaminants (qPCR probably okay but microarray would not be okay).

Hope this helps.

Typically researchers are interested in the 260/280 and the 260/230 ratio.  Protein has a high absorbance at 280 while nucleic acids have a high absorbance at 260.   On the other hand contaminants like phenol have a high absorbance at 230.  High A230 or A280 indicates that your nucleic acid prep has a high amount of contamination which may or may not affect downstream assays.  Typically your 260/280 ratios should be around 2 (ranges 1.8-2.1) and 260/230 ratios are optimally  2.  260/280 is almost always around 2 but I have observed a wider range of variation for 260/230 depending upon the person performing the purification process, quality of reagents, etc... Keep in mind that these ratios can be greaty skewed if your concentration of nucleic acids is fairly low, especially for the 260/230 ratio so if it looks suboptimal it might still be okay for downstream assays assuming they are not ultrasensitive to the presence of contaminants (qPCR probably okay but microarray would not be okay).

Hope this helps.

The ratio 260/280 reveal the purity of RNA preparation. for RNA it should be around 2.0 and for DNA it should be 1.8. this ratio is ralated with the amounts of protein to RNA or DNA in the preparation. The ratio 280/230 is unknow for me

If you're following the standard guanidium isothiocyanate/phenol method for RNA isolation (TRIzol-type), then your major contaminants are going to be protein, phenol and guanidium. Brian very nicely covers the protein issue, above, though I would add that the chief candidate for absorbance at 230 is generally guanidium rather than phenol.

Phenol tends to absorb at 270, so is tricky to assess ratiometrically (it's right between protein and RNA absorbance maxima), so for that simply look at your trace. If the actual peak is at 270 rather than 260, or the trace has a slight shoulder around 270, then you have a phenol problem. This is usually fairly rare, though (in my experience).

Guanidium contamination (low 260/230 ratio) is much more common, and this is because ethanol/isopropanol precipitation (the final RNA isolation step) can quite easily precipitate guanidium salts. Cold isopropanol precipitation is particularly bad for this (always do isopropanol precipitations at room temperature), but some carryover is not uncommon no matter how careful you are.

Both phenol and guanidium do very bad things to proteins, so you really don't want them around if you're planning on any downstream enzymatic reactions with your RNA (like cDNA synthesis).

If you have guanidium contamination, you can usually remove this by simply reprecipitating your RNA: make your RNA up to a usable volume (say, 500ul) with DEPC-treated water, add 0.1 vols of 3M sodium acetate, pH 5.5, and either:

1 vol of ROOM TEMP isopropanol, followed by incubation at room temp for 20 mins

or

3 vols of ICE COLD ethanol, followed by O/N incubation at -20 or 1 hour at -80

then collect precipitated RNA by centrifugation at 13krpm at 4 degrees in a benchtop centrifuge, and wash 2x with ice cold 70% ethanol.

This usually loses 20-30% of your RNA, but also loses 90-100% of your guanidium, so it's less RNA but much better quality.

If you have phenol contamination, you probably need to do a chloroform extraction: phenol is highly soluble in chloroform, so just make your RNA up to 500ul with water (as above), then add 500ul of chloroform and vortex vigorously. Spin for 10 mins, take aqueous phase, precipitate as above. 

There is no need to know about the ratio A280/230 during nucleic acid extraction. ONLY A260/230 and A260/280 are important. These two ratios indicate the level of purity of your nucleic acids. Depending on the extraction method or the kit used, the values can vary but the optimal ratios are respectively 2 and 1.8.