Hi Priyanka,

I assume you are talking about the 260:280 ratio, which should be 2 for RNA and 1.8 for DNA. I upload a file for you here from ThermoScintific. You may take a look at it, it is very easy and precious. It says you might have contamination of phenol, proteins etc.

Isolation of RNA should not necessarily take place under laminar flow. I do it on the bench and it works fine for me. What you could do in this case is that you designate/dedicate a bench, set of pipettes, set of tips only for RNA isolation. Nothing else should be made in this place. Spray eveything with RNase-Away solution that inactivate these devilish enzymes.

Best of luck.

In the samples with lower values, you will still have DNA/RNA, just means you have some contamination i.e. for 280 this means protein while for 230 this means extraction chemical (phenol etc).

I've made cDNA with samples that had 1.0 260/230 and the qPCR worked fine.

Some methods such as microarrays are sensitive to high extraction chemicals so these ratios are important for certain applications whilst other applications are not so sensitive. 

thank you Karim and Can for your valuable suggestions. never before have i used RNase solution so I would use it and give a try but I clean everything with DEPC water before starting isolation.

Can I would definately give a try for qPCR with my these samples