You could lower the volume of eluate to 20 uL if you are using a higher volume, you can also load the same column multiple times with the same sample. That would mean that at the bind step you don't proceed to wash-wash-wash-elute, instead you would do bind-bind-bind-wash-wash-wash-elute. I would email Qiagen\exiqon to make sure that is ok to do.

Why don't you simply ethanol precipitate your eluate?

Shneha,

If you don't have limitation of the sample try starting with larger sample/quantity.

As Can wrote above smaller volume should give higher concentration, yet at the expense of losing some of the material (i.e. - lower recovery; look at the kits specifications they usually mention the efficiency they see with their test sample).

As to precipitation that is a good possibility, be aware that when the concentration is low, and the total quantity not high enough, it is likely that you will lose material during the precipitation. To enhance that you may use Glycogen, a carrier that is known to help in enhancing precipitation efficiency.

I also suggest you to refer to the following paper, that may be helpful for your task, if you have not seen it ye. Article Comparison of Methods for miRNA Extraction from Plasma and Q...

Sneha

As people already suggested you can try starting with higher amount of sample. Addition of glycogen while you precipitate the RNA would be helpful in precipitating low amount of RNA as well as short RNA species. you can also try increasing the time of precipitation, but be careful not to allow precipitation for to long, as other contaminants from your sample may also get precipitated.

good luck..