What are you using to isolate the macrophages? If it's ficol/lympholyte (excuse spelling), you may find that this is the cause of the issue. 

My solution to this was more trizol, more chloroform and a much more thorough wash with 70% Ethanol. 

That said, the droplet will eventually re-suspend in enough water and heat, you could then re-precipitate but with LiCl, which should be less likely to precipitate the Ficoll! 

Thanks for your answer Nathan.

I isolate the precursors of the macrophages from the bone marrow and differentiate them in vitro (6 days) with a special medium, so unfortunately your suggestion might not fit my problem. Thanks anyways!

Fair enough! I'd still have a skim through the media components and see if you have any kind of hydrophilic polysaccharide, as this sounds a lot like the problem I had when I used ficoll! 

I have contacted a few years ago Life Technologies Technical support regarding the same issue although the starting material was completely different. This was the answer I received:

"The problems you are seeing with the bubble after the precipitation might be related to the quality of the chloroform you are using in combination with a high RNA/salt content of you samples. In order to prevent this from happening we would recommend you to use a HPLC grade purified chloroform (99.5%) and to use less input material (less cells) in your samples. This will prevent the bubble from appearing."

I hope this helps!!

Thanks to all of you for your suggestions. I finally I chosed the "easy" solution by using RNA extraction kit with columns. It works perfectly with macrophages.

I am using TRIzol reagent of Invitrogen. I found one published about relation 260/280 that the measure is 1.60.

When you are using TRIzol, you must be sure in a cleaned area.

TRIzol should keep freezing and when you have to use, you defrost and the isolation of macrophages at room temperature.

Incubate when you add trizol 5 minute,previously homogenizate the sample with relation acording of manofactured, incubate 3 inute when you add cloroform, then centrifugate 12000g - 15 min - 4°C, It´s critical. When you add isopropanol you put in ice for 15 min. the centrifugate.