Hi Paweł Małecki, concentrating RNA in formamide can be challenging, as formamide is a potent denaturant and can interfere with common RNA precipitation methods. However, there are a few possible approaches you can consider:

• Ethanol precipitation: You can try adding ethanol to your formamide-extracted RNA samples to precipitate the RNA. Generally, adding 2.5 to 3 volumes of ethanol and incubating at -20°C or -80°C for an extended period (e.g., overnight) can facilitate RNA precipitation. After precipitation, centrifuge the samples, remove the supernatant, wash the RNA pellet with ethanol, and then air-dry or vacuum-dry the pellet.
• Lithium chloride (LiCl) precipitation: LiCl can be used to concentrate RNA in formamide-extracted samples. Add 2-3 volumes of ice-cold 4 M LiCl to the sample, incubate at -20°C or -80°C for several hours, and then centrifuge to pellet the RNA. Wash the pellet with ice-cold 75% ethanol, air-dry or vacuum-dry, and resuspend the RNA in an appropriate buffer.
• SpeedVac concentration: Another option is to use a SpeedVac concentrator to remove the formamide and concentrate the RNA. Transfer the formamide-extracted RNA to a suitable tube or plate, place it in the SpeedVac, and evaporate the formamide under vacuum at a low temperature. Be cautious not to over-dry the RNA, as it may affect its integrity.

Please also consider that concentrating RNA in formamide can potentially lead to some RNA degradation or loss. Therefore, it's crucial to optimize the concentration method and validate its effects on RNA integrity and yield using appropriate quality control techniques like agarose gel electrophoresis or spectrophotometric analysis.

I'd try isopropanol to remove the formamide. Should be more efficient than ethanol (at least from experience with precipitating nucleic acids from aqueous solutions). Basically, any solvent that is partially miscible with formamide should do the job, as long as the RNA doesn't dissolve in it. So, in the end, it will precipitate.