Removing a stubborn odor

I had a subletter that smoked in a bedroom for a few years. He vacated a year ago and despite my efforts, the room still smells a little. Not like cigarettes per say just kind of stale. I’ve repainted, replaced the mattress, aired it out for countless days. I suspect the smell is in the floorboards (only b/c it’s the most logical thing left). I covered it in baking soda and let it sit for 2 days. It helped a little. I’m going to try it again but was hoping there was a trick I didn’t know. I’m looking for DIY solutions if at all possible.