You can strip the BTEX in liquid phase to pass it to the gas phase, I think you could use tedlar bags to store the gaseous samples. But due to the volatility of BTEX I do not know how long they last in the stock market. That's according to the BTEX. I do not know how to do it with formaldehyde. I think it can be the same way, but I do not know how much it interacts chemically.