right i tried that and it didnt seem to work when adjusting the pot it just seemed to be all the way one way or all the way the other. i dont know if i need a higher rotation pot so i can be more accurate with it? im currently using something like a 300 degree at the moment
I really don't think you need an op amp for this. The Arduino pins have very high input resistance (around 100 Mohms), so detecting the difference between 1.7Mohms and infinity is easy. I used a voltage divider made from two 4.7Mohm resistors with an atmega328p in a recent project. However, you will need to take precautions against picking up interference. I suggest you connect your ionisation probe between the input pin and ground, use a 4.7Mohm pullup resistor to +5V, and also connect a 0.1uF capacitor between the input pin and ground.
You can get finer control by putting fixed resistors in the top and bottom of the pot so the wiper covers a smaller of voltages. Also try reducing the gain a bit