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.
i did try this but did not seem to work
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
i haven't altered the gain at all, i presume to lower the gain i reduce the value size of the 100k which is connected to the output of the op amp?
right i think i've got it working in a matter of speaking with out altering the gain the problem i have now is it seems to be too sensitive still so putting my finger between the probes is enough to activate it.