We have a control product that reads a PH sensor with very high impedance. A few suggestions:
1. Handle with care. Have assembly people wear gloves.
2. Clean the boards really good, dry, then coat the analog section.

I thought our analog engineer was paranoid, but he took an un-coated board and connected a sensor in a solution reading 7.1. He quickly placed his hand a few inches from the board, which caused the reading to change by 40-50%!