pH sensor project: trouble with RF interference
pH probes provide relatively high voltage (hundreds of mV), however they have extremely high impedance - 10's or 100's of M\$\Omega\$ for a typical glass probe.  

You must ground the BNC shell to the housing (or an electrostatic shield around the electrometer amplifier), and that should be connected to circuit ground. 

It's normal to have a small low-leakage capacitor on the op-amp input, perhaps 1nF NP0 or film, and a series resistor of some M\$\Omega\$, which will filter out any RF that gets past the shielding.