Capacity Embedded in PCB for High-Sensitive Electrometer Legitimate?
If you define your sensor as a Charge Meter with a sensor of 100 fF =\${10^{-13}}~~\$  [F] and  a constant current of \$10^{-15} [A]~~\$  then 

\$\frac{dV}{dt} = \frac{Ic}{C} = \frac{10^{-15}[A]}{10^{-13}[F]}={}{10^{-2}}{}[\frac{V}{s}] = t_R~~=\frac{0.35}{f_{-3dB}}\$  from 10% to 90% [V]

thus \$ f_{-3dB} = \frac{0.35}{10^{-2}}~=35 ~ \$ [Hz]

This \$ f_{-3dB}~~\$may be too high for your expectations.

Potential Solutions: 


reduce stray C by using active guarding around tracks with same CM voltage buffered adjacent to signal tracks such that ΔV=0 thus ΔIc=0 between guard tracks and even less to stray conductors.  
use chopper stabilized methods in datasheet.
use lower input bias current Op Amps.
reduce stray leakage R by using air tracks to reduce surface dust creepage R as explained in datasheet
Compute track PCB capacitance which is 4x air to reduce conductor capacitance to sensors using wider separation and air tracks perhaps with AWG 30~40 magnet wire.
use gold leaf sensors and measure displacement like Coulomb did which had a long decay time [s]
use circuits employed for ESD meters to measure discharge or charge with S&amp;H with suitable protection. They often use a charge amplifier (TIA) commonly used for large piezo accelerometers to measure vibration charges to convert to voltage but maybe also BW limited.


Comments?