Does the 6517B measures pA current by discharging the source

I have a charged liquid stream flowing into an insulated metal container. My Keithley 6517B is connected to the container to measure  a pA current produced by the charged liquid stream. The stream is constant so the current should be constant, but the electrometer shows the current decreasing in time. Does the electrometer discharge the container as it measures the current? Does it discharge the current faster than the liquid stream can replenish it? Why else would the electrometer show a decreasing current?

By the way, the technical support "ticket" on the Tektronix website doesn't work!!!
 

Need you to clarify on connections.  Where is the HI and the LO?

Your experiment sounds a little bit like Ion Beam Measurements.
Check out section 4.3.5 in our handbook:
Keithley Low Level Measurements Handbook

You may need to create a bias condition to attract the charge to flow through the amp measure function of the electrometer.