Picoamperemeters or also called pA-Meters are sensitive amperemeters, which, as the name implies, can measure very low currents. The lowest current range is 2pA full scale with a resolution of 10aA (10E-17A or 10 Attoampere). Looking at the elementary charge of 1,6x10E-19 Coulomb, about 100 electrons are responsible for 1 digit on the display ...
Picoamperemeters are working with current feedback methods. The shunt resistor is in the feedback loop of a differential amplifier. The burden voltage equals the differential voltage of this OP-Amp and is about 200μV. With this technique low currents still can be measured at small voltage potentials of eg. 10mV.