Amplifying an ionization current with an op amp
Ion currents can be amplified from a plasma or ion source in a vacuum. They are called Ion Langmuir probes. The problem is the probe changes the ion current, if you have a very low density plasma, this may become very hard to measure the current. To give you an idea, a transimpedance amplifier can measure in the pico-amp range reasonably well, which would still be 10^6 or one million electrons per/second to measure 1pA.

Usually mass spectrometers have ion rates much much lower than that (with an ion corresponding to an electron). So if you connected a plate directly to an amplifier, you'd need a lot of ions (in the thousands to millions) (to either recombine with electrons OR knock off electrons to produce a current).

A better way is an electron multiplier which has a series of plates, with each plate knocking off more electrons, it can gain up the current from one electron or ion into thousands or more, which makes it possible to count individual ions. The other problem is because the ions have a space charge, it might be hard to develop a current high enough with only ions, because an ion beam 'separates' the more ions are bunched together.