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My observation: Previously at my work I was assigned a project in which I needed to amplify an AC signal from a piezoelectric electric sensor. I simply made an inverted op amp circuit to accomplish this. I now have been assigned a project involving a mass spectrometer (RGA). In the spectrometer there is an electron multiplier to amplify the partial pressures. The fact that an op amp was not used to provide gain for the ionization current perplexed me.

My question: Why can't one use an op amp to amplifier the current from the output of the quadruple mass spectrometer? My suspicion is that the reason is something to do with ion current being in a vacuum. I understand that inside the RGA is a electron multiplier, I just want to know why an op amp would not accomplish the same result.

Notes: My background is in Computer Science and my knowledge in physics is elementary, so sorry in advance if my question is based off of a misconception.

Links: https://www.mksinst.com/docs/ur/hpq3-ds.pdf (The RGA that the mass spectrometer will be hooked up to. - Could not get my hands on the data sheet for the actual mass spectometer.)

winny
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J Maklen
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  • Please provide a link to the specifications for the output of the mass spectrometer. – Elliot Alderson Jul 31 '18 at 19:04
  • Electron multipliers are typically very fast, and opamp design to accommodate the necessary speed might be difficult. However, without real details, there is no way of knowing. – Scott Seidman Jul 31 '18 at 19:14
  • I cant get my hands on the datasheet for the actual mass spectrometer but I could get the info on the RGA it will be hooked up to. https://www.mksinst.com/docs/ur/hpq3-ds.pdf To clarify, I believe the electron multiplier is inside the RGA. @ElliotAlderson – J Maklen Jul 31 '18 at 19:25
  • @JMaklen That 'RGA' has digital output (Ethernet); all the amplification is inside it already, along with digital converters. – Whit3rd Jul 31 '18 at 19:34
  • I should of made myself more clear. I know that but I want to know WHY an electron multiplier was used over a op amp for this application. Thanks! @Whit3rd – J Maklen Jul 31 '18 at 19:39
  • Are you sure you're not talking about a PHOTOMULTIPLIER?? – Scott Seidman Jul 31 '18 at 19:44
  • @ScottSeidman 100% sure I am not. RGA = Residual Gas Analyzer. – J Maklen Jul 31 '18 at 19:49
  • From your linked doc: "This is possible in a single scan or peak jump data acquisition with less than 20ms between the maximum and minimum signal levels and ****without**** the need for an expensive electron multiplier which needs frequent calibration and replacement" – Scott Seidman Jul 31 '18 at 19:54
  • @ScottSeidman Oh is see, not sure how I missed that. Still doesn't really answer my question if I still wanted to amplify the ionization current with an op amp.(Even though it doesn't really matter for my specific application) – J Maklen Jul 31 '18 at 20:02

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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.

Voltage Spike
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Accounting for typical ionization and collection efficiencies of quadrupole mass spectrometers, the range of ion currents produced (for pressure ranging from 1e-12 to 1e-6 mbar approx) will range from femtoAmps to nanoAmps. A current detector with at least 6 order of magnitude dynamic range is needed to measure these signals.

A typical RGA unit can also work without the electron multiplier but with a reduced dynamic range. In this configuration, the ion current collected on a metal cup (Faraday Cup) is sent to a (logarithmic) trans impedance amplifier to produce measurable voltages and then read by an ADC. However, the noise floor of this scheme is typically around picoAmps and signals well below this can not be measured reliably.

Electron multiplier amplifies this tiny ion current by a factor ranging up to 1e6 (depending on the bias voltage) and hence extending the dynamic range of current measurement to sub-femtoAmp levels. Additionally, since electron multipliers are fast response detectors (nanosecond response), they can also be used to directly count the ions arriving per unit time, providing enhanced measurement sensitivity.

A very good description of these points is provided in the SRS RGA series manual.

prs
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Some beliefs can be given:

The fact that a part of the circuit is flying ions in a vacuum allows an opamp circuit to amplify the arriving current as well as if the current came along a wire. Simply let the ions to land into a cup which is connected to the minus input of an opamp. A feedback resistor R from output to the same input would determine how many volts you get output for a given ion current I. U=-I*R

I think there's not enough current available in your application. Leakage and noise in the opamp circuit would cover the result totally.

Ion bombardment can rip off electrons from some material and that electron flow can be amplified with secondary emission in the same way as it happens in microchannels of a photo multiplier. That's your electron multiplier, I guess.

I believe noise and leakage in electron multiplier is much lower than in opamp circuits. Otherwise also night view goggles would use opamp arrays and a multiplexer to make proper video instead of microchannel amplifiers which output to a phosphor in front of the eye.