Difference between Voltage and Current ?

[Paulhurleyuk]

I'm an administrator for a large, multi instrument chromatography system that has lots fo HPLC's and a few GC's.

One of my users recently showed me a validation protocol they had recieved which quoted responses in pA (which we think must be pico Amps ?)

Our Perkin Elmer Autosystem GC's / Dionex Chromeleon produces mV (milli Volts). Now knowing a small amount of basic physics Current and Voltage are linked, but very different quantities.

Is pA a current ? Is this produced from only certain types of detectors ? (we use FID exclusively). I'm puzzled.

Thanks

Paul.

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Paul Hurley

[CE Instruments]

The signal from a Flame ionisation detector is created by the increase in conductivity of the flame when a compound is burnt. The voltage is fixed and the current change is measured, the change is in Pico Amps. That said the detector output will be in millivolts on a nominal 1 volt scale for analogue output. Digital connection direct to PC can display as the orginal current change in pA.

[jwmol]

Most CDS systems would allow you to use a Y-axis multiplier to convert from the GC analog output back to the signal unit at the detector source (in case of an FID: pA).

e.g. if 1,000 mV on the output terminal of the GC would be equal to 1,000,000 pA, you would introduc a factor of 1000 and enter the right unit in the detector settings of the CDS.

I do not know if Chromeleon can do this.

Most people do not bother as the concentration results for the peaks will be no different with this setup. An analytical method is calibrated against concentration e.g. mg/l or %. Hence the pA / mV question is more about convenience if you want to see the baseline signal level in a relevant unit in order to judge the behaviour of your FID.

JW