FID signal

[thohry]

Hi all,

I am used to operating Agilent GC and the Agilent FID signal is in pA. But some other vendor's FIDs have the unit in uV. I am not sure if the two units have something in common and if we could transfer from one to another.

Thanks

[rb6banjo]

One is a current and one is a voltage. I assume you could just use V = IR but you'd have to know the resistance of the flame. Can you share with me how you think this conversion might be useful to you?

[Peter Apps]

As something leaves the column and passes through the flame it ionises and the flame resistance drops. This leads to an increase in current through the flame, which is converted to a voltage signal by an electrometer.

Whether in pA or mV, the signal still has to calibrated against analyte quantity to be any use for analysing samples.

Peter

[thohry]

Thanks both for replying me.
@ rb6banjo : I just want to see the similarity between the two. For those that use pA I know the baseline signal of about 10 - 15 pA are OK, but for those that use uV, I have no idea.

[rb6banjo]

The best way to address your concern would be to measure the signal-to-noise ratio for an analyte at a small concentration. It's a bit of work but it's a better comparison of the instruments.

[antonk]

Definitely, these pico-amperes rather virtual than real.

Instrument ADC measures voltage - it is converted to pA using virtual (or real) feedback resistance of amplifier.

There are at least two way to correspond these mV to nA.
1. Make certified mixture and inject in both instruments. Compare SNR (signal-to-noise ratio) and get LOD (limit of detection).
2. Take certified low-current calibrator (an instrument with precise regulated current source) and take a measurement to determine what is scale for 1 pA in instrument measuring current, and what resistance is taking in account in instrument measuring voltage.