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Why is $\ce{H2}$ used in an FID, apart from the fact its combustion does not contaminate the flame?

In other words, is the temperature of the flame important or critical? Is the stability of the temperature?

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  • $\begingroup$ What alternative would you suggest ? E.g.a methane-air flame in FID would be like e.g using methylchloride as the makeup gas in ECD. $\endgroup$ – Poutnik May 19 at 12:43
  • $\begingroup$ @Poutnik One alternative would be electric arc ionization. Another extreme UV. What is special about H2? $\endgroup$ – Dirk Bruere May 19 at 13:21
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    $\begingroup$ It would be even much worse than methane. The key is to keep as low baseline ionisation as possible. That does not happen with methane flame and definitely not with arc or UV ionisation. With the arc, there would be additionally very high noise, several orders higher than useful signal. $\endgroup$ – Poutnik May 19 at 13:26
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The key to the answer is understanding how FID works.

The hydrogen flame has a minimal flame ionisation, what is needed for the low signal baseline.

Incoming organic molecules from the HPGC column create in the flame a lot of ions and increase the flame electric conductivity.

Using alternatives causing higher ionisation would decrease FID sensitivity that is not great even at ideal conditions, compared to e.g. ECD.

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Another fundamental aspect of using hydrogen in gas chromatography is the so-called van Deemter curve. The curve shows the linear velocity of the gas on the x-axis and plate heights on the y-axis. It can be shown that by using hydrogen one can obtain the best efficiency as compared to any other gas.

And of course, hydrogen is required for burning the organic molecules exiting the column. It is a clean flame (no carbon background).

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