There are many variables in GC, with flame ionization detector (FID) temperature being one of them. An average GC user can’t possibly optimize all variables, so most of us use manufacturer recommendations. Agilent recommends that the FID temperature be set 20 °C above the maximum oven temperature to prevent condensation of molecules in the FID jet. However, as a curious operator of GC, I have always wondered: Is there an optimal FID temperature to achieve the highest FID sensitivity? As temperature changes, do peak heights change? Does the baseline noise change with FID temperature?
Sensitivity in GC is usually defined as the peak height-to-noise ratio. Examining peak heights alone wouldn’t provide a good measure of sensitivity if the baseline noise was also changing, or vice versa.
We performed an experiment to answer this question: does FID sensitivity change with FID temperature? This was done by injecting 0.1 uL of pure methanol using a cool on-column inlet on an Agilent 7890A GC with a Polyarc/FID and varying the FID temperature from 150 to 450 °C. All variables were kept constant between each experiment with the exception of FID temperature. Six injections were performed at each temperature, and the baseline noise and peak heights were measured in each of the 42 experiments. Because the Polyarc reactor converts organic molecules to methane before detection in the FID, the FID was seeing an equivalent amount of methane in every experiment.
That brings us to the results shown here. As the FID temperature is increased from 150 to 450 °C, the peak height for methane increases monotonically. The peak height is about 15% higher at 450 °C than at 150 °C. Does that mean we should operate the FID at 450 °C? Not so fast. The second plot shows how the baseline noise changes with temperature. At moderate temperatures the baseline noise is approximately constant, but at 450 °C the baseline noise increases by about 20%.
The third figure shows the peak height-to-baseline noise ratio (i.e. sensitivity). Here, we can see that an FID temperature of 400 °C gives the highest sensitivity, and thus 400 °C is the optimal FID temperature. So, the next time you modify your GC method, change the FID temperature to 400 °C. Only in the event that your oven temperature is greater than 380 °C would I recommend the FID be operated at 450°C.
If you have performed or seem similar studies in the past, I would be love to learn more. Further work is required to understand why FID sensitivity changes with temperature.
Note: The variability in the data is caused by the injection-to-injection repeatability of the autosampler, and the error bars reflect the standard deviation in the six data points taken at each temperature. This information only applies to Agilent FIDs and may be different for other manufacturers.
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