In Flame Emission Spectroscopy, what part of the flame is used to measure the concentration of the analyte? Flame Emission Spectroscopy is an analytical measuring device that relates the intensity of the emitted light relative to to the concentration of the sample.

As I am reading an article about FES, I noticed how the author purposely inserted the different parts of the flame and their activity.



The above image was from an article in Chemistry LibreTexts about FES. 

The author said "The Primary Combustion Zone is where the initial decomposition occurs and molecular fragments are observed. The Interzonal Region is the hottest part of the flame and atomic fragment are observed. The Secondary Combustion Zone is cooler overall and a conversion is seen from atoms back to stable molecules and oxides."

I have read the article but it was never mentioned what part of the flame does the transducer or sensor was placed on.
I have read the article but it was never mentioned what part of the flame does the transducer or sensor was placed on.


It was never mentioned because no sensor or transducer is actually placed on or in the flame. What spectroscopist do is that they focus the light (coming from the flame) into a device called the monochromator. The job of the monochromator is to separate the wavelengths of the light from the flame. The desired wavelength is then focussed on the detector. Thus the detector is really far from the actual flame.

Now the key question is which part of the flame is useful. There is no single correct answer. The criterion is we scan the height of the flame and check which part of the flame is showing the maximum intensity of the desired signal. This is done by changing the height of the burner. Most spectrometers do it automatically but old lovely instruments had knobs for manual adjustment.

Normally for most elements one would like to see the hottest part of the flame, where the concentration of the atomic emission is the highest.