Infrared Gas Analyzer: How to Accurately Measure Concentration

[Charles123]

An infrared gas analyzer gives you the concentration of a given molecule that absorbs radiation in a specific infrared band, in a gas, by determining the attenuation of radiation of the frequency in case, when in a path between emitter and receiver is a sample of the gas in analysis. My question is, since the molecule will re-emit the radiation, and it can do this in any direction, some of it can reach the receiver. Therefore, in order to not account for this radiation, that would otherwise indicate a minor concentration of the molecule, what is done? Is it just a question of time lag of the re-emitted radiation in comparison to the radiation that reaches the receiver directly?
Thank you
Regards

 

[CWatters]

I don't know for sure how its done but the radaition spectra has a different intensity pattern to the absorbtion spectra.

Appears they are quite complex devices..
http://en.wikipedia.org/wiki/Infrared_gas_analyzer

 

[mikeph]

It's very unlikely the molecule will return to its original state, and also that any radiated light is oriented directly at the detector, so the signal will be tiny.

Stimulated emission is a much bigger effect, where the light can make a molecule emit an identical photon in the same direction as the original one. Even so, this effect is very small in the infrared (~0.01% I think).

 

[Charles123]

Thank you for your answers!
CWatters, even that is the case, it can still be emission in the same wavelength, isn`t that the case?
"It's very unlikely the molecule will return to its original state", why is that MikeyW?
Regards

 

[sardar]

I can explain that in order to accurately measure the concentration of a molecule using an infrared gas analyzer, a few factors are taken into consideration. One important factor is the path length of the sample gas between the emitter and receiver. This path length is carefully calibrated to ensure that the attenuation of the radiation is only due to the absorption by the molecules in the gas sample.

Additionally, the instrument is designed to only detect the radiation that is emitted in a specific direction, usually towards the receiver. This is achieved through the use of filters and mirrors that only allow specific wavelengths and angles of radiation to reach the receiver. This helps to eliminate any re-emitted radiation from the gas sample that may reach the receiver and give false readings.

Furthermore, the instrument is usually operated in a controlled environment with minimal external radiation sources. This helps to minimize any interference from external sources and ensures that the detected radiation is only from the gas sample being analyzed.

In summary, accurate measurement of concentration using an infrared gas analyzer involves careful calibration of path length, use of filters and mirrors to eliminate re-emitted radiation, and operating in a controlled environment. It is not just a question of time lag, but rather a combination of these factors that ensure precise measurements.