ppm estimation of gas measurement

Hi, someone should help please. I have absorbance (A) spectra (calculated with A = I/Io, Beers lambert law) of some gas measurement using a gas cell. I will like to estimate the measurement in ppm. How can I estimate this in Matlab?

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[Frank]

Coincidentally, I was just searching for a ready made matlab unit conversion m file for this conversion. There are a lot of online conversion sites for ppm to concentration and back but most of them assume standard temperature and pressure and you may have (like I do) various lab conditions. I just resigned myself to writing a little conversion code that does it for my purposes. If I do it this evening, I will return and post it, but you can do it too if you want. If you have the absorbance, which is usually expressed as log10(Io/I) [what you wrote is what is called the transmittance]. Then the Beer Lambert law has A = epsilon*L*c where epsilon is the molar attenuation coefficient of the species you are measuring, L is the optical path length of the device, and c is the molar (or mass) concentration of the species in units compatible with the other quantities. You need to convert your absorbance to concentration first by plugging in the values of L and epsilon corresponding to your device and whatever species you are measuring. Once you have concentration the conversion to ppm is relatively straightforward, depending on whether your target species is dilute in otherwise ambient dry air. If you are happy to use the standard temperature and pressure you can use 22.4 for the molar volume, but otherwise you need to use the ideal gas law (maybe with adjustments for RH and your particular species if your accuracy depends on that). In the simple case you can just calculate it as C_ppm = C_mass/vol*(22.4)/(Mol. weight of your species). Good luck.

[Doyinsola]

Hi Frank, thanks. Yes, the absorbance formular I wrote up there is a mistake. I have the corrected conversion. Also, I am familiar with the Beers lambert law formular you wrote for calculation of concentration in ppm but the challenge for me is how to estimate the epsilon which I have seen that they use a standard concentration for but in my case I have no standard concentration as what I have measured is a smoke from an ablation process which means all gases I have identified are initially unknowned but identified based on the absorbance spectra compared with a reference (HITRAN). In case this is also same for you and what you have figured out can help me with estimating the epsilon, I will be happy if you share the code here when you do this evening.

Thank you and I anticipate.

[Frank]

I think your question is more suited for expert molecular spectrosopists than matlab code help. Folks here can help if you have some code you are tying to make work but you need help with the details of the matlab code implementation. There are a lot of instrumental and spectroscopic details that are involved in your problem. I think better places to find help with a possible matlab implementation would be 1) searching the file exchange for some specialists who have worked this specific problem 2) searching reasearchgate for discussions of the problem or 3) searching available literature for someone who has done this. I am fairly multidisciplinary, but molec spec is not my strongest suit. A quick search of available pubs turned up this recent paper that may help you set up how you want to do it

Gas phase multicomponent detection and analysis combining broadband dual-frequency comb absorption spectroscopy and deep learning

https://www.nature.com/articles/s44172-023-00105-z

Once you figure out how you want to do the calculation given your specific apparatus and develop some code, I think the folks here could help you make it work. Good luck.

[Doyinsola]

Thanks for your well detailed response. I will check it out