Absorption spectrometry is useful for analysing metals in the samples. But it's not recomended for non metals though. I know that it's difficult to analyse non metals because most of them absorbs in the same wavelenght as atmosphere (oxygen). Anyway, I would like to know if ionization energy plays a role in this analytical technique. I can't find any strong source.
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Your intuitive argument is not that far off. I am sure you are talking about atomic absorption spectrometry (AAS). Analyzing metals is routine but analyzing non-metals is indeed near impossible because one has to work in the vacuum UV region, but some non-metals like phosphorous can be determined with very low sensitivity (=large detection limits). Indirect methods for non-metals exist though.
The main point to remember in AAS is that you want free atoms not ions. So nobody is interested in ionizing metals or non-metals (hence their ionization energy), rather it is desired that ionization be suppressed in AAS using ionization suppression buffers. What one is interested in AAS in the electronic transition from the ground state to the first excited state. Ionization is far far away.
Look at this data from Perkin Elmer's AAS Cookbook for P, and its corresponding harsh conditions to even atomize phosphorous. A nitrous oxide is flame is required.
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$\begingroup$ So the argument of ionization energy is invalid. It all makes sense now. $\endgroup$ Commented Oct 21, 2022 at 0:20
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2$\begingroup$ I think so. There is Saha equation that talks about ionization but that is for stars but some people did connect to flame spectroscopy as well. I studied this decades ago. $\endgroup$– ACRCommented Oct 21, 2022 at 0:27