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In ICP-OES (Wikipedia page), you introduce a sample into a very hot plasma and then run a spectral analysis on the resulting light emissions. Liquid samples are sprayed into the flame. From the Wikipedia article:

A peristaltic pump delivers an aqueous or organic sample into an analytical nebulizer where it is changed into mist and introduced directly inside the plasma flame.

From that I can imagine that you would probably dissolve solid samples (the technique is widely used in metallurgy and mineralogy) in something suitable to obtain a liquid sample. If that's correct, I'm wondering how the feeding apparatus is constructed to withstand possibly aggresive solvents, like nitric acid or aqua regia. The injection tube is apparently made from fused silica, I guess that's a probable material for the feed as well.

And how about gases? Are they analysed with ICP-OES as well? Are they simply pumped into the injection tube? For example, can we just introduce a stream of air into the plasma and get the emission spectrum for 78% nitrogen, 21% oxygen (I guess the argon will be hidden because the plasma gas is argon)? I guess since every sample will be beyond vaporized anyway by the intense heat, it can't make much difference, but are gas samples too "thin", i. e. do they contain too little atoms per volume for useful analysis?

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  • $\begingroup$ From that I can imagine that you would probably dissolve solid samples in something suitable to obtain a liquid sample. Under normal circumstances, that is absolutely correct. As mentioned on the answers, acids are not a problem - in fact, concentrated acids are usually used to "digest" organic samples in sample preparation. Then the samples are diluted to be suitable for the ICP. There's a whole science behind sample preparation, though. Each sample and each analyte require very specific sample preparation methods in order to be properly detected in equipments such as an ICP-OES. $\endgroup$ – Alex Apr 18 '13 at 16:55
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About the measurement of solids

Injections tubes are indeed often made out of fused quartz. Dealing with aggressive solvents is actually not that much of an issue because of the extremely low amounts of material that can be detected with ICP-OES. Therefore, you can simply dilute your aggressive solvent before use. You can look for example at this research note where they dissolve a sample in $\ce{HCl}$ and $\ce{HNO3}$ and then dilute it before use.

About the measurement of gases

Yes, it is possible to measure gases directly, but it is not recommended and some adaptations to the normal procedure are needed. In this research paper they investigated aerosol analysis by ICP. The main issue was keeping sampling lines at elevated temperatures to allow proper calibration. A secondary problem arose because their target analyte, metallic compound in flue gases, which are volatile at (or close to) room temperature, are typically highly unstable or explosive. Therefore, they recommend dissolving areosols in a liquid solution of water and some organic solvent (1-methoxy-2-propanol in their case) and use this as feed.

For the measurement of 'normal' gases it seems that the problems they encounter will not occur, because you don't need elevated temperatures in your calibration. So in principle that means that you can indeed measure air. It should be noted that in some machines a shear flow of dried air or nitrogen is used to cut the plasma (see wikipedia) which will disallow measurement of air, as this Perkin Elmer instrument (overview table in slide 7 of this presentation). Some other instruments, like the one in the research paper I referenced, do not use such a shear flow and will thus be able to detect air.

Gas samples are not at all 'too thin', because the normal nebulized feed that goes into the plasma is similarly dilute.

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  • $\begingroup$ :) That first research note incidentally is about the exact application I am interested in, specifically, the analysis of steel alloys. $\endgroup$ – Hanno Fietz Apr 16 '13 at 10:54

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