How are samples prepared for analysis with ICP-OES? Can it analyse gases?
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.