Compounds are injected into the chamber in several forms using several techniques. Gases, liquids and solid compounds are used. Single compounds or mixtures can be injected. These can be pure or diluted (to provide for a accurate and manageable injection amount). Liquids and solids require heating or nebulizing.
Gases are purchased as pure compounds or diluted (in nitrogen usually). These are purchased commercially prepared, in gas cylinders of single compounds or mixtures. There is an injection shed used to house the gas cylinders, closer to the smog chamber. The gases are injected into the return loop of the chamber sample manifold. Electrically activated solenoid valves allow for control of the time of the injection, after the injection rate is measured.
Liquids can be injected by heating or nebulizing using a Colison nebulizer. Liquids with low boiling points can be injected directly into the return of the sample manifold (which is always heated). Liquids or solids with higher boiling points can be heated
(using a hot-air gun or heating bath) in a "U-tube" and delivered using simple connecting tubing and using air or nitrogen as a carrier gas. Very high boiling point compounds are injected this way directly under the chamber, through the floor with a minimal amount of tubing (which is also heated, see photos). Mixtures of liquids can also be injected this way.
Nebulizing is useful when heating does not work or causes decomposition of the compounds. The nebulizer forms a fine mist without heating. The nebulizer is operated in the updraft of one of the mixing fans, mounted to a metal pole about 2 meters above the chamber floor, to assure evaporation and to prevent condensation on the outside of the nebulizer (see photo).
Whereas liquid injection is very precise and accurate, the nebulizer is less so, since not all of the liquid is injected.
Test injections are necessary to calibrate the injection as a function of time, and internal standards are necessary as well (compounds which can also be injected by direct heating). Very light or non-polar compounds are not suitable for nebulizer injection, since they de-gas faster than they nebulize.
