|MadSci Network: Chemistry|
The best way to measure the level of most minerals, including iron and chloride, is by a process called Inductively Coupled Plasma, or ICP for short.
Basically, ICP works by sending a fine spray of a mineral solution into a chamber filled with hot gas. The gas "excites" the minerals, causing them to give off energy at a certain wavelength (each mineral gives off its energy at a specific wavelength). The strength of the energy given off indicates how much of a mineral is contained in solution.
I have included the URL for a site which describes how this process works and includes pictures of the process.
Try the links in the MadSci Library for more information on Chemistry.