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Microwave spectroscopy is the use of microwave signals to determine the what elements exist in a sample of material.
This page came about from a Microwaves101 contributor (and inventor!), David Patterson.
...This is a chemical analyzer I've built using cryogenic microwave tricks. The whole thing cools down to about 6 Kelvin; I put molecules into the box (the big square is one side of the box), hit them with microwave pulses, listen more or less for the "echo", and the echo tells me what kind of molecules they are. Those horns can twist around because the polarization is really important, and I don't trust dual polarization horns [do you? Their mode quality seems crappy to me]. I call it the RABBIT, not because it stands for anything, but because my girlfriend really likes rabbits. The components on the top are a cryogenic amplifier (which you can buy) and a cryogenic protection switch (which you can also buy, but it doesn't say anywhere that it works at 4 Kelvin. Here's a trick: The HMC547 works just great at low temperature, even though it's only rated to -40 C. But caveat emptor.. I've tested exactly one unit. )
Check out David's patent application.