Nuclear Magnetic Resonance (NMR)
Basically, atoms of the sample are subjected to an external magnetic field. But instead of aligning with that field, they precess (wobble). By disturbing these atoms with an alternating field, the atoms can be made to "flip". When this field is at the same frequency as the precess rate, the atom is in resonance. As the atom passes in and out of resonance, energy is absorbed and emitted.
Years ago, while volunteering at a local library, I came across a book that was a collection of articles from Scientific American's "Amateur Scientist" column. One that stuck with me was an April 1959 project to demonstrate nuclear magnetic resonance. While by today's standards it is hopelessly obsolete, but it sparked a "What if" moment. What if the vacuum tubes were replace by technology readily available today, such as MMICs and low-noise transistors and op-amps? I had high hopes of something akin to a "Star Trek"-style tricorder. But, after reading about how NMR works, I'm sorry Mr. Spock, not yet. Now, since the emission frequencies involved are directly proportional to magnetic field strength, a strong field will increase the precess rate, and vice versa. My plan is to use the Earth's own magnetic field (Earth's Field NMR), allowing for low precess frequencies (at the expense of resolution) to make a portable unit.
