Types of spectroscopy

There are many ways to use a spectrometer to analyze materials, and each uses different light sources, sample preparation, and procedures. We're trying to summarize each briefly for those who are just getting started; please help to refine and improve the summaries below (and add case studies from relevant research notes). Also see our listing of different use cases for spectrometry.

Absorption spectroscopy

Known full-spectrum light (the sun, a halogen lamp) passes through a sample (made transparent by putting it in solution or cutting a very thin slice of it) and we measure what colors of light was blocked by the sample. Usually you take a "baseline" of just the light, with perhaps a pure water sample in the sample container (A), and then take a real reading (B), then subtract B from A to get the absorption spectrum. See the above diagram.

Reflectance spectroscopy

Like absorption spectroscopy but you shine light onto a sample and measure what's reflected. Instead of passing through the whole sample, light interacts with just the surface of the material. Less sensitive, but in theory you could just go outside and measure what sunlight reflects off of things. This is most commonly used in earth observation satellites.

Fluorescence spectroscopy

• Fluorescence occurs when photons are absorbed by atoms, and electrons are bumped up to higher orbit -- as they release energy when they drop down to their original energy state, they release a photon. Because the electrons fall specific distances, you always get the exact same color. The color of the photon indicates how far it fell, which relates to the size of the atom.
• Can use any high energy light, including sunlight (although it is hard to see flourescence in presence of so much reflectance)
• seems to be best for hydrocarbons, chlorophyll, and ravers

Flame emission spectroscopy

• Only for identifying pure elements like mercury, sodium, maybe arsenic? See more here: http://publiclaboratory.org/notes/straylight/10-14-2012/classroom-flame-spectroscopy