What is spectroscopy?

Spectroscopy is a type of chemical analysis done by shining light on a sample to determine what is inside. Chemists commonly measure the absorbance, how much light is absorbed by the sample, or the transmittance, how much light passes through the sample. An analogy of how spectroscopy woks, is that f you imagine that light is food and the sample is a room full of people, a complete spectrum of light would be like giving all the food in a grocery store to the people in the room. As you might imagine, almost all of the food would be wasted. But some of the food would be eaten. Imagine that you knew there was only one person who would eat asparagus. If all of the asparagus came through the room uneaten, that person could not be in the room, but if some were missing, you would know that person was in the room. Furthermore, no matter how much broccoli you put in the room, the asparagus-lover would never eat any, and so you could never know if she was in the room or not.

Similarly, in a chemical analysis, many different kinds (wavelengths or energies) of light (a spectrum) are shone through a sample. Some of the light is absorbed. By knowing what wavelengths of light are absorbed (or "eaten") by the sample, we know what is inside. But, if we are looking for a specific molecule or characteristic and shine the wrong wavelengths of light through a sample, no matter how much light we put through, we will never learn anything about the sample. Because different molecules and characteristics of molecules absorb at different energies of light, there is a need for different forms of spectroscopy.

Each different for of spectroscopy uses a different part of the electromagnetic spectrum, shown below, to investigate specific characteristics of a sample. For example, infrared spectroscopy (IR) is used to investigate how the molecules in a sample vibrate, while ultraviolet spectroscopy (UV) is used to investigate how certain chemical bonds in a molecule are arranged. Remember, IR spectroscopy can not be used to investigate the information that UV provides and vice-versa.

As you can see, the visible portion is just a small part of the spectrum. Energy and frequency both increases from left to right, while wavelength increases from right to left, making it inversely proportional to energy and frequency. So, the highest energy light (or radiation) is gamma radiation, while the lowest energy is microwave. This is important because, as explained above, molecules only absorb light if it is at exactly the correct energy. In spectroscopic terms, UV can not be used to investigate the vibrations of molecules because it is not the correct energy, even though UV light has more energy. Similarly, even if we had the most powerful IR lamp in the world, we could not use it to investigate the electronic transitions that make UV spectroscopy investigates because IR light is not the correct wavelength, or energy, to investigate electronic transitions.

Spectroscopy in Chemistry

If we could collect data on one molecule from every type of spectroscopic instrument available today, we would have a unique fingerprint of that molecule. But this is not practical or even feasible, so it is up to the chemist to determine what technique is best suited to the sample she is investigating.

The types of spectroscopy covered in this web presentation are IR, UV-vis, and NMR, and there is a brief description of each type of spectroscopy and an example spectrum at each page for the technique. The measurement of pH is not considered spectroscopy because there is no light going through a sample to be measured.

At Heidelberg, Germany in 1860, Gustav Kirchoff and Robert Bunsen (of Bunsen burner fame) discovered that each element absorbs and emits it own unique spectrum (1). This technique later led to the discovery of helium in the sun before it was known to exist on Earth.

1) Suplee, Curt. Milestones of Science. National Geographic Society. Washington, D.C., 2000.