Absorption Spectroscopy: Definition & Types

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson, we'll learn about absorption spectroscopy. Read on to learn how this spectroanalytical procedure works, what it can be used for and different kinds of absorption spectroscopy, then test your knowledge with a quiz.

Introducing Spectroscopy through Color

All material can absorb certain types of waves. We see this happening all the time when we look at something and see a color. For instance, look at the following picture and describe the color:

A vibrant azalea

You would probably say that the flower is yellow. Sunlight, which is shining down on the flower, is 'white light' - it contains all of the colors. When the light hits the flower, the flower absorbs all of the colors except for yellow. The yellow gets bounced back to our eye.

This same principle works with spectroscopy, the measurement of matter based on how much of a particular material is emitted or absorbed. Light waves pass through a material, and the light that makes it through can then be measured with a sensor. We know that everything that didn't make it through the material must have been absorbed by that material. Often we measure the amount of light emitted or absorbed, but spectroscopy can also measure other waves, such as gamma-rays, sound, x-ray, and infrared.

Spectroscopy and Absorption

While defining spectroscopy, we used the word 'absorb' several times, as well as 'emitted.' Absorption refers to how much light (or other waves) can be taken in by the material being measured. Emission, on the other hand, refers to how much light (or other waves) can be released by the material being measured - emitted light is usually altered from its original state by the material, and that altered light is what's measured during spectroscopy.

Both absorption and emission spectroscopy are useful measurement tools, but in this lesson, we will focus on absorption spectroscopy, which is a procedure that uses the absorption of light waves by free atoms to determine chemical elements.

How Does Absorption Spectroscopy Work?

Different materials absorb different wavelengths based on their molecular and chemical make-up. For example, let's look at the flavoring vanilla. The chemical that makes the flavor vanilla is called vanillin, the structure of which can be seen below:

The structure of vanillin
Structure of vanillin

It has a phenol group (the hexagon with the OH attached). The phenol group will absorb wavelengths of 2.2 to 3.0 microns. So, if infrared waves are passed through the vanillin, the specific wavelengths around 2.2 and 3.0 will be absorbed.

If we had not known beforehand what this substance was, absorption spectroscopy would have revealed that it has a phenolic group. Based on the absorption, the OCH3 and aldehyde groups would also be apparent, allowing us to figure out that the substance is vanillin.

Types of Absorption Spectroscopy

Absorption spectroscopy can be used with any type of wave, including infrared, gamma, microwave, x-ray, visible light, sound, atomic, and even radio waves. However, not all of these waves have a practical use. The most common types of waves measured by absorption spectroscopy are infrared, atomic, visible, ultraviolet (UV), and x-ray.

Each spectrophotometer works using the same techniques. However, spectrophotometers are categorized according to the type of wave being measured. This is what we mean when we refer to 'types' of spectrophotometers. For example, a visible light spectrophotometer measures visible light rays, while a UV spectrophotometer measures ultraviolet light. They work in the same way - measuring the waves emitted or absorbed -- but they measure different kinds of waves. Note that some spectrophotometers can measure more than one kind of wave.

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