Atoms combine with each other to form molecules. Molecules move in various ways. Although we will not discuss their travels through space, we will discuss certain internal motions. Molecules are not rigid, but are held together by “flexible” chemical bonds. Such bonds can stretch, vibrate, scissor, rock, wag, or twist. Each of these motions energies found in the infrared radiation.
What is Infrared Radiation?
Infrared radiation constitutes that part of the electromagnetic spectrum just above the microwave, and just below the visible (light).1 Each of the motions referred to above requires a slightly different frequency and energy, the exact values of which depend upon the particular molecule involved. When infrared energy passes through some of the chemical, quantities of some frequencies are absorbed. Energy not absorbed passes through and is measured according to each frequency. A graph can be drawn that represents what was absorbed.
How an Infrared Spectrum Acts as a Fingerprint
Measured amounts of infrared energy pass through a carefully prepared sample of chemical, which will interact with it. Specific frequencies are absorbed, but not completely. The quantity absorbed depends upon the nature of the group doing the absorbing. If there is more than one exact same group, the amount absorbed increases accordingly. The remaining energy is less than what entered the sample.
One way to graph the results is from the amount of energy absorbed – such a graph is called an absorption spectrum. An alternative way to graph the results is to base it on what passes through the sample – a graph based on this is called a transmission spectrum. The values of the energy peaks do not carry values, such as ergs. This is because such units are divided out. Mathematically, the quantity of energy that passes through the sample is divided by the energy before entry, thus cancelling out whatever units were used, and the result is multiplied times 100%. The XY-graph plots this unitless result against wavelength, rather than against frequency.
To simplify, if you go view a graph from left to right, you are going from higher frequency (higher energy) to lower frequency (lower energy). And if you see a large peak (hanging down for a transmission spectrograph, shooting upwards for an absorption spectrograph), it means that more of the energy of that frequency was absorbed, with less transmitting through. Now let’s examine a transmission spectrograph: look at the sample fingerprint or chart, here. How is this a fingerprint? Well, consider a different chemical, here. Different from the first one, isn’t it?
Using Infrared Fingerprints to Solve a “Crime”
Now all of this would be of no practical value, except that it is possible, using “fingerprints” or an Infrared Spectrum, to identify a particular molecule. Thus, if you have a mysterious compound you have discovered but you can’t tell which one it is in a lineup, you may be able to run an infrared spectrum on it and discover just what it is you have! Yes, the mystery can be solved once again – using fingerprints!
1 There are other forms of spectroscopy or methods of producing chemical fingerprints. For instance, spectroscopy involving visible light is called colorimetry, since molecules absorb different frequencies or colors of visible light. Also, there is ultraviolet spectroscopy, because the same thing can be said about ultraviolet light.