The science behind the scenes - Chemical photography: seeing in a different light
Chemical photography is a specific technique that allows scientists to look within objects to discover the chemicals inside. This has uses in all sorts of areas: forensic science, creating new medicines, and even discovering fake artworks!
Chemical photography began life as a military technique. The military use infrared light detectors to guide their missiles, and infrared light, and techniques that detect it, are the basis of chemical photography. When infrared is shone on objects, detectors can measure which frequencies are absorbed. This is called infrared spectroscopy. The pattern of absorption - the chemical photograph - can then be decoded by scientists to tell them what chemicals are present in the object.
How does it work?
Infrared light is used, because it has been found that you can get more information about the chemicals (and the types of bonds within them) than if using light in the visible range.
In infrared spectroscopy, infrared light is shone through an object, and then the light is detected after it has emerged on the other side. Comparing the light before and after it passes through the object shows which frequencies within the light have been absorbed, and if you use an infrared array detector then it shows the spatial distribution of this absorption in the sample - the chemical photograph.
The material making up the object will be full of chemical bonds. Chemical bonds vibrate in different ways (twisting, bending and stretching), depending on the type of bond present (e.g. CO2, N2). Some examples are shown below: CO2 can vibrate in many different ways, but N2 will only vibrate in one way.
Different types of vibrations absorb different frequencies of infrared radiation. By looking at the infrared spectrum, scientists can tell which chemical bonds are present within the object, and hence what the object is made from.
No vibration:
Symmetric stretch:
Asymmetric stretch:
Bending modes:
Attenuated total reflection (ATR)
Attenuated total reflection (ATR) is a special way of measuring an infrared spectrum. One problem with infrared spectroscopy is that it can only be used with very thin samples of material, because the infrared beam of light has to pass through and out the other side where it is detected. If scientists want to investigate thicker objects, they can use ATR.
In this technique, the beam of light is angled through the object and bounces off one surface of it (possibly more than once), zigzagging through the object until it emerges at another point and is detected. This type of reflection inside the object is termed total internal reflection.
In the diagram below, using ATR to examine the tablet shows how much of it is a polymer, how much is the drug, and how much is additives.
The story behind infrared light
Sir William Herschel was a British astronomer, and a Fellow of the Royal Society, who is famous for discovering the planet Uranus, and also for discovering infrared light. He did this by passing sunlight through a glass prism. As sunlight passed through the prism, it separated into a colour spectrum, which contains all of the visible colours that make up sunlight (as in a rainbow).
Herschel was interested in measuring the amount of heat in each colour. As he measured the heat in different colours, he noticed that the temperature increased from the blue to the red part of the spectrum. He then placed a thermometer just beyond the red part of the spectrum, where there was no visible light, and found that the temperature was even higher than any he measured in the visible spectrum. Herschel realised that there must be another type of light beyond the red, which we cannot see. This type of light is known as infrared - infra is derived from the Latin word for ‘below’.
Why is this important?
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