Spectroscopy: Definition and Types

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  • 0:01 Inside an Analytical Lab
  • 0:50 Electromagnetic Radiation
  • 2:30 The Spectrometer
  • 4:01 What Can Spectroscopy…
  • 7:26 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

In this video lesson, we will introduce and discuss different spectroscopic techniques used by scientists in an analytical lab. We will understand the importance of these techniques and show how they can be employed in identification and analysis.

Inside an Analytical Lab

TV and filmmakers love chemical analysis. I am sure you instantly recognize the labs in shows like CSI, Bones and NCIS. These labs are filled with all sorts of instruments, and the scientists are painstakingly analyzing the information that comes out of them. Unlike TV, where just one piece of analysis coming from one instrument instantly solves the crime, in real life, scientists need many different types of chemical analysis. Each lab instrument provides a different piece of information to help solve the puzzle. In this lesson, we are going to learn about the important topic of spectroscopy, which is a scientific technique for analyzing the properties of material by measuring its interaction with light.

Electromagnetic Radiation

We are going to look at four techniques commonly used to help with structural analysis: ultraviolet-visible spectroscopy, infrared spectroscopy, nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry. The first three are known as absorption spectroscopy, and we will deal with these first.

Spectroscopic techniques involve absorption of specific energies of light or, more accurately, electromagnetic radiation. Absorption, here, simply means that energy has been transferred from the radiation to the molecule. This energy change causes a transition or an excitation from a lower energy level to a higher energy level. Each absorption corresponds exactly in energy to a very specific excitation depending on the wavelength of the light. If we understand these excitations, then we can begin to figure out the structural features of that molecule.

The electromagnetic spectrum
The electromagnetic spectrum

Here we have the electromagnetic spectrum. You can see the different types of radiation, from radio waves on the left at low energy all the way through to gamma rays on the right at very high energy. The radiation we are interested in here, is ultraviolet and visible, infrared and, finally, radio waves for NMR.

So how do we use electromagnetic waves to help us analyze material? Great question! Let's move on and see how data is acquired using a spectrometer.

The Spectrometer

We have just learned that spectroscopy involves the absorption of specific energies of electromagnetic radiation. All types of absorption spectrometers share common features. The radiation source depends on the type of radiation being used, and the sample holder will vary depending on the sample being analyzed. Using a series of lenses and photocells, the spectrometer compares the electromagnetic radiation shining on the sample to the radiation that just passes through. This provides us with the output. The output can either be a number or a spectrum.

Let's have a quick look at a typical infrared spectrum. All absorption spectra provide two kinds of information that help us identify the chemical structure of a mystery material. The absorption wavelength, or frequency, gives the energy associated with a particular excitation. This can be related to a specific part of a molecule. For example, this helps us identify functional groups. And the absorption intensity provides information on both the ease of transition and the concentration of the absorbing part of the molecule. Hopefully, you can see that this information is starting to give us a fingerprint of the molecules contained in a given material. Using the information from different techniques can help us solve the mystery.

What Can Spectroscopy Tell You?

The development of spectroscopic methods of analysis has provided scientists with a tool kit to help identify complex unknown molecules.

And one of the most important advantages of spectroscopy is that it is non-destructive. This means that, if you are working on a crime scene, you are not destroying the evidence in the process of analyzing it! This is a huge plus. A court judge would be not happy if you told her you had destroyed something vitally important. For this reason, spectroscopic techniques are used before more damaging types of analysis.

Okay, so let's briefly look at the sort of information the different techniques can help us with. Ultraviolet-visible spectroscopy is hugely versatile. UV-visible radiation transfers the right amount of energy to cause electron transitions from the ground state to the excited state. It is particularly useful in the analysis of colored materials, such us dyes and paints. In forensic analysis, it is often used to identify pigments in textiles.

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