Tomorrow I’ll be doing a small presentation about this analytical method in chemistry class. We’ll be doing presentations in small groups, so it’s not really a big thing. But – never, ever underestimate things! Therefore I’ll be presenting it to you first! Explaining things to someone else helps you learn it yourself, that’s why discussing the subject with your classmates is a great thing to do before exams etc. ~Here we go!
When a the light-beam goes through the prism it splits in different wavelengths (see below) which is shown in different colours, in reality too.
There are many analytical methods, both because there are many things to analyse and because there sometimes are different ways to get the same (or almost same) results. Before we go on, we’ll ourselves analyse something – the name of this method. The middle part, “spectro-” comes from the word “spectra”, which gives us the hint that this is about light. The first part, “emission” is what you could call the opposite of “absorbtion”, instead of something getting soaked up we have something leaving from somewhere. The third part, “-metry” indicates that we are measuring something. So what result did we get, analysing this?
“Measurement of emitted light-thingy”
Seems easy, but it is a bit more complicated than that. Examplia Gratia; we cannot call ourselves chemists (or physicists for that matter) when calling something “light-thingy”. I’ll answer some questions…
Q: What is light?
A: Light is electromagnetic radiation – energy being transported. It can be described as a particulate flow or wave phenomenon. Either way, emitted light flows from the source, in the “shape” of so-called photons. Photons are tiny tiny particles which are generated e.g. when the electronic structure of atoms or molecules is changed.
There are also different kinds of light. The type of light depends on which wavelength it has. There are light which we can not see with just our eyes, light with very low wavelength, so-called ultra-violet light, light with very high wavelength, so-called infra-red light and the so-called black light. Between ultra-violet and infra-red is the light which we can see. This light has different colours, depending on what wavelength it has.
Q: How do we measure light?
A: Light can measured it by wavelength. The wavelength is the distance between two peaks (crests) or two lows (throughs), its unit is either meters with a prefix (very small distances) or “Angstrom” which was introduced by the swedish physicist Anders Jonas Ångström. Intensity, (which shouldn’t be called brightness) is simply the height from the “zero-point” of the wave, the so-called equilibrium, to the highest point of the wave. Also called, Amplitude – if you’ve studied sine waves. It can also be measured in hertz, which tells the frequency – the number of cycles per second of a periodic phenomenon. The shorter wavelength, the higher frequency, vice versa.
We can either measure the wavelength of the emitted light or the intensity of it. But what’s really studied when using this method is atoms and molecules – not just “simple” light. Which of the two things measured depends on if we want to know quality or quantity.
- Quality is what something is made out of.
- Quantity is how much there is of something.
Q: Why are we interested in the wavelength?
A: Well, by knowing the wavelength we know what kind of light it is. But there’s something more to this, energy levels.
When I say “energy levels”, I of course have to explain further (if you’re not a genius :)). To make it easier – we’ll use an example with a simple atom. As I suppose, when you’ve read this much – you know what an atom is built of, (nucleus containing positive protons and neutral neutrons this surrounded by a ‘cloud’ made of negative electrons). Though, there is an other way of picturing the atom – the Bohr model. Niels Bohr was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics for which he received the Nobel prize in physics in 1922. He improved earlier models of the atom, saying that every atom has a certain number of electron orbitals and that each electron orbital has a particular energy level.
Q: Why are we talking about energy levels?
A: When energy, (either thermal, resulting from collision, or radiational, resulting from absorption of electromagnetic radiation), is applied to an atom, it can (if a sufficient amount) “lift” an electron from its ground state to a higher state. Which means that it’ll move to a shell more far from the nucleus. This is called excitation – the electron gets excited and is in an excited state! Though, the new structure is very unstable and decays rapidly, the electron quickly jumps down to its ground state again. When it does, the energy that took it to the outer shell is emitted again, as photons – light. Depending on what element the atom is – we’ll get different kinds of light. Different meaning difference in wavelength, colour!
This is why we are interested in measuring the wavelength – by doing that, we can get to know what stuff is made of.
Q: What about intensity then?
A: In simple terms you could call ‘intensity’ brightness, but this is not completely correct. See, intensity – measured in luminance depends both on the emitted light and the area absorbing it. What we can say is, that by measuring the intensity, we can determine the concentration of an element. Higher concentration of a specific element in say, a star – emits light with a higher intensity of the light with the specific wavelength for that element.
I won’t explain the procedure in detail since it is hard to find information about it and much of the sources are quite confusing. This because spectroscopy which is the basis for emission spectrometry is a common search-result. There are some differences.
Q: What is it used for then?
A: As you’re told, we can determine what things are made out of with this method. Mankind has had (and still has) good usage of this method. It is the most commonly used procedure for the measurement of trace elements in rocks, water, soil, manufactured goods and biological specimens. Daily usage, all over the world. What has brought us even more is perhaps the use of it regarding astronomy. It is one thing to understand the planet we’re living on, touching, but how about really knowing what the little sparkling points in the night sky are made of? We can, with emission spectrometry (and a bigger brain than I have perhaps), from the light the stars emit towards us, even though they are light-years away – determine what they are made of!
Isn’t it amazing?
Yours, a tired SockFusion, head full of Physics & Chemistry