Atomic spectroscopy (idea)

Atomic Spectroscopy is the technique of observing the amount of light absorbed or emitted by a specific substance in order to determine the concentration of a species in the substance.

The Theory

Electrons can exist in one of two states:

• Ground State. In this state, the electron contains the least energy possible, orbiting as close as it can to the nucleus.
• Excited State. In this state, the electron contains more energy than in its ground state, orbiting further from the nucleus of the atom.

An electron in its excited state can "decay" to its ground state, giving off the excess energy in the form of a photon of light, the wavelength of which is determined by the equation:

λ = h / E

Similarly, a photon of light with this wavelength may be absorbed by the electron, exciting it. Due to quantum mechanics, only certain wavelengths of light may be absorbed or emitted. These wavelengths are (generally) unique to each element, and therefore we can use these wavelengths to identify whether or not an element exists in a substance and, if so, its concentration. Analysis by shining a light at the substance and observing the wavelength(s) absorbed is known as absorption spectroscopy, while energising the substance and observing the wavelength(s) emitted is known as emission spectroscopy.

From our measured values we can obtain the absorbance of a substance (for absorption spectroscopy at least), defined as:

A = log₁₀( P₀/ P_t)

where P₀ is the power of the light beam from the cathode lamp, and P_t is the power of the light beam once it has passed through the substance. We can also show this as:

A=ε c l

where ε is a constant, c is the concentration of the element in the substance and l is the length through the substance that the ray of light passes. Rather than manually measure the length (which will generally stay constant throughout the experiment), we usually write:

A = ε' c (where ε' = ε l)

Thus we can find out the concentration by the equation:

c = (log₁₀( P₀/ P_t))/ε'

Thankfully, if you need to do Atomic Spectroscopy, you probably have access to a machine that does all of this for you.

The Equipment

You will need the following:

• An atomiser.
• A monochromator.
• A photomultiplier.
• Some sort of electronic recorder.
• For absorption spectroscopy, a light source.

A commercial spectrograph will have all of these built in, making your job a lot easier.

The equipment is set up something like this:

 _______                     Atomiser          _______________  Monochromator
|_______| - - - - - - - - ____________ - - - - +              |
Lamp                      \          /         |              |    Photomultiplier
                           |________|          |______________+ - )---|
                                                                      |    ___
                                                                      |---|___| Recorder

The Method

NB: Depending on your equipment, some of the following steps will be automated. If you're lucky, it'll all be automated and you can read the concentration straight from the machine.

Set the monochromator to a wavelength that is absorbed or emitted by the element you are observing. If you are using absorption spectroscopy, start the lamp and make sure it is shining through the atomiser.

In order to calibrate the machine, get several samples of differing (but known) concentrations of the element you will be observing, and place them in the atomiser. Measure the intensity of light for each sample and plot c versus A in order to get a nice straight line with gradient equal to ε'.

Take your sample of unknown composition and place it in the atomiser. Take your measurement, and from that and the graph you will be able to find the concentration of your element.

Complications

Because of the narrowness of the bands of light absorbed or emitted and the size of the light spectrum, it is very rare for spectral interference (that is, when two elements absorb or emit light of the same frequency) to occur. A lot more common is chemical interference, in which another element interferes chemically. An example of this is the phosphate ion, PO₄^3-, which bonds with calcium to form calcium phosphate, Ca₃(PO₄)₂. If we were to test a solution of Ca₃(PO₄ )₂ for calcium, we would get a very low value for c despite the presence of calcium, as the bonded PO₄ interferes chemically. There are two ways to stop this happening:

1. Add a releasing agent which forms a more stable compound with the interfering species, thus freeing the element being analysed.
2. If we are analysing a metal, add a complexing agent which forms a compound with the metal, but which readily decomposes into its component atoms in the flame.

Another way to solve this problem is to increase the temperature of the flame until it is hot enough to decompose the compound. However if the flame is too hot it will ionise the element being analysed, changing its spectral "signature". A way of solving this problem is to add an ionisation suppressant, that is, an element that ionises more rapidly and thus suppresses the ionisation of the element.