L8 - Atomic Absorption Spectroscopy

Objectives

Atomic Theory - A Background and Links to Module 1

• Atoms have valence electrons, which are the outermost electrons of the atom.
• Atoms can be excited when irradiated, which creates an absorption spectrum. Remember the Emission Spectra lessons in Module 1 and depth study.
• When an atom is excited, the valence electron moves up an energy level.
• The energies of the various stationary states can then be determined by these emission lines.
• The resonance line is then defined as the specific radiation absorbed to reach the excited state.
• The Maxwell-Boltzmann equation gives the number of electrons in any given orbital.
• It relates the distribution to the thermal temperature of the system
• Planck proposed radiation emitted energy in discrete packets (quanta)

An Introduction to AAS

• AAS is a highly sensitive spectroscopic technique that can detect Parts per Million (ppm) or Parts per Billion (ppb) of metal ions in solution.
• It can only be used to detect the concentrations of metal ions.
• It can not detect anything else - even if they have colour.

How AAS works

• Firstly you need to know which metal you are looking for. AAS cannot be used to indiscriminately identify unknown metals.
• As an example you might suspect a soil sample has mercury in it.
• When this is determined then you can calibrate the AAS to test for Hg only by selecting a Hg Cathode Lamp (Cathode lamps emit spectra of light that are ONLY created by that element)
• When an electric current is passed through the Hg Cathode Lamp the lamp emits it's highly specific spectrum of light and directs it at the vaporised sample in the flame.
• When the sample in the soil is burned any Hg atoms will be Atomised and their electrons will be excited to vacant energy levels.
• If the sample contains Hg then these atoms will absorb the light from the cathode lamp. Only these Hg atoms can absorb those particular wavelengths of light.
• The light is then passed through the monochromator (a prism) and then a slit. This slit can select the specific wavelengths of light required for Hg analysis. This is then detected and displayed as a number.
• This number is called absorbance and is proportional to concentration as a derivation from the Beer-Lambert law.
• If the concentration of Hg is high then a lot of light will be absorbed and very little will be transmitted and detected.

Calibration Curves

• Much like colorimetry AAS can only be used if you use a calibration curve.
• A calibration curve is when we use different known concentrations of Hg and place each concentration into the flame and measure their absorbances.
• As concentration is proportional to absorbance we will obtain values that are relative to the samples and should then be able to plot a graph.
• This should show a linear relationship between concentration and absorbance.
• When we test our sample we are able to determine the concentration of Hg by interpolating the results.

AAS Past Paper Questions