Atomic Absorption relies on the principle that each atom absorbs light of a particular wavelength, and hence at that wavelength the quantity of that absorption is proportional to the elements concentration. The first technique used in this analysis was Electrothermal Atomisation, known as Graphite Furnace-AAS. The components of a Graphite Furnace AAS are similar to that of Flame AAS, they more or less use the same components, with the exception of the component used to heat the sample.
For a Graphite Furnace Atomic Absorption Spectrometer the schematic is: Hollow Cathode Lamp â†’ Graphite Tube â†’ Monochromator â†’ Detector
The Hollow Cathode Lamp (HCL) is a selective/specific light source made from the element of interest. The HCL contains an inert gas which is usually Argon or Neon. Ionization of the inert gas takes place, which causes acceleration of the gas into the Cathode. This causes the metal atoms (Al) of the cathode to sputter into the gas phase. The collision of sputtered atoms with the Argon atoms or electrons, excite the metal to higher energy levels. It is the process of decay that occurs which causes excited electrons to emit light in their return down to lower energy levels which is shone through the sample in the Graphite tube.
Ar + e- â†’ Ar+ = 2e-
M(s) + Ar+ â†’ M(g) + Ar
M(g) + Ar+ â†’ M*(g) + Ar
M*(g) â†’ M(g) + hv (light)
The monochromator isolates the absorption line obtained from the sample, giving an accurate depiction to the detector. While the detector measures the intensity of the light and produces an electrical signal equivalent to the intensity.
The sample is injected directly into the graphite tube, which is then heated. The tube provides thermal energy enough to break bonds within the sample and produce free atoms of the analyte. The sample undergoes three stages of heating, which are all imperative to the analysis of the sample:
Step 1: drying of sample (which is the removal of water and the solvent) happens at approximately 125 degrees celsius. Step 2: ashing of organic matter (removal of organic and inorganic material, essentially any matrix) happens at approximately1300 degrees celsius. Step 3: Vaporization of analyte atoms (free analyte atoms in light path) happens at approximately 2400 degrees celsius
Step 2 is important for preventing Spectral Interference in the analysis.
Due to the fact, there is no flame used in this particular technique there is no combustion products, hence there improved sensitivity of between 10 and 10^3.
1. From stock standard of Aluminium provided, prepare concentrations of 10ppb, 20ppb and 50ppb. 2. Add 2ml of 10% HNO3 to each standard and make up to volume in polycarbonate flasks using Ultra High Purity water (UHP water). 3. Prepare a blank: 2ml of 10% HNO3 diluted with UHP water.
4. Take 50ml samples of tap water from: a laboratory sink, a tap in C block, a sink from alternative lab. 5. Add 1ml of 10% HNO3 to each sample as a preservative. 6. Ensure all solutions are mixed well before loading into carousel. 7. Load carousel in order of: blank, then set of standards filling all inner positions. 8. Load samples into outer carousel using small cups.
9. Run analysis.
Optimising the GF-AAS is important to ensure maximum efficiency of detection. This is done via running the standards over a range of pre-treatment and atomisation temperatures to determine the conditions for maximum response. Running the blank is important in establishing the base-line reading this acts as a correction factor, as you can determine how much absorption you find in your samples as a result of the solvent.