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Study Notes: Flame Atomiser

The efficiency and reproducibility of the atomisation step in large measure determines the method’s sensitivity, precision and accuracy and is the most critical step in AAS. The aim is to generate a supply of uncombined analyte atoms in the ground state and to expose this atom population to light at a characteristic absorption wavelength. Absorbance depends on the number of unexcited atoms in the flame.

The atomiser is effectively an analogue of the sample holder (cuvette) in UV-Vis spectroscopy and the atomiser provides a vehicle for atomisation and the container for the atomic vapour (atom reservoir).

Only liquid samples can be used as the sample must be non-particulate and in a form able to be introduced in to the flame of the atomiser. In graphite furnace the sample must be able to be introduced in to the graphite furnace assembly.

Prior to introduction into the flame the liquid sample is introduced into a nebuliser where it is broken into small droplets. A sub-population (10%) of these droplets is then introduced into the flame. The sample is usually carried in the oxidant and fuel for the flame. The nebuliser and burner are usually part of a burner-nebuliser system.Skip flash movie

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Pneumatic nebuliser

The rate at which the solution is aspirated into the flame is usually less than 6 mL/min. At higher rates the stream tends to saturate, is not effectively atomised by the flame, and the flame is cooled by the excess solution. This process increases chemical interferences.

Uptake rate for standards/sample into the nebuliser must be constant and must have the same physical characteristics. For instance, the presence of ethyl alcohol or high salt concentrations will modify the surface tension for samples and therefore standards needs to be matched to the sample. The lower the surface tension the greater the uptake.

The flame position is adjusted for each element to yield a maximum absorbance and is part of optimization procedure for AAS.

Flame-absorbance profile for three elements

The most common burner type is the laminar flow or pre-mix burner that has a long path length. As the beam of light is passed through the broad flame a long optical path results which provides added stability for readings.

Cut away showing the workings of the burner

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The atomic vapour cell (flame & GF) are the most likely sources of trouble in an analysis and require the most maintenance. Good AAS results depend on both sample preparation and instrument operation.

Common problems include poorly adjusted nebuliser and impact bead, misaligned burner slot, clogged nebuliser and improperly filled drain.

Sample preparation problems include inappropriate solvent, poor dissolution, particulate matter, high salt, surface tension problems and the elements present in the dissolution buffer that may cause interferences.

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