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Study Notes: Graphite Furnace Atomiser

Flame atomisation is not very efficient because:

  1. A large proportion of the sample ends in the drain or is not completely atomised, and
  2. The residence time of individual atoms in the optical path of the flame is brief, ~10-4 seconds.

The graphite furnace atomiser which is also called an electrothermal atomiser utilises an electrically heated cup or tube made of graphite, although tubes are usually used in modern equipment. The heated graphite furnace provides the thermal energy to break chemical bonds within the sample and produce free ground state atoms of the analyte.

Aqueous sample are acidified, usually with nitric acid, to a pH of 2.0 or less. The sample (usually ~20 µL but less than 100 µL) is added to the graphite furnace either manually or automatically and evaporated at a low temperature, then ashed at a higher temperature. After ashing the current is increased causing the temperature to rise to 2000 – 3000°C and the sample atomises in a few milliseconds.

The following diagram outlined the essential features of Graphite Furnace Atomic Absorption (GFAA). The sample is placed on the platform in the graphite furnace before temperature cycling through the following steps occurs:

1. Drying Step: 80 – 200° C Removes solvent from sample
2. Ashing Step: 350 – 1600° C Removes organic and inorganic material
3. Atomisation Step: 1800 – 3000° C Generation of free analyte atoms in light path

Cross-sectional view of a graphite furnace

Graphite furnace AAS has the following advantages and disadvantages compared to flame AAS.

Graphite Furnace Advantages
Flame Advantages
Superior sensitivity - detect analytes at concentrations 10 - 100 times lower than flame
Simple
High conversion efficiency of sample into free atoms Convenient
Low sample volumes ~20 µL Excellent Results
Absorption signal is a well defined peak Relatively short measurement time ~14 seconds
Peak heights and areas can be used for quantitative measurements  
Temperature is only factor involved in production of free atoms compared with flame which also must factor in the composition of the flame, gas flow and oxidant-fuel ratio  
Directs analysis of some types of liquid sample that would be inappropriate for flame AAS  
Low spectral interference due to generally higher temperatures  

 

Graphite Furnace Disadvantages

 

Flame Disadvantages

Longer measurement time than flame up to ~330 seconds depending on the temperature program used Sensitivity Limitations
  Restricted Analytical Scope
Limited dynamic range Loss of sample during process hence higher initial sample volumes required
High matrix interference Short residence time of analyte in optical path
  Flame composition and temperature have a bearing on the results obtained

Flame atomisers are not efficient whereas the graphite furnace atomiser is efficient due to the higher conversion efficiency of sample into free atoms. Higher efficiency means less sample required and superior sensitivity. Solid and liquid samples may be measured using a graphite furnace atomiser.

Apart from the atomisers themselves, flame and graphite furnace AAS use the same componetry. Instruments are commonly able to switch from one atomiser to the other quite readily via use of interchangeable atomisers.

It is a given that the atomic vapour cell (both flame and graphite furnace) are the most likely sources of trouble in an analysis and require the most maintenance. Graphite tubes need to be replaced every 200 to 800 burns due to pitting of the inside surfaces. This leads to production of data that is poorly reproducible and a loss in sensitivity.

Modern AAS instruments are computer interfaced and print off results via chart recorders or digital displays including peak height/peak area calculations

A simplified view of the GFAA set up is given below.

Diagram of the basic components of a Graphic Furnace Absorption Spectrophotometer

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