“The only source of knowledge is experience”
— Albert Einstein
Flame atomic absorption spectroscopy is a well established and precise method for elemental analysis giving concentration results in mg/L (ppm) levels. However, better sensitivity is achievable using electro- thermal atomisation with a graphite furnace.
Limitations of flame AAS
- Burner – nebuliser is a rather inefficient sampling device. Majority of the sample gets drained and the small fraction reaching the flame has a short residence in the light path
- High sample consumption of the order of 3-5ml/min
- Matrix interferences limit applications particularly in analysis of biological and geological samples
- Analysis limited to ppm concentration ranges
Benefits of graphite furnace analysis
- Entire sample is atomised and the atoms are retained in the atomisation graphite tube for extended user controlled time periods
- Microlitre quantities of sample are sufficient and the quantity can be increased to 50 – 100 μl to enhance sensitivity
- Temperature programming steps help remove the solvent and major matrix interferences
- Detection limits typically 100 – 1000 times better than flame techniques are achievable thereby giving routinely analysis in μg/l(ppb levels)
Graphite furnace components
Graphite tube – serves as a sample cell as well as a heating element
Electrical contact cylinders – provide electrical connection to the tube.Current flow provides heating of tube and sample
Water cooling housing – serves to cool the assembly
Inert gas – protects heated tube from atmospheric oxidation. External gas stream surrounds the outside of the tube and internal gas flow purges the tube. Flow is reduced or completely stopped during atomisation to increase sample residence time and improve signal output.
Quartz windows – at each end of the tube help to seal the tube and allow light to pass through
Power supply programmer – controls current supplied to tube as covered by user program
Transverse heating provides uniform heating of graphite tube across its length. In end to end heating there can be temperature gradient along the tube length. L’vov platforms delay the vaporization and atomisation of the sample until furnace atmosphere has reached equilibrium conditions.
Stabilised temperature platform furnace (STPF) was pioneered in 1970’s by Perkin Elmer. It is a combination of graphite tube quality, design and operational parameters to improve atomisation and detection. Tube lifetime improvement is provided by using high-quality graphite for the tubes., Platforms maximise power heating to virtually eliminate interferences and internal gas stop increases sensitivity.
Limitations of graphite furnace analysis
- Longer analysis time in comparison to flame analysis
- Lesser number of elements analysed by furnace technique – around 40 as compared to about 70 in flame technique
- Higher cost of graphite furnace assembly but it is also available as a switching option with flame operation in most commercial instruments
- Higher and more complex background levels require expensive background correction options
The next module will introduce you to dispersion and resolution of light with the help of a monochromator.