Sampling Techniques for Atomic Absorption Spectrometry

Sampling Techniques for Atomic Absorption Spectrometry

Sampling Techniques for Atomic Absorption Spectrometry

 

Fig. I.1: A burner system used in atomic absorption spectrometers

Two systems are commonly used to produce atoms from the sample in atomic absorption spectrometry:

  • Aspiration

    • &

  • electrothermal atomization (graphite furnace).

 Aspiration involves sucking a solution of the sample into a flame. Figure I.1 shows a typical burner and spray chamber. Ethylene/air or ethylene /dinitrogen oxide are used. A flexible capillary tube connects the solution to the nebulizer. At the tip of the capillary, the solution is “nebulized” that means is broken into small drops. The larger drops fall out and drain off while smaller ones vaporize in the flame. Only 1% of the sample is nebulized.

 

Electrothermal atomization (graphite furnace, GFAAS) is where a drop of sample is placed into a graphite tube that is then heated electrically. This is by far the most advanced and widely used high sensitivity sampling technique for atomic absorption known also as graphite furnace. In this technique, a tube of graphite is located in the sample compartment of the atomic absorption spectrometer, with the light path passing through it. A small volume of the sample solution quantitatively placed into the tube through a sample injection hole located in the center of the tube wall. The tube is heated through a programmed temperature sequence until finally the analyte present in the sample is dissociated into atoms and atomic absorption occurs.

There are several advantages of the graphite furnace technique comparing to aspiration:

Fig. I.2: Graphite tube. A drop of sample is placed into the graphite tube and then is heated electrically. Since almost all the sample is dissociated into atoms a high concentration of atoms is obtained. This is by far the most advanced and widely used high sensitivity sampling technique for atomic absorption known also as graphite furnace.

  • The detection limits for the graphite furnace  fall in the ppb range (ng/l) for most elements. The sample is atomized in a very short period of time, concentrating the available atoms in the heated cell and resulting in the observed increased sensitivity.
  • The graphite furnace is much more automated than the other techniques. Even though heating programs can be very sophisticated, the entire process is automated once the sample has been introduced and the furnace program initiated. Automatic samplers are used to facilitate the process that can be completed unattended.
  • Interference problems have been minimized with the development of improved instrumentation
  • The graphite furnace can determine most elements measurable by AA in a wide variety of matrices.

 

Relevant Posts

Atomic Absorption Spectrometry (AAS)

Light sources for atomic absorption

 

References

  1. (a) A. Walsh, Anal. Chem., 63, 933A–941A, 1991 (b) S.R. Koirtyohann, Anal. Chem., 63, 1024A–1031A, 1991 (c) W. Slavin, Anal. Chem., 63, 1033A–1038A, 1991.
  2. D. Harvey,  “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
  3. D.A. Skoog, F.J.  Holler, T.A.  Nieman,  “Principles of Instrumental Analysis”. Saunders College Publishing: Philadelphia, 1998.

 

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