2.3 Specialized Sample Introduction Techniques and Analysis
2.3.4 Electrothermal Vaporization (Graphite Furnace Atomic Absorption, GFAA)
The graphite furnace, formally known as an electrothermal vaporization unit, uses a typical FAAS unit but replaces the burner head with a furnace system. No flame is used in the operation of this system; instead the metal in the sample is atomized by heating the cell with electrical resistance to temperatures not obtainable in flame systems. The heart of the system is illustrated in Animation 2.6. All systems use an automatic sampler to ensure reproducible results between replicate analyses. In GFAA, samples are usually digested (in acid) to assure homogeneity of the injected solution. A sample is placed into the graphite furnace cell through a small hole in the side. Cells range in size depending on the brand of the instrument but are usually about the diameter of a standard writing pencil (~ 0.5 cm) and 2 to 3 cm in length. Argon gas is passed through the cell to pass vapor and analytes into the radiant beam and the furnace and sample are then cycled through a three-step heating process. First, the water is driven off by resistance heating at 107 °C (which partially blocks the beam of the HLC source but this is not recorded). Next, the sample is “ashed” at 480 °C to degrade any organic material in the sample (again this absorbance signal reduction is not recorded). Finally, the cell is rapidly heated to 2000 °C where the analytes are placed in their volatile elemental state where they absorb radiant light from the Hollow Cathode Lamp; this signal is recorded. The system is then prepared for another run by heating the cell to 2500 °C to remove excess material before cooling the chamber with tap water back to room temperature. Then another standard or sample can be added and the process is repeated.
One obvious advantage of this system over the cold vapor and hydride technique is automation that reduces the cost of analysis. Another advantage of the GFAA technique over FAAS or FAES is the improvement in detection limits, typically in the low parts per billion or high parts per trillion ranges. Most of the elements that are analyzed by FAAS can be analyzed by GFAA but high background concentrations of a few rare earth and alkaline earth elements in the graphite tubes limit their detection limits.
An illustration of sample introduction and the heating steps is given in Animation 2.6.
Animation 2.6 Illustration of a Graphite Furnace Sample Introduction System.
The Zeeman Effect (Correction): In the graphite furnace samples containing high amounts of solids and organic matter can be introduced which lead to high background levels and spectral interferences when these compounds are heated and degraded during the atomization step. These problems can be overcome by using a Zeeman correction calculation. The Zeeman Effect capitalizes on the observation that the absorption profile of an element is split into several polarized components in the presence of a strong magnetic field. When absorption measurements using radiation from a deuterium or mercury vapor lamp are taken with and without a magnetic field present in the reaction cell, the presence of spectral interferences can be corrected for (subtracted out) and this difference in signals is referred to as the “Zeeman background corrected” atomic absorption signal. The physics behind this process is beyond the scope of this text and it is only important to note that the Zeeman correction is available and used in most graphite furnace systems.
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