Page 57 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: FYK/LSX Revised Pages P2: FWQ/FPW QC: FYD
Encyclopedia of Physical Science and Technology en001d42 April 28, 2001 15:9

Atomic Spectrometry 781

FIGURE 13 Graphite furnace electrothermal atomizers and their corresponding temperature profiles and analytical
signals. (A) and (C) correspond to furnace geometry and the tube temperature profile for transversely heated atomiz-
ers. (B) and (D) correspond to furnace geometry and the tube temperature profile for longitudinally heated atomizers.
The appearance time of the analytical signal for sample atomization from the furnace wall or the platform is highlighted
in (E).

sample is pulse vaporized and the atomic vapor diffuses center of the tube, it may condense at the cooler ends of
rapidly out of the tube. the graphite furnace.
Regardless of the furnace design, the sample is usually
vaporized from a small graphite platform that is placed
2. Tube Design and Heating Characteristics
inside the graphite tube (Fig. 13A and B). The sample is
Figure 13 illustrates two popular tube designs for elec- placed on the platform through a small hole in the top
trothermal vaporization. The transversely heated graphite center of the furnace tube. Figure 13E illustrates the dif-
tube (Fig. 13A), also known as an integrated contact ferent atomization profiles expected when the sample is
cuvette, is heated through the electrical contacts at the vaporized either from the furnace wall or the platform.
side of the atomizer tube. This allows for a more uniform This figure helps to illustrate the function of the platform
heating profile along the length of the graphite tube, as and explain why it is a useful tool. The graphite tube takes
shown in Fig. 13C. In longitudinally heated furnaces a finite time to heat up, as indicated by the tube-wall tem-
(Fig. 13B), the atomizer is heated through the electrical perature versus time plot. The vapor temperature in the
contacts at the ends of the graphite tube. Although simpler furnace follows the tube-wall temperature, and if the sam-
in design, these furnaces tend to have a significant thermal ple is placed on the bottom wall of the tube, it heats up at
gradient along the length of the graphite tube (Fig. 13D). the same rate as the tube. In most cases, the sample is va-
Consequently, because the analyte is vaporized at the porized while the tube is still heating up and results in the