refractory oxides and oxidation of the graphite. The axis of the furnace is aligned along the optical path
of the radiation from the lamp and as the vaporized sample is contained within this region, maximum
sensitivity can be attained. The sample (1–50 mm^3 ) is deposited on the bottom inner surface of the tube
near the centre and the temperature raised to about 2500 K from cold within 1 or 2 minutes. The heating
cycle can be controlled so as to allow solvents to evaporate or organic residues to be ashed before
raising the temperature rapidly to that required to produce an atomic vapour. Thus the absorbance signal
from a flameless source will constitute a peak whose height or area is related to the analyte
concentration (Figure 8.29). Refinements to the simple tube design have been dictated by precision
losses deriving from uneven heating across the tube. Whilst the graphite is heated rapidly by the flow of
the electric current, the centre of the tube is heated by radiant energy only, and its temperature may lag
considerably behind that of the graphite. Hence elements volatilized from the internal surface of the
tube may condense in the cooler centre. To overcome this problem the use of a L'Vov shelf or platform
has been introduced. By depositing the sample on this, rather than the tube walls, it is heated essentially
by radiant energy alone and the condensation problem is removed. Figure 8.30 shows the arrangement
of a L'Vov shelf. Although Figure 8.29 shows the tube being heated by a longitudinal passage of
electric current, transverse heating is also used. A more uniform temperature profile is produced by the
latter, with the consequent improvement in precision. The carbon rod is a simpler device having a small
shallow recess for the sample machined in the top surface. It is positioned with the sample recess just
below the optical axis, otherwise its operation is
Figure 8.29
Graphite furnace for atomic absorption analysis and typical output signal.