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Abstract:
The temperature of the nitrogen gas phase in a graphite tube furnace for atomic absorption spectrometry has been determined using coherent anti-Stokes Raman scattering (CARS). Subtle details of the temperature evolution at various locations in the tube have been identified. Under steady-state conditions, the temperatures of the tube wall and of the gas phase near the tube centre are essentially identical. The longitudinal gradient of the gas phase temperature between the tube centre (heated to 2700 K)andthetube ends is around 1200 K. This is less than that predicted by model calculations. During rapid heating, typically used for atomization of the analyte, the gas follows the wall temperature very closely and with essentially the same heating rate. Irregularities in this heating pattern, such as an intermediate slowing of the heating rate 0.3 s after start of heating, are most probably caused by gas expansion during the period of rapid tube heating. A pronounced radial temperature gradient was observed in the gas phase of tubes with inserted platform during the rapid heating phase, but not in tubes without a platform. The gradient in the gas phase disappears within about 0.5 s after the tube wall has reached the preset temperature. When the platform technique is used and the temperature program selected with care, volatilization of the analyte can be delayed until the tube wall and the gas phase have almost reached their final temperatures and are close to thermal equilibrium.
