TEMPORAL AND SPATIAL TEMPERATURE DISTRIBUTIONS IN TRANSVERSELY HEATEDGRAPHITE TUBE ATOMIZERS AND THEIR ANALYTICAL CHARACTERISTICS FOR ATOMIC-ABSORPTION SPECTROMETRY

The important role which temperature plays in atomic absorption spectr ometry (AAS) for the formation and detection of atoms in the absorptio n volume is discussed and the literature is reviewed. Non-homogeneous temperature distribution in the absorption volume is in contradiction to one of the prerequisites for the application of Beer's law used in AAS to convert absorbance into analyte concentration or mass, and is p articularly troublesome for an ''absolute analysis'' envisaged for ele ctrothermal atomic absorption spectrometry (ETAAS). Coherent anti-Stok es Raman scattering (CARS) is used to study the gas-phase temperature distribution in a state-of-the-art transversely heated graphite tube a tomizer (THGA). The effect of the internal gas flow on the size of the heated atmosphere is studied by steady-state temperature measurements . Temporally and spatially resolved measurements make it possible to s tudy the temperature field within the atomizer volume in all three dim ensions during the rapid heating of the furnace to final temperatures in the range 2173-2673 K. The role of the integrated platform of the T HGA on the temperature field is investigated by temperature measuremen ts of the gas phase in the presence and absence of the platform. The p latform is identified as the major source of temperature gradients ins ide the tube volume, which may be as high as 1000 K in the radial dire ction during rapid heating. These gradients are most pronounced for he ating cycles starting at room temperature and gradually decrease with increasing starting temperature. Shortly after the tube wall reaches i ts final temperature, the gas-phase temperature equilibrates and appro aches the wall temperature. Because of the unavoidable contact with th e cold environment at the open ends of the tube, minor temperature gra dients are observed in the gas phase also in longitudinal direction, w hich can be further reduced by restricting the openings with end caps. The results obtained for the THGA are compared with those obtained ea rlier for a longitudinally heated graphite tube atomizer (HGA), includ ing some analytical applications of these two furnace types. Both the temperature characteristics and the resulting analytical characteristi cs substantiate the superiority of the THGA in comparison to the HGA a s an atomizer for ETAAS.