MATRIX VAPORS AND PHYSICAL INTERFERENCE EFFECTS IN GRAPHITE-FURNACE ATOMIC-ABSORPTION SPECTROMETRY .2. SIDE-HEATED TUBES

A model for vapour losses in a transverse heated graphite atomizer (THGA) was developed, taking into account diffusion and convection. The latter includes an upward directed flow at the tube ends and an axial flow from the tube outside towards the injection port. The validity of the model was proved by experimental results, generated by using (i) various temperature and gas-flow conditions during atomization and (ii) standard and modified, end-capped, THGA-tubes. By adding end-caps with an aperture of 3 mm diameter to the THGA-tubes, diffusional losses through the tube ends were limited. As a result we eliminated interference effects due to analyte trapping on the surface of matrix particles, formed in cooler zones at the ends of the tube. For standard THGA-tubes, using recommended conditions, this interference could decrease the analyte signal by 10-20% in the presence of mug masses of matrices like Ag, Cu, Pd etc. In addition, the sensitivity was increased 1.7 times by using end-capped tubes.