Corrosion of transversely heated graphite tubes by mineral acids

Corrosive changes of transversely heated graphite atomizer (THGA)-tube and platform surfaces were studied by scanning electron microscopy in combination with tube lifetime measurements under recommended conditions for vanadium determination. This was done for the four mineral acid matrices HNO3, HF, HCl and HClO4. Rising corrosion and reduced tube lifetime are observed for these matrices in the sequence HNO3 < HF much less than HCI < HClO4. Morphological changes related to the corrosive attack are different for each acid matrix and so are the effects on the analytical behaviour of the tubes. The results are compared to relevant data for vanadium and chromium measurements, which are applied for routine quality control of THGA- and LHGA-tubes by Perkin Elmer. The average mass loss of the investigated tubes per analysis cycle is also determined and is a further essential parameter of tube corrosion. Mass loss is mainly caused by carbon evaporation and particle emission during atomization and tube scavenging steps. Changes in electrical resistivity of the investigated tubes before and after the lifetime experiments were found, however, they were within the specified range for the quality control of THGA-tubes. Hence, they do not affect the temperature setting by voltage control in the relevant spectrometer systems. (C) 1999 Elsevier Science B.V. All rights reserved.