Surface Corrosion Studies on High-Purity Quartz Vessels for Digestive Sample Preparation

 High purity quartz is the most common material for pressure vessels of high pressure digestion systems. Such systems are in worldwide use especially for matrix decomposition and mineralization procedures with either conventional heating or heating in a microwave field. Usually, the quartz vessels are used for hundreds of decompositions and corrosive attack is often already visible with the bare eye. On the other hand it is well known that adsorption as well as leaching phenomena increase with rising surface roughness of a material. This is especially critical in trace and ultratrace work. For the latter such interactions with the vessel wall might even become the limiting factor in ultratrace determinations at the ng/g-level and below. Therefore, systematic morphological and compositional investigations by high resolution scanning electron microscopy were performed on the inner surfaces of decomposition vessels which were subjected to a rising number of digestions under pressure in a Multiwave digestion apparatus. Milk powder was digested with nitric acid for these investigations. Rising corrosive attack was observable but did not lead to severely damaged surfaces. The latter were found on a quartz pressure vessel for an HPA-digestion apparatus which was in intensive use over a long time. Recrystallization was also observable in this case. In addition to investigations by scanning electron microscopy, FT-NIR-Raman spectroscopy was also applied for the surface characterization of the various quartz vessels. The onset of recrystallization is detected much earlier with this method than by morphological changes in the scanning electron microscope (SEM). It appears that the combination of surface morphological and topochemical investigations in the SEM with vibrational spectroscopy is ideal for an optimal characterization of corroded quartz surfaces. Received January 15, 2001. Revision April 14, 2001.