PTFE for Mercury Checking Heavy Metal Analysis with Perfluorinated PTFE Digestion Vessels
In 2010 the annual emission of mercury into the atmosphere amounted to around 1,960 t, with the sources being mainly anthropogenic in origin. Despite efforts to reduce the emissions further large quantities of mercury have found their way into the food chain. The analytical investigation of the heavy metal with a robust digestion has thus gained increased importance.
Mercury is a naturally occurring element of mineral origin. In 2010, 1,960 t of mercury were discharged into the atmosphere, of which about 30 % were anthropogenic in origin. The main causes are coal combustion, the production of chlorine, the manufacture of steel and cement, and the extraction of gold. The largest proportion of annual emissions (60 %) comes from the so-called re-emission of mercury making its way back into the atmosphere from the soil, water, sediment or waste. In aquatic systems elemental mercury is transformed into methylmercury by bacterial processes, from where it finds its way into the food chain.
Mercury and its compounds exhibit negative effects on the nervous, cardiovascular and immune systems. The blood-cerebral barrier is passed, leading to enrichment of the tissue. Moreover, mercury compounds cross the placental barrier and may thus influence a child’s development. On the basis of new findings current values for elemental impurities for drug products have been established in USP chapter 232. In addition, as early as 2001 the EPA determined a value of 0.1 µg/kg body weight per day as the maximum tolerable daily dose of methylmercury.
Finding and Determining Mercury in Routine Analysis
The high toxicity of mercury makes monitoring a matter of necessity. This principally requires the use of technology that demonstrably makes robust digestion possible without the mercury vaporising at high temperatures. Older references (e.g. DIN EN 13805) recommend the use of quartz glass for lossless digestion. It was long supposed that the surface roughness and porosity of PTFE leads to lower results. However this presumption has been refuted during the last few years by the use of isostatically compressed TFM-PTFE. There is now greater readiness to accept this rather than fluoroplastics, since even current norms like EPA 3052 or EPA 3051A permit the use of these materials.
Under the brand name TFM-PTFE is concealed a so-called second-generation PTFE in which a modifier in small quantities is integrated in the linear chain of the polymer. This modified version is processed like conventional PTFE, while exhibiting a distinctly better property profile. The chemical resistance, thermal stability and broad temperature range are comparable with those of standard PTFE.
But the modified has a substantially reduced melting viscosity, a significantly decreased microporosity and a lower Stretch-Void-Index (SVI). Pore-free products with low permeability are the result.