PTFE for Mercury Checking Heavy Metal Analysis with Perfluorinated PTFE Digestion Vessels

Author / Editor: Dr. Kerstin Dreblow, Wolfgang Steinbrecher, Dr. Michael Henes / Tobias Hüser

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.

Figure 3: Digestion vessels from Berghof are completely made of TFM-PTFE
Figure 3: Digestion vessels from Berghof are completely made of TFM-PTFE
(Pictures: Berghof; Juergen Lippert; © T. Michel, © tanatat - Fotolia.com)

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.

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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.

Figure 3: Digestion vessels from Berghof are completely made of TFM-PTFE
Figure 3: Digestion vessels from Berghof are completely made of TFM-PTFE
(Pictures: Berghof; Juergen Lippert; © T. Michel, © tanatat - Fotolia.com)

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.

Figure 1: TFM-PTFE
Figure 1: TFM-PTFE
(Picutre: Berghof)

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.

The process of isostatic compression moulding was developed by Berghof more than forty years ago in order to achieve optimal compression and excellent material properties. In the case of conventional, single-axis moulding the material is generally compressed vertically in a mould with a force plug. There is no horizontal compression crosswise to the direction of compression. With isostatic compression moulding on the other hand the force is applied to the material evenly and simultaneously from all directions via a hydraulic medium and uniformly compressed.

In this way optimal compression is achieved, resulting in minimal porosity, an improved surface structure and maximum tensile and compressive strength. No preferred directions are created and isotropic material properties are retained. In particular the tensile and compressive strength of the material is consistent in all spatial directions. Further processing of PTFE and TFM-PTFE by moulding follows the stages compression, sintering and cooling.

Quality Benefits of Perfluorinated PTFE

The advantages of isostatic compression moulding can be made clear by means of appropriately enlarged REM images. With the chain polymer a markedly finer and smoother structure is achieved. This results in a clear reduction in the risk of contamination and memory effects. Berghof microwave digestion systems are characterised by durable digestion vessels entirely manufactured of isostatically compressed polymer. Additional non-acid-resistant plastic materials are not used, guaranteeing simple and safe handling. The vessels consist of just a few components and can be opened and closed without the need for tools.

Figure 2: Moulding procedures compared
Figure 2: Moulding procedures compared
(Picture: Berghof)

Digestion of Reference Materials in Mercury Checking

Mercury exhibits a large toxicological potential which explains the necessity for monitoring. In the past the applicability of vessels made of fluorplastics for wet chemical digestions has been controversial discussed. However, since a few years there is a greater readiness to accept such materials through modifications and improved properties. To investigate the memory effects in TFM-PTFE vessels the total mercury content of the following reference material was determined:

  • Pine needles from the 15th Needle/Leaf Interlaboratory Test 2012/2013 of the Federal Research and Training Center for Forests, Natural Hazards and Landscapes, Vienna Mercury: 0.0338 ± 0.010 mg/kg

For digestion 500 mg of sample material were weighed in the TFM-PTFE vessel in each case. The samples were mixed with 3.0 ml HNO3 (65 %), 1.0 ml H2O2 (30 %), 0.5 ml HCl (37 %) and 10 ml bidistilled water and digested at 210 °C with the Speedwave Four microwave digestion unit.

Analysis of the mercury content was carried out using ICP-MS. In order to obtain information about possible carry-overs or contamination by mercury on the one hand reference materials with varying mercury contents were examined, while on the other hand blind digestion was undertaken at the start and finish of the test series. Between the individual digestions the vessels were simply rinsed with bidistilled water. The digestions were conducted on four consecutive days.

Table 1: Comparison of PTFE and TFM-PTFE
Table 1: Comparison of PTFE and TFM-PTFE
(Picture: Berghof)

Examining the Mercury Concentration in Pine Needles

At the beginning and end of the series of experiments blind digestion was carried out in the digestion vessels. The blind values were under the detection limit, so that it may be presumed that there was no contamination by mercury. A total of four repeated digestions were undertaken in the same vessel on different days. Between the individual digestions the vessels were simply rinsed with bidistilled water.

The reproducibility of the methods employed was specified in order to determine the precision. The recovery rates of between 99 and 118 % are adequate and are within the acceptance criterion of ±30 %.

It can also be seen from the measurement data that there are no carry-overs or contamination whatsoever on the walls of the vessel that might lead to flawed results. Traces of mercury can therefore be determined to be fault-free even where isostatically compressed TFM-PTFE vessels are used. A rinsing of the vessels with water is also sufficient and additional blind digestions are not essential.

Long Live, High Quality: Perfluorinated PTFE Digestion Vessels

Digestion vessels made of isostatically compressed TFM-PTFE last several years without deteriorating in quality while at the same time the containers are characterised by their low cost compared to those made of other materials. For this reason they are therefore the preferred vessels for use in modern laboratories.

Result of pine needle analysis after digestion (straight line: certified value 0.0338 ± 0.01 mg/kg; broken line: upper and lower tolerance limit)
Result of pine needle analysis after digestion (straight line: certified value 0.0338 ± 0.01 mg/kg; broken line: upper and lower tolerance limit)
(Picture: Berghof)

Demanding analytic issues such as those involving trace analyses of mercury or other heavy metals can be safely handled by Berghof microwave digestion units with TFM-PTFE vessels. In order to prevent memory effects or contamination additional complex cleaning procedures are generally not required. Users profit from increased productivity and a simplification of daily routine. ●

* Dr. Kerstin Dreblow works for Berghof Products + Instruments GmbH and Dr. Michael Henes works for Berghof Fluoroplastic Technology GmbH, Eningen u.A./Germany. Wolfgang Steinbrecher works for Berghof Analytik + Umweltengineering GmbH, Tübingen/Germany.

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