Comparison of digestion methods for determining the total metal content in dust samples

Status:

completed 07/2018

Aims:

The digestion methods used as conventional methods for occupational safety and health purposes require modification and adaptation to the state of the art. The methods are described in the IFA folder and the MAK collection. Microwave digestion methods are now often used. The decisive benefits of the microwave digestion methods are decreased digestion times, greater reproducibility, and a reduction in cross-contamination and interferences in analysis. In consideration of the elevated requirements for the limits of quantitation, precision and correctness of numerous metals and their compounds, and the progressive reduction of the limit values, the last two of these points in particular are highly relevant. In order for the method to be established, checks must determine that the results from the two methods are comparable. A key problem is the high variability in metals and metal compounds in workplace atmospheres in metalworking plants. This must be considered when the method is in development, by the study of dusts from the most diverse of working areas.

Activities/Methods:

The comparability was examined in round-robin tests. Suitable dust materials were first required for development of the method and performance of the relevant comparative tests. These materials must be as homogeneous as possible and must possess a defined particle size distribution resembling those that may arise at workplaces. It is also important that they contain a wide range of metals and metal compounds.

The metrological services of the MGU supplied the IFA with eleven dust deposit samples that had been collected at different workplaces. To ensure that the available coarse dusts were comparable to atmospheric samples, the former had to be ground and homogenized. The particle size must not exceed 100 µm (maximum particle size of inhalable dusts). The dusts obtained were characterized by means of microscopy, ICP-MS and TXRF, and the dusts with a suitable metal content selected for development of the method. These dusts were used to perform parallel series of conventional digestion and microwave digestion tests (variables: acid mixture, time and dust mass). ICP-MS was used for quantitative analysis of the metal concentrations. The results of analysis were compared according to the recovery of 20 metals. The microwave digestion method yielding results correlating most closely to those of the conventional method used to date was selected.

Two round-robin tests were organized by the IFA in order to review the microwave digestion method following its development. The national round-robin test, in which members of the DFG's working group for air analyses participated, was used to assess the new digestion method. The participants were required to adhere closely to the IFA's specifications. The international round-robin test conducted with partner institutes (including HSL, NIOSH, IRSST, INRS, SUVA, ÖSBS) had the purpose of comparing the different digestion techniques used by the institutes, and also of assessing the comparability of the conventional method and the microwave method developed at the IFA.

Results:

In the new method, the dust is digested for 45 minutes in the microwave oven at 60 bar (max. 240 °C) with 10 ml concentrated nitric acid. The method yields results that compare closely to those from the conventional method of open digestion used to date. In order to assess and establish the new method, two round-robin tests were launched by the IFA. Experienced laboratories from the DFG's working group responsible for air analyses and international OSH institutes took part in the round-robin tests. Selected dusts were to be studied with the use of the conventional open digestion method and, if technically possible, the new microwave pressure digestion method. The participants were at liberty to use further in-house digestion methods. Two dusts were studied in the round-robin test, in which German members of the DFG working group responsible for analysis methods (BAuA, IGF, BGN, ALL, and VW AG) participated. Four dusts were studied in the second round-robin test, in which foreign OSH institutes participated (NIOSH, IRSST, ÖSBS, SUVA, HSL, WOHL, INRS and STAMI).

The results for the metals reveal good comparability between the conventional digestion method used to date and the microwave pressure digestion method developed by the IFA. Exceptions are antimony, tin and tungsten, for which the microwave method is not suitable. The IFA microwave pressure digestion method is suitable in principle for determining the overall metal content in dust samples, and will be published in the near future as a recommended method of equal validity. Comparison between all digestion methods also yields satisfactory results. It was seen however that the use of a digestion medium of lower concentration and a comparatively low digestion temperature and time may yield lower metal concentration results.

Last Update: