Sampling and counting rules for the characterisation of airborne NOAA by EM

Status:

completed 11/2024

Aims:

Materials in the nanoscale range (1 to 100 nm) have increasingly been used in recent years in a range of industrial sectors. In order to assess exposure to such substances in workplace atmospheres, analytical methods must first be developed. In addition to gravimetric analysis of the mass concentration, methods must also be established for determining the numerical concentration of nanoscale particles. As yet, no uniform collection and counting criteria exist that yield comparable results.

Firstly, collectors currently in use and other types, are to be tested and their properties and collection efficiencies compared. Two essential aspects must be considered during the development of suitable counting criteria for the scanning electron microscopy analysis of air samples for nanoscale particles. Firstly, the criteria must selectively cover one or more fractions of these particles that are relevant in terms of their impact upon health. Secondly, application of these criteria in a number of different laboratories must deliver reproducible and comparable results, in order for subjective influences of the analysers to be excluded as far as possible. The counting criteria must be suitably adapted, as the counting criteria to be established for nanoscale fibres (such as carbon nanotubes) differ from those for nanoscale granular particles.

The objective of the project is to harmonize the procedure and describe it in two standards addressing collection and counting criteria for nano-objects and their agglomerates. Part 1 is to describe the requirements for collection of the nano-objects, Part 2 the counting criteria for scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Activities/Methods:

The IFA participated in the "Sampling and counting rules for the characterisation of airborne NOAA by EM" project conducted by CEN TC 137. A number of institutions were involved in this project, which was coordinated by NEN, the Dutch Standards Institute. Firstly, a literature survey was conducted of current progress in the development of collection, analysis and counting methods for nanoscale substances, and a selection of collectors to be studied and a preliminary set of counting criteria were drawn up. For comparison of the collectors and counting methods, a number of nanoscale materials were then selected, and sample holders were loaded with these materials in special dust test chambers. The preliminary counting criteria were applied and compared in the participating laboratories by the use of electron microscopy methods. An interpretation of the analysis results supported finalization of the catalogue of suitable counting criteria. These criteria were subsequently incorporated into the standards documents to be prepared by CEN TC 137.

Results:

The aerosol samplers analysed differed considerably in their sampling efficiency in combination with the substrates used. The recommendation is for FAP samplers (used for sampling to determine the exposure to inorganic fibre dusts in accordance with DGUV Informative document 213-546 or for indoor measurements in accordance with VDI 3492) to be used, under defined sampling conditions and with use of specific filters for subsequent interpretation on the scanning electron microscope for counting nano-objects and their aggregates and agglomerates (NOAA).

Comparative interpretations yielded differences in the understanding of the preliminary counting criteria. The rules for recording and interpreting images were then specified more closely.

The findings regarding the collectors and the counting criteria for nano-objects, aggregates and their agglomerates were compiled in a technical specification (DIN CEN/TS 18117:2025-04, Workplace exposure – Detection and characterisation of airborne NOAA using electron microscopy – Rules for sampling and analysis).

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