Analytical method for the measurement of toner particle concentrations

Status:

completed 04/2012

Aims:

Recent years have witnessed an ongoing discussion concerning the pollution of the air in offices by toner dusts. Established measurement methods have been used to determine exposure for example to metals, volatile organic components and ozone. The concentration and particle size distribution of nanoparticles in the breathable air can also be determined. Very little is known however about the actual concentration of toner particles in the breathable air. Since the toner particles (with an average particle size of between 3 and approximately 10 µm, depending upon the manufacturing process) occur in conjunction with other dust particles, they are difficult to measure selectively. In the course of a BfR study of the German Federal Institute for Risk Assessment into emissions from office equipment (2007), scanning electron microscopy (SEM) was used to perform preliminary analyses of samples containing such particles. The results were however unsatisfactory and not quantitative. In order to improve the quantitative measurement of workers' exposure to toner particles emitted from office machinery, a measurement method was to be developed which measures the toner particles as selectively as possible. The measurement results are to yield the toner particle count per m³, and by calculation from that for an estimated particle density, the particle mass concentration.

Activities/Methods:

A method was developed based upon the established phase contrast microscopy method documented in BGI 505-31 for determination of the number concentration of respirable fibres. Sampling was performed as for dust measurements, either at the person or statically (for example by means of the GSP-10 or PM4G samplers). Sampling filters for the inhalable dust fraction were used for analysis. Following collection, the membrane filters were cleared by acetone vaporization. The toner particle number concentration was determined by evaluation of a portion of the filter area under the microscope. Reference filters were prepared from material samples of the toners employed in the working area. These were then used to define optical parameters for identification of the toner particles on the prepared filters. Following internal pilot tests in the laboratory, the method was trialled in dust measurements in offices in order to test its suitability for use in the field.

Results:

An analysis method was developed for determination of the number concentration of toner particles in workplace atmospheres. Based upon the analysis of inhalable dust filters, the number concentration and the calculated mass concentration of toner particles are determined. The method has revealed significant cross sensitivities in the trial phase. Optical microscopy is of only limited suitability for distinguishing between toner and other dust particles. In the particle size range below 3 µm in particular, it has clearly led to false positive results. Problems also occur during preparation of the filters for analysis. The use of acetone vaporization to clear the filters results in the toner particles being partly liquefied, fusing with each other, or disintegrating into smaller particles. To enable the influence of the preparation and the typical size and morphology of the toner particles to be evaluated on a case-by-case basis, a reference sample of the toner used in the working area is required. The method is still at the trial stage.

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