Visibility of fine fibres of asbestos during routine electron microscopical analysis

Our plain-English summary of Visibility of fine fibres of asbestos during routine electron microscopical analysis, by A.P. Middleton (The Annals of Occupational Hygiene, 1982).

The study

• Author: A.P. Middleton
• Journal: The Annals of Occupational Hygiene
• Published: 1982;25(1):53-62
• Cited by: 11 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Most analyses for asbestos in samples from the non-occupational environment are carried out using techniques based upon either scanning (SEM) or transmission (TEM) electron microscopy. The mode of image formation and resolution obtainable differ between SEM and TEM and the present study was undertaken in order to provide some indication of the comparability of observations made by the two techniques, with particular reference to the visibility of fine, sub-micrometre fibres. The results indicate that, in general, at a given magnification finer fibres are visible when using TEM than when using SEM. When using SEM, fibre visibility is improved by the use of higher magnifications, by the use of slower raster speeds and by the use of gold rather than carbon for specimen coating. The use of photomicrographs leads to improved fibre visibility in both SEM and TEM images. The study suggests that comparable results can probably be obtained from the two techniques, although fibres ≲0.1 µm dia may be close to the limit of visibility in the SEM. In order to facilitate inter-laboratory comparison of results it would seem appropriate for standard samples to be available so that optimum conditions can be determined for the observation of fine fibres.

PubMed has no abstract for this study. This is the authors' abstract, copied from the article.

In plain English

This limited laboratory methods study compared how well scanning (SEM) and transmission (TEM) electron microscopes show very thin asbestos fibres. The SEM tests used airborne chrysotile asbestos collected on a filter; the TEM tests used a sample deposited from a liquid suspension onto a grid. On the SEM, the thinnest visible fibre was noted at 500 to 5000 times magnification on gold-coated and carbon-coated specimens; two observers used slow scanning, and one repeated the work at TV-rate scanning. In general, TEM showed finer fibres than SEM at the same magnification. On the SEM, higher magnification, slower scanning and gold coating improved visibility. However, gold coating markedly degraded X-ray analysis of a UICC chrysotile 'A' fibre; carbon coating had little effect. Photographs improved visibility on both instruments. The author concluded that TEM easily shows the finest fibres, about 0.02 to 0.05 micrometres across. Seeing fibres under 0.1 micrometre on the SEM appeared to need operation near its detection limit, so some may be missed. The author noted that results from random fields of view may have depended on whether fine fibres were present there.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory methods comparison (empirical assessment of fibre visibility in SEM versus TEM)
• Who was studied: No people were studied. For the SEM, a sample of airborne chrysotile asbestos was collected on a Nuclepore filter and divided into gold-coated and carbon-coated halves. For the TEM, a sample deposited from a liquid suspension on a 400 mesh grid with carbon support film was examined. Two observers did the SEM slow-scan work, and observer 2 also did it at TV-rate scan. Results were compared with unpublished results from another laboratory (Teichert, 1981).
• Exposure: Chrysotile asbestos fibres. A fibre of about 0.15 micrometre diameter from UICC chrysotile 'A' was used for the X-ray analysis tests. It was analysed uncoated, then carbon-coated, then gold-coated.
• Main results:
  • In general, at a given magnification, finer fibres were visible with TEM than with SEM.
  • On the SEM, especially for direct on-screen viewing, fibre visibility improved with higher magnification, slower raster (scan) speed, and gold rather than carbon coating.
  • Carbon coating had comparatively little effect on the X-ray (EDXA) spectrum of the fibre, while gold coating had a marked harmful effect.
  • Micrographs (photographs) improved fibre visibility compared with on-screen viewing.
  • Because of their low contrast, SEM negatives would need an initial magnification of at least 5000 times for the finest fibres to be visible.
  • Results agreed reasonably well between observers 1 and 2 and with the other laboratory, with differences of detail.
  • Imaging of the finest fibres (about 0.02 to 0.05 micrometres) was well within the capabilities of a modern TEM.
• The authors' conclusion: The author concluded that observing fibres less than 0.1 micrometre in diameter in the SEM appears to require operation near the limit of detection, so some such fibres may not be observed.
• Limitations the authors state:
  • The author described the study as limited.
  • Results from random fields of view (Tables 1 and 3) may have been influenced by whether sufficiently fine fibres were present in particular fields. The author said the second exercise (Table 2) was not subject to this limitation.
  • In the second exercise, a fine fibre known to be present may have been noticed at a lower magnification than it otherwise would have been.
  • The study did not assess how differences in sample preparation between SEM and TEM affect the observed fibre sizes; it was limited to already prepared samples.

Related studies

• Scanning electron microscopical investigations of broncho-alveolar casts after intratracheal asbestos fibre instillation — B. Voss, T. Kerenyi, et al. (2000)
• Nephrogenic adenoma: a form of adenomatous metaplasia of the bladder. A clinical and electron microscopical study — E.A. Molland, P.A. Trott, et al. (1976)
• Electron microscopical investigation of asbestos fibers — A.M. Langer, A.D. Mackler, et al. (1974)
• Electron microscopy in the diagnosis of percutaneous fine needle aspiration specimens — E.J. Wills, S. Carr, et al. (1987)
• Diagnostic electron microscopy using fine needle aspiration biopsies — B. Mackay, T. Fanning, et al. (1987)
• The classification of asbestos fibres by scanning electron microscopy and computer-digitizing tablet — S.F. Platek, R.D. Riley, et al. (1992)

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