An examination of MCE filter morphology and implications on preparation and analysis of air samples for asbestos

Our plain-English summary of An examination of MCE filter morphology and implications on preparation and analysis of air samples for asbestos, by M. Getman, J. Webber, S. Bowser (Journal of Occupational and Environmental Hygiene, 2017).

The study

• Authors: M. Getman, J. Webber, S. Bowser
• Journal: Journal of Occupational and Environmental Hygiene
• Published: 2017;14(9):140-144
• Read it: PubMed · Publisher

Abstract

Morphological imperfections of mixed-cellulose ester filters and their possible influence on sample preparation and analysis of asbestos were examined. Filters were identified with large regions of non-porous "dead zones" which could negatively affect fiber deposition and, therefore, fiber recovery and analysis by Transmission Electron Microscopy (TEM). As these imperfections are effectively erased during the preparation of the sample, they may not be readily observed by TEM. Un-collapsed filters as well as those collapsed using dimethylformamide (DMF) and two acetone techniques were examined. In order to minimize negative sampling and analytical bias, it is suggested that MCE samples be collapsed utilizing a 35% DMF solution, etched with a correctly calibrated low-temperature etcher, carbon coated using a rotating and tilted stage, and analyzed with a strong analyst-independent grid square opening randomization scheme.

In plain English

This was a laboratory study of air-sampling equipment, not a study of people or health effects. The New York State Asbestos Laboratory examined the mixed-cellulose ester (MCE) filters in cassettes used to collect air samples for asbestos testing by transmission electron microscopy (TEM). Three brands of cassettes with 0.45 µm filters, called only X, Y and Z, were loaded with amphibole asbestos fibers. The authors viewed them under a scanning electron microscope before and after the preparation steps: collapsing with a 35% dimethylformamide (DMF) solution or one of two acetone methods, then plasma etching. Filters X and Y had a uniform structure. Filter Z had regions with little to no permeability, which the authors called "dead zones." Most evidence of these disappeared after collapsing, so an analyst might not notice them. DMF gave better surfaces than either acetone method. After etching, filter Z showed uneven pitting. The authors wrote that such flaws could interfere with airflow or cause lost fibers. The article reports how the filters looked, not fiber counts. The authors recommended DMF collapsing, a calibrated low-temperature etcher, a rotating and tilting stage for carbon coating, and random, analyst-independent selection of grid squares.

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 examination of filter structure by scanning electron microscopy (SEM), comparing three filter brands and three preparation methods
• Who was studied: Three brands of air-sampling cassettes with 0.45 µm mixed-cellulose ester (MCE) filters, anonymized as X, Y and Z. The work was done at the New York State Asbestos Laboratory, Wadsworth Center, New York State Department of Health. No people were studied.
• Exposure: Filters loaded with aerosol-deposited amphibole asbestos fibers. They were then collapsed by 'fast blast' acetone, controlled acetone (250 µL), or 35% dimethylformamide (DMF), and plasma etched for a calculated 5% mass reduction.
• Main results:
  • Before collapsing, the Z filter had regions with porosity and regions with little to no permeability ('dead zones').
  • The X and Y filters had a uniform composition. The authors wrote that the Z filter's non-uniform surface was potentially interfering with airflow, which made its effectiveness as a collection medium suspect.
  • Most evidence of the dead zones in the Z filter disappeared after collapsing, so an analyst would not know about possible uneven fiber deposition.
  • DMF collapsing was judged superior to both acetone methods. Of the acetone methods, controlled acetone gave fewer surface imperfections than fast blast.
  • After collapsing, filter X had the most uniform surfaces, followed by Y. Filter Z was consistently of inconsistent quality.
  • After plasma etching, filter Z showed more pitting in some regions and less or none in others. The authors wrote that this could compromise the carbon replicate film and cause loss of fibers.
• The authors' conclusion: The authors concluded that a uniform 0.45 µm MCE filter with a lace-like structure, collapsed with DMF before etching, gives the best surface for carbon coating. Because laboratories often cannot choose their filters, they recommended four steps: collapsing with 35% DMF, using a correctly calibrated low-temperature etcher, using a rotating and tilting stage during carbon coating, and using analyst-independent random selection of grid squares.

Topics

Medical terms in this study, explained in our Lexicon: Deposition, Transmission Electron Microscopy (TEM).

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