Sample preparation method for visualization of nanoparticulate captured on mixed cellulose ester filter media by enhanced darkfield microscopy and hyperspectral imaging

Our plain-English summary of Sample preparation method for visualization of nanoparticulate captured on mixed cellulose ester filter media by enhanced darkfield microscopy and hyperspectral imaging, by N.M. Neu-Baker, A.C. Eastlake, S.A. Brenner (Microscopy Research and Technique, 2019).

The study

• Authors: N.M. Neu-Baker, A.C. Eastlake, S.A. Brenner
• Journal: Microscopy Research and Technique
• Published: 2019;82(6):878-883
• Cited by: 3 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

A significant hurdle in conducting effective health and safety hazard analysis and risk assessment for the nanotechnology workforce is the lack of a rapid method for the direct visualization and analysis of filter media used to sample nanomaterials from work environments that represent potential worker exposure. Current best-known methods include transmission electron microscopy (TEM) coupled with energy dispersive x-ray spectroscopy (EDS) for elemental identification. TEM-EDS is considerably time-, cost-, and resource-intensive, which may prevent timely health and safety recommendations and corrective actions. A rapid screening method is currently being explored using enhanced darkfield microscopy with hyperspectral imaging (EDFM-HSI). For this approach to be effective, rapid, and easy, sample preparation that is amenable to the analytical technique is needed. Here, we compare the sample preparation steps for mixed cellulose ester (MCE) filter media specified in NIOSH Method 7400-Asbestos and Other Fibers by Phase Contrast Microscopy (PCM)-against a new method, which involves saturation of the filter media with acetone. NIOSH Method 7400 was chosen as a starting point since it is an established technique for preparing transparent MCE filters for optical microscopy. Limitations in this method led to the development and comparison of a new method. The new method was faster, easier, and rendered filters more transparent, resulting in improved visualization and analysis of nanomaterials via EDFM-HSI. This new method is suitable for a rapid screening protocol due to its speed, ease of use, and the improvement in image acquisition and analysis.

Published 2019. This article is a U.S. Government work and is in the public domain in the USA.

In plain English

This laboratory study compared two ways of preparing mixed cellulose ester (MCE) air-sampling filters for viewing nanoparticles by enhanced darkfield microscopy with hyperspectral imaging. One was the clearing step of NIOSH Method 7400, an established method for asbestos and other fibers. The other, a new method, soaked the filter in liquid acetone under a coverslip. Filters were loaded with silica nanoparticles or multiwalled carbon nanotubes; blanks were included. No asbestos was studied. The new method took about 1 to 2 minutes per sample. NIOSH Method 7400 took about 10 to 15 minutes for the first sample and about 3 minutes for each later one, but often upwards of 30 minutes when the vaporizer overheated and had to cool. Under the microscope, NIOSH Method 7400 left an opaque background; the new method did not. Silica nanoparticle signals were about 11,000 arbitrary units against a background of about 200 with the new method, versus about 1,700 against 1,200 with NIOSH Method 7400. The authors concluded the new method was inexpensive, rapid and simple, but said further work must ensure particles are not lost.

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 of two filter sample preparation techniques
• Who was studied: Laboratory-generated MCE filter samples, not people. Each method was used to prepare 16 filters: nine exposed to silica, four exposed to multiwalled carbon nanotubes, and three blanks. No dates or sampling locations are given beyond the authors' affiliations (SUNY Polytechnic Institute and NIOSH).
• Exposure: Filters were loaded with aerosolized silica nanoparticles (10 to 20 nm in diameter) or multiwalled carbon nanotubes (average diameter 9.5 nm, average length 1.5 micrometers). Two preparation methods were compared: NIOSH Method 7400 (acetone vapor plus triacetin) and a new liquid acetone saturation method. No asbestos was used.
• Main results:
  • Preparation time for NIOSH Method 7400 was about 10 to 15 minutes for the first sample and about 3 minutes for each later sample.
  • NIOSH Method 7400 often took upwards of 30 minutes when the vaporization chamber overheated and had to cool down. Typically only three samples could be prepared before such a wait.
  • The new method took about 1 to 2 minutes per sample.
  • Filters from both methods looked transparent to the naked eye. Under darkfield microscopy, NIOSH Method 7400 left a visible opaque filter background, while the new method left none.
  • Silica filters prepared by NIOSH Method 7400 showed nanoparticle spectra of about 1,700 arbitrary units against background pixels of about 1,200 arbitrary units.
  • Silica filters prepared by the new method showed nanoparticle signals of about 11,000 arbitrary units against a background of about 200 arbitrary units, with an exposure time of about 0.2 seconds.
  • To reach a similar signal-to-noise with NIOSH Method 7400, acquisition time per line had to be increased by upwards of 7.5 times. This added considerable time and did not always yield improved results.
  • On a filter with multiwalled carbon nanotubes prepared by the new method, nanotube pixels reached about 10,000 arbitrary units, against a background pixel of about 80 arbitrary units.
• The authors' conclusion: The authors concluded that the new method is inexpensive, rapid and simple. They said it clears the filter more completely than NIOSH Method 7400 and improves hyperspectral data quality.
• Limitations the authors state:
  • Further work is needed to make sure particles are not lost with the new technique.
  • Hyperspectral imaging still needs a detailed comparison with TEM-EDS for specific fiber sizes of a given material.
  • The authors state that the imaging approach cannot replace TEM-EDS. They describe it as a screening tool to identify which samples need more intensive analysis.
  • Methods for cleaning ordinary slides and coverslips of contaminating particles have not yet been assessed.

Topics

Medical terms in this study, explained in our Lexicon: Mixed Cellulose Ester Filter, Transmission Electron Microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), TEM/EDS, NIOSH Method 7400, phase contrast microscopy (PCM).

Related studies

• An examination of MCE filter morphology and implications on preparation and analysis of air samples for asbestos — M. Getman, J. Webber, et al. (2017)
• Efficiency of sampling and analysis of asbestos fibers on filter media: implications for exposure assessment — D.A. Vallero, J.R. Kominsky, et al. (2009)
• A routine for the control of the performance of microscopists evaluating airborne asbestos fibre samples on membrane filters by phase contrast microscopy — J.R. Hoyes, R. Clayton (1980)
• The influence of external aerodymanic factors on the measurement of the airborne concentration of asbestos fibres by the membrane filter method — A.M. Johnston, A.D. Jones, et al. (1982)
• A comparison of historic asbestos measurements using a thermal precipitator with the membrane filter-phase contrast microscopy method — G. Burdett (1998)
• A total sample preparation method for the measurement of airborne asbestos and other fibers by optical and electron microscopy — N. Kohyama, S. Kurimori (1996)

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