On the threshold conditions for electron beam damage of asbestos amosite fibers in the transmission electron microscope (TEM)

Our plain-English summary of On the threshold conditions for electron beam damage of asbestos amosite fibers in the transmission electron microscope (TEM), by J. Martin, M. Beauparlant, S. Sauvé, et al (Journal of Occupational and Environmental Hygiene, 2016).

The study

• Authors: J. Martin, M. Beauparlant, S. Sauvé, G. L'Espérance
• Journal: Journal of Occupational and Environmental Hygiene
• Published: 2016;13(12):924-935
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Asbestos amosite fibers were investigated to evaluate the damage caused by a transmission electron microscope (TEM) electron beam. Since elemental x-ray intensity ratios obtained by energy dispersive x-ray spectroscopy (EDS) are commonly used for asbestos identification, the impact of beam damage on these ratios was evaluated. It was determined that the magnesium/silicon ratio best represented the damage caused to the fiber. Various tests showed that most fibers have a current density threshold above which the chemical composition of the fiber is modified. The value of this threshold current density varied depending on the fiber, regardless of fiber diameter, and in some cases could not be determined. The existence of a threshold electron dose was also demonstrated. This value was dependent on the current density used and can be increased by providing a recovery period between exposures to the electron beam. This study also established that the electron beam current is directly related to the damage rate above a current density of 165 A/cm2. The large number of different results obtained suggest, that in order to ensure that the amosite fibers are not damaged, analysis should be conducted below a current density of 100 A/cm2.

In plain English

This laboratory study tested whether the beam of a transmission electron microscope (TEM), used to identify asbestos, alters the chemistry of amosite fibers. It did not study people or disease. The authors used amosite fibers from South Africa's Penge mine and treated the magnesium/silicon (Mg/Si) x-ray signal ratio as a damage marker. In 11 fibers analyzed under conditions where no damage was suspected, the Mg/Si ratio averaged 0.153, with a standard deviation of 0.008. On some fibers the ratio fell by 75% once current density passed a threshold that varied between fibers. For most of 14 fibers this threshold lay between 100 and 600 A/cm2, and the authors could not link it to fiber diameter. Damage increased with exposure time. In a test on one fiber, pausing the beam between exposures reduced damage. Above 165 A/cm2, damage rose with beam current. The authors concluded that several beam settings must be controlled and that a conservative approach is to work below 60 A/cm2. All fibers came from one sample; the authors noted that other samples may vary more.

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 experiment (analytical method study) on asbestos fibers in a transmission electron microscope; not a study of people or health outcomes
• Who was studied: Individual amosite (grunerite asbestos) fibers from the UICC reference sample, Penge mine, Northern Province, South Africa. Fiber counts by test: 11 fibers for chemical homogeneity, over 15 fibers for threshold tests, 14 fibers with a statistically significant threshold, 7 fibers for time tests, and 15 fibers for the no-damage operating map.
• Exposure: Electron beam exposure in a field-emission TEM operated at 200 keV, with varied beam current, beam diameter, current density (A/cm2), exposure time and recovery time; damage measured as change in the magnesium/silicon (Mg/Si) EDS x-ray intensity ratio
• Main results:
  • In 11 amosite fibers analyzed under conditions where no damage was suspected, the average Mg/Si ratio was 0.153 with a standard deviation of 0.008
  • At a constant beam current of 8.3 nA, reducing the beam size below 60 nm quickly led to similar damage on the three fibers tested
  • In one type of fiber response, the Mg/Si ratio fell 75% (from about 0.16 to about 0.04) above a certain current density
  • In a second type of response, the Mg/Si ratio fell about 40%, but the difference was within statistical error, so the authors could not state from these data that damage occurred
  • In a third type of response, no damage was observed even at very high current densities
  • Among 14 fibers with a statistically significant threshold, most had a current density threshold between 100 and 600 A/cm2, and some had a very high threshold
  • The authors could not show a correlation between fiber diameter and current density threshold
  • In time tests on 7 fibers, damage increased with exposure time, especially at higher current densities. Below 100 A/cm2, exposure time could be very long before damage appeared.
  • On one fiber, allowing recovery periods (beam blanked for 30 seconds or two minutes between 30-second exposures) reduced the rate of damage compared with continuous exposure. The longer recovery time reduced it more.
  • Above a current density of 165 A/cm2, the electron beam current was directly related to the damage rate
  • From the no-damage conditions mapped for 15 fibers, the authors gave an example: a 60 nm beam at 200 A/cm2 should allow analysis without damage, regardless of beam current
  • The authors found that exposure time, beam size, current density and electron beam current all must be controlled to avoid fiber damage
• The authors' conclusion: The authors concluded that, because the damage threshold differs between fibers and is hard to predict, a conservative approach is to work at a current density below 60 A/cm2. They considered current density the most limiting parameter.
• Limitations the authors state:
  • All fibers came from the same sample. Greater variation would be expected for fibers from different samples and geological locations.
  • The magnesium signal is absorbed, especially in thick, iron-rich fibers, which makes comparisons between different fibers difficult
  • When the magnesium signal was weak, error increased. Exposure time could not be lengthened to compensate, because time itself causes damage.
  • Very high current densities could not be tested because the Si(Li) EDS detector saturated
  • For some fibers, a threshold could not be confirmed or ruled out

Topics

The National Library of Medicine indexes this study under subjects our site covers: Amosite.

Medical terms in this study, explained in our Lexicon: Amosite, energy dispersive X-ray spectroscopy (EDS).

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