ABSTRACT
Transmission electron microscopy (TEM) is a powerful and efficient tool for the analysis of asbestos fibers. Although this analysis technique is common and several standard methods exist for asbestos analysis, questions remain about the optimal conditions to be used. Because asbestos fibers are relatively sensitive to the electron beam, it is important to better understand the phenomena of damage in order to avoid them. This study specifically investigates the effect of the acceleration voltage on damage to four different types of asbestos fibers: chrysotile, amosite, crocidolite and anthophyllite. The results support the conclusion that, contrary to what is usually recommended, it is best to use an acceleration voltage of 200kV rather than 100kV in order to avoid damage. The findings shed light on possible damage mechanisms, the most predominant of which seems to be caused by an induced electric field.
ABSTRACT
Damage to asbestos fibers by the transmission electron microscope (TEM) electron beam is a known limitation of this powerful method of analysis. Although it is often considered only in terms of loss of crystallinity, recent studies have shown that the damage may also change the elemental composition of fibers, thus causing significant identification errors. In this study, the main objective was to assess whether temperature is a factor influencing damage to asbestos fibers and, if so, how it can be used to minimize damage. It was found that lowering the temperature to 123K can inhibit, for a given time, the manifestation of the damage. The significant decrease of atom diffusion at low temperature momentarily prevents mass loss, greatly reducing the possibility of misidentification of anthophyllite asbestos fibers. The results obtained in this study strongly suggest that the predominant mechanism damage is probably related to the induced-electric-field model relegating radiolysis to the status of a subsidiary damage mechanism.
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.
