Scanning electron microscopy and hyperspectral imaging in man made vitreous fibers characterization

Fibers can be classified into natural and man-made (i.e., synthetic) fibers, further categorized as organic or inorganic. Man-made vitreous fibers (MMVFs) are non-crystalline, fibrous inorganic substances (silicates) primarily made from rock, glass or processed minerals. This group includes glass fibers (i.e., glass wool or continuous glass filament), rock (stone)/slag wool and refractory ceramic fibers. A fiber is defined as a particle with a length to width ratio of at least 3:1. MMVFs can be classified as non-hazardous or hazardous based on their chemical and geometric characteristics (i.e., inhalable/respirable). Respirable fiber have a diameter (FD) < 3 μm, length (FL) > 5 μm and a length to width ratio (FL/FD) > 3:1, capable of reaching the deepest part of the lung. The most commonly used analytical techniques to date are the same as those used for asbestos fibers. Scanning Electron Microscopy (SEM) is a powerful imaging technique using an electron beam to illuminate a sample, producing high-resolution images. It enables detailed visualization of MMVF individual fibers, allowing to analyze their morphology, size distribution, structure, aspect ratio and surface features. SEM, when equipped with energy-dispersive X-ray spectroscopy (EDS), can also determine the elemental composition of MMVFs, thus aiding in understanding their chemical makeup and potential environmental impact. Hyperspectral Imaging (HSI) is a non-destructive imaging technique that combines conventional imaging with spectroscopy, not requiring any physical or chemical samples manipulation. It captures images of a sample, at multiple wavelengths, across the electromagnetic spectrum, providing detailed spectral information for each pixel. HSI facilitates a rapid and accurate identification and analysis of different components in MMVF samples, based on their spectral signatures. It can also generate spatial maps of MMVF distribution, providing insights into fiber dispersion within a material or product. Additionally, HSI offers information about the size, shape and orientation, assisting in their characterization. Both SEM and HSI are valuable techniques for studying MMVFs. SEM provides detailed structural and elemental information at the micro and nanoscale, while the most diffused HSI devices operating at lab and micro scale offer information about fiber distribution and characteristics based on detected spectral signatures with a maximum spatial resolution of the order of microns. However, SEM requires complex sample preparation (similar to other “standard” fiber analysis techniques such as Fourier-transform infrared spectroscopy (FTIR), XRay Diffraction (XRD) and Optical Microscopy), and longer analysis time. These drawbacks can expose operators to risks and contaminate the laboratory. In contrast, HSI does not require sample preparation, reducing both analysis time and exposure risks. The study shows the morphological and compositional differences of several MMVF samples, by SEM-HSI combined analysis. The results can contribute to a more accurate and rapid characterization of fibers, especially regarding their potential hazards to humans and the environment. The goal is to achieve a reliable, robust and fast analysis using HSI alone, without the need for SEM or preparative operations, as required by traditional techniques.