Discrimination of single acrylic fibers focusing on zinc for forensic investigation using nanobeam synchrotron radiation X-ray fluorescence imaging and X-ray absorption fine structure analysis

The identification of single acrylic fibers is critical in forensic science. Spinning solvents and dyes containing zinc are used to produce acrylic fibers. However, the information has not been used to identify acrylic single fibers for forensic purposes. This study aimed to discriminate between single acrylic fibers by focusing on zinc. A single type of red acrylic fiber from the Forensic Science Fiber Collection (Microtrace LLC, USA) was used as the standard sample. In contrast, nine commercially available colored acrylic fibers were used as samples. Owing to the limited beam time of synchrotron radiation X-ray analysis, total reflection X-ray fluorescence (TXRF) was used as a trace element screening method. Nanobeam synchrotron radiation X-ray fluorescence (SR-XRF) imaging of thinned acrylic single-fiber cross-sections was conducted to visualize the distribution of trace elements within a single fiber. X-ray absorption fine structure (XAFS) analysis was conducted to investigate the chemical states of zinc in the single acrylic fibers. The nanobeam SR-XRF imaging enabled the visualization of zinc derived from dyes and spinning solvents in single-fiber cross-sections. The images were classified into three distinct patterns. Two types of X-ray absorption near edge structure (XANES) spectra of the zinc absorption edge, reflecting the difference in the chemical state of zinc, were obtained from the acrylic single-fiber sample. In conclusion, the distribution and chemical state of zinc were found to be powerful indicators for distinguishing single acrylic fibers.