Technical note: Forensic investigation of mosquito nets

Mosquito nets are among the sample types that often occur in forensic investigations. In the case of various crimes (e.g. break-in, vandalism, burglary), they can be used as supportive evidence of a link between the control sample and the suspect's clothing, or the tool used to commit the crime. The primary aim of our work was to determine how different the individual samples are from a forensic aspect. The material residues generated in real-life criminal cases are typically microscopic in size (below 1 mm), which limits the applicability of macroscopic observations. Therefore, our focus shifted from macroscopically observable characteristics to instrumental techniques suitable for microscopic analysis. In addition to documenting macroscopic properties, three analytical methods were used, refractive index determination, infrared spectroscopy, and Raman spectroscopy. A total of 50 mosquito net samples were examined, of which 35 are fiberglass. The members of 434 out of the total 595 possible pairs can be distinguished from each other using only refractive index measurement. The fiberglass samples can be classified into 7 groups based on the infrared spectra of their plastic coating. The discrimination power of this method was found to be 73 %. Based on the Raman spectra, it was possible to make 10 groups from the 35 samples with a 76 % discrimination power. Any of the two techniques together resulted in a discrimination power over 80 %, and examining the three techniques, this value was found to be as high as 88 %. In less commonly real cases with adequate sample size and condition, macroscopic properties can also be assessed, increasing the overall discrimination power up to 98 %. Among the purely plastic samples, infrared analysis allows only limited differentiation due to the similarity of their polymer structures. Based on the infrared spectra, out of the 15 purely plastic samples, 11 are made of polyethylene-terephthalate, 3 are made of polyethylene, and 1 is made of polypropylene. However, if the sample size and conditions are appropriate, macroscopic features can provide a discrimination power of over 92 %. Based on these results, we can conclude that the applied methods have a sufficiently good discriminating power in the case of mosquito net samples.