Elemental analysis of white cotton fiber evidence using solution ICP-MS and laser ablation ICP-MS (LA-ICP-MS)

The purpose of this research is to investigate a method for the forensic elemental analysis of cotton fibers for the purpose of increasing the discrimination between otherwise similar cotton evidence using microwave digestion inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A quadrupole ICP-MS and UV laser ablation (266 nm) instruments were used for the analysis. A cotton standard reference material (IAEA V-9) was used to validate the developed methods producing good accuracy with typically <10% bias and good precision (typically <5% RSD) for the element list: ²⁵Mg, ²⁷Al, ⁵⁵Mn, ⁵⁷Fe, ⁸⁸Sr and ¹³⁷Ba. It was found that the LA-ICP-MS method resulted in improved precision over the solution ICP-MS method. Twenty-four raw cotton samples and five white cotton T-shirts were analyzed with the developed methods. It was also found that all the raw cotton samples from different sources were distinguishable from each other, as were all the cotton T-shirts resulting in zero type I errors and zero type II errors for the pairwise comparisons. The laser ablation method was slightly faster than the solution-based analysis, requiring 3 h for the laser analysis of 10 samples vs. 3.5 h for the solution analysis, including sample preparation time. One additional advantage of the LA-ICP-MS method was the extremely low sample consumption (1.75 μg were consumed vs. 250 mg for the solution-based method) and the fact that the LA samples are amenable for reanalysis.