Dietz Marianne E, Stern Libby A, Mehltretter Andria Hobbs, Parish Ashley, McLasky Velvet, Aranda Roman
Counterterrorism and Forensic Science Research Unit, FBI Laboratory, Quantico VA 22135, USA.
Sci Justice. 2012 Mar;52(1):25-32. doi: 10.1016/j.scijus.2011.05.003. Epub 2011 Jul 5.
Forensic interest in adhesive tapes with polyvinyl chloride (PVC) backings (electrical tape) derives from their use in a variety of illicit activities. Due to the range of physical characteristics, chemical compositions, and homogeneity within a single roll of tape, traditional microscopic and chemical analyses can offer a high degree of discrimination between tapes, permitting the assessment of potential associations between evidentiary tape samples. The carbon isotope ratios of tapes could provide additional discrimination among tape samples. To evaluate whether carbon isotope ratios may be able to increase discrimination of electrical tapes, particularly with regards to different rolls of tape of the same product, we assessed the δ(13)C values of backings from 87 rolls of PVC-based black electrical tape (~20 brands, >60 products) Prior to analysis, adhesives were removed to prevent contamination by adhering debris, and plasticizers were extracted because of concern over their potential mobility. This result is consistent with each of these tapes having approximately the same plasticizer δ(13)C value and proportion of carbon in these plasticizers. The δ(13)C values of the 87 PVC tape backings ranged between -23.5 and -41.3 (‰, V-PDB), with negligible carbon isotopic variation within single rolls of tape, yet large variations among tape brands and tape products. Within this tape population, carbon isotope ratios permitted an average exclusion power of 93.7%, using a window of +/-0.3‰; the combination of carbon isotope ratio measurement with additional chemical and physical analyses raises the discrimination power to over 98.9%, with only 41 out of a possible 3741 pairs of tape samples being indistinguishable. There was a linear relationship between the δ(13)C value of tape backings and the change in δ(13)C value with the extraction of plasticizers. Analyses of pre- and post-blast tape sample pairs show that carbon isotope signatures are within 0.3‰ of pre-blast values, indicating that carbon isotope values are largely preserved during an explosion.
对带有聚氯乙烯（PVC）背衬的胶带（电工胶带）进行法医鉴定的兴趣源于其在各种非法活动中的使用。由于单卷胶带的物理特性、化学成分和均匀性范围，传统的显微镜和化学分析可以在胶带之间提供高度的区分能力，从而能够评估证据胶带样本之间的潜在关联。胶带的碳同位素比率可以在胶带样本之间提供额外的区分能力。为了评估碳同位素比率是否能够增强对电工胶带的区分能力，特别是对于同一产品的不同卷胶带，我们评估了87卷基于PVC的黑色电工胶带（约20个品牌，>60种产品）背衬的δ(13)C值。在分析之前，去除粘合剂以防止粘附碎片造成污染，并提取增塑剂，因为担心其潜在的迁移性。这一结果与这些胶带中每种胶带具有大致相同的增塑剂δ(13)C值以及这些增塑剂中碳的比例一致。87个PVC胶带背衬的δ(13)C值在-23.5至-41.3（‰，V-PDB）之间，单卷胶带内的碳同位素变化可忽略不计，但不同胶带品牌和胶带产品之间存在较大差异。在这组胶带中，使用±0.3‰的窗口，碳同位素比率的平均排除能力为93.7%；碳同位素比率测量与其他化学和物理分析相结合，将区分能力提高到98.9%以上，在可能的3741对胶带样本中只有41对无法区分。胶带背衬的δ(13)C值与增塑剂提取后δ(13)C值的变化之间存在线性关系。对爆炸前后胶带样本对的分析表明，碳同位素特征与爆炸前的值相差在0.3‰以内，这表明碳同位素值在爆炸过程中基本保持不变。
