A quantitative assessment of the chemical variation in food grade polyethylene cling film, a common wrapping material for illicit drugs, using attenuated total reflection-Fourier transform infrared spectroscopy

Abstract

Cling film is commonly encountered in forensic investigations as a wrapping for drugs of abuse. The evidential value of a match between a known and a questioned piece of cling film is directly related to the level of the chemical variation both within and between samples. However, little is known about the chemical variation in cling film. To address this, attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) was used to quantitatively assess the chemical variation in food grade low-density polyethylene (LDPE) cling film. Following acquisition of full scan IR spectra between 4000 and 650 cm⁻¹, a spectral region of potential discrimination between 1560 and 1200 cm⁻¹ was identified. The complex series of weak, overlapping absorption bands in this region were deconvoluted and peaks fitted. Bivariate plots comparing each pair of normalised peak areas showed significant overlap of the data points for the different samples, indicating no between-sample chemical variation. Further, the same plots demonstrated that the extent of within-sample chemical variation was at least as great as that between samples. Thus, in the sample set evaluated in this study, it was not possible to discriminate between food grade LDPE cling films using ATR-FTIR spectroscopy due to a lack of quantifiable chemical variation. The study therefore highlights the possibility of false positive matches in the analysis of LDPE cling films and provides a methodology to evaluate the extent of chemical variation in a relevant reference population prior to undertaking casework.