Characterization of document paper using elemental compositions determined by inductively coupled plasma mass spectrometry

Abstract

Comparison of the origin of successive pages of a multiple page document can provide important forensic evidence in cases of document fraud. The use of elemental compositions, determined by inductively coupled plasma mass spectrometry (ICP-MS), to characterize paper is reported. Samples of commonly used office document paper (white, A4, 80 gsm) from seventeen different sources, were analysed for their elemental composition, after microwave digestion in nitric acid–hydrogen peroxide solution. The limits of detection, instrumental precision and potential matrix interferences were established for the 23 most abundant elements found in paper. The variability of distribution for these elements within a ream of paper was determined. Nine elements (Na, Mg, Al, Mn, Sr, Y, Ba, La and Ce) were selected as suitable discriminators because their concentrations were sufficiently high and uniform. All seventeen papers could be distinguished by statistical t-tests (99% confidence) using only two of these elemental concentrations (Mn and Sr). The results are also presented using ternary plots which use three inter-element associations to display the differences in paper composition. Four separate batches of a single product line from the same mill, manufactured at monthly intervals, were also discriminated using elemental concentrations. Three unknown samples, chosen randomly from among the seventeen papers, were correctly identified from their elemental composition; two unknown samples from the four batches were similarly identified. The results show that elemental analysis, using ICP-MS, provides an effective and robust technique for the discrimination of document paper.