The potential for application of ink stable isotope analysis in questioned document examination
Introduction
Understanding the authenticity and origins of documents is of widespread commercial and legal interest. As such, questioned document examination (QDE) is an important forensic discipline because it focuses on answering questions related to a document's history [1], [2]. Examiners often investigate paper documents or records to determine authenticity, such as the Hitler diaries “discovered” in East Germany in 1983 [3] or counterfeit currency surfacing in the global marketplace [4]. Document examination can also be useful in investigations of material origin, as when the FBI searched for the origins of the 2001 Amerithrax letters [5].
Questioned document examination is a comparative science in which trained examiners observe characteristic features of two or more samples in order to compare and contrast different documents. Casework investigations may involve scrutiny of the applied media (ink) and the substrate (paper), both of which can be examined using a variety of non-destructive and destructive analysis techniques. Non-destructive examination often relies on optical observation via microscopy, photography, and additional spectral analysis methods [6], [7], [8], [9]. Destructive QDE techniques typically rely on chemical analysis methods; examples include chromatography [10], [11], elemental analysis [12], and mass spectrometry [13], [14], [15].
Chromatography—specifically, high performance thin-layer chromatography (TLC)—is among the most common destructive analysis techniques used in QDE for identifying ballpoint pen inks because analysis is relatively straightforward and results are rapidly generated [16], [17], [18], [19]. Yet the identification of gel pen inks via TLC is often not possible [20]. This is because gel pen inks contain pigment-based colorants as opposed to the dye-based colorants found in most ballpoint pen inks [21]; these pigments are not soluble in the organic solvents associated with TLC analysis methods. Given the reduced potential for tampering, gel ink pens are marketed as an appealing choice for signing documents like wills or contracts. As such, document examiners frequently encounter gel inks during investigations [20]. However, authentication of text written with a gel ink pen is challenging because of (1) the difficulty in removing pigmented ink from the substrate once it is applied and (2) the lack of validated chemical analysis techniques to distinguish different gel inks. Several methods for characterizing and differentiating gel inks have been investigated [20], [22], [23], [24], but to date no single approach has been deemed preferable for the discrimination and identification of gel ink pens.
In this study we explored a novel chemical analysis technique for pen inks: stable isotope ratio analysis. One of the key features of stable isotope analysis is the ability to relate and distinguish chemically identical materials through measurement of naturally occurring small differences in the stable isotope abundances within the materials. Stable isotope analysis can provide an additional isotope “fingerprint” to further characterize a piece of evidence. Measurement of stable isotope ratios has previously proven useful in the investigation of many different materials of forensic interest [25], [26], [27]. For example, stable isotope analysis has been used to differentiate packaging tapes [28], [29] and paints [30] as well as drugs [31], [32], [33] and explosives [34], [35], [36]. Related to QDE, recent work by Jones et al. focused on the stable isotope analysis of commercially available office paper and demonstrated that carbon isotope ratios could be used to discriminate different papers [37], [38], [39].
Here we show the potential for stable isotope analysis to be applied as a quantitative testing method for characterizing pen inks during questioned document examination. We first investigated ink isotope ratio variation within packages of pens, by purchase location, and among pens of various ages before measuring the stable isotope ratios of pen inks surveyed in one location. We then focused on the stable isotope analysis of inks on paper. Finally, we present some limitations of the technique as explored in this work and discuss future work to further develop stable isotope analysis for application in QDE casework.
Section snippets
Stable isotope analysis and statistical analysis
Samples were analyzed for nitrogen (N), carbon (C), hydrogen (H), and oxygen (O) element concentrations and stable isotope ratios. Stable isotope ratios are reported in δ-notation, where δ = RA/RSTD − 1 and RA and RSTD are the ratios of the rare to common stable isotopes (e.g., 15N/14N, 13C/12C, 2H/1H, and 18O/16O) in the sample and an international standard, respectively. The international standard for nitrogen is atmospheric air (AIR); for carbon, it is Vienna-Pee Dee Belemnite (V-PDB). The
Within-package isotope ratio variation of pen ink
We used the stable isotope ratio data from 10 packages of ballpoint pens to calculate the within-package standard deviation (SD) for each of the four measured elements (N, C, H, and O). Standard deviations were 0.05‰–0.15‰ for δ15N, 0.02‰–0.15‰ for δ13C, 0.9‰–3.3‰ δ2H, and 0.26‰–0.98‰ for δ18O values (Table 1). We compared these within-package standard deviations to the standard deviations of the laboratory reference materials used to correct and normalize measured isotope ratio data (shown in
Characterization of pen ink using stable isotope ratio analysis
In this study, we explored the novel application of stable isotope analysis in questioned document examination for the characterization of ballpoint and gel inks. We showed it was possible to measure the stable isotope ratios of N, C, H, and O “as is” in all surveyed samples, establishing that this analysis technique could be broadly applied to both ballpoint and gel inks. We are unaware of stable isotopes ratio studies on pen inks—or, alternatively, other dyes or pigments—and we were thus
Acknowledgments
We thank Michael Lott for his assistance in instrument modification required for the analysis of pen ink on paper. Two anonymous reviewers provided feedback that improved this manuscript.
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