MeV TOF SIMS determination of deposition order between optically distinguishable and indistinguishable inks

In the forensic investigation of questioned documents, it is often very important to know the deposition order of ink traces from two different writing tools at their intersection on a paper. In the present work, intersections of inks from several writing tools were studied using optical techniques that are standardly applied for questioned documents examination in a forensic laboratory, and an accelerator-based Ion Beam Analysis (IBA) technique called Secondary Ion Mass Spectrometry using MeV ions (MeV SIMS) that is applied in an accelerator facility. MeV SIMS provides molecular information about the studied inks from writing tools, which is an added value and can be also applied for the determination of deposition order but was so far relatively rarely used in forensic studies. Aim of this paper is to compare performance of optical techniques and MeV SIMS for several combinations of intersecting lines. Cases were divided into those in which optical techniques can distinguish used inks and those which are optically completely indistinguishable. In the latter cases, we show that although mass spectra of used inks (from blue ballpoint pens) had extremely small differences, these in combination with advanced and most importantly objective multivariate algorithms could be very beneficial in resolving the deposition order at the intersection of optically indistinguishable inks. In general, MeV SIMS proved to be more efficient for oil-based inks while difficulties were encountered with water-based ones, similar to optical methods.

在可疑文件的法医检验工作中，明确两种不同书写工具在纸张上形成的墨迹交叠处的沉积顺序，通常具有至关重要的意义。本研究采用法医实验室常规用于可疑文件检验的光学检验技术，以及加速器设施中应用的基于加速器的离子束分析（Ion Beam Analysis, IBA）技术——即使用兆电子伏（MeV）离子的二次离子质谱（MeV SIMS）——对多种书写工具的墨迹交叠区域展开了研究。MeV SIMS可获取研究中所用书写工具墨迹的分子层面信息，这一特性具备额外应用价值，也可用于墨迹沉积顺序的判定，但目前在法医研究中相对较少得到应用。本文旨在针对多种交叠线条组合场景，对比光学检验技术与MeV SIMS的检验性能。本次研究将检验场景分为两类：一类可通过光学检验技术区分所用墨迹，另一类则无法通过光学手段完全区分墨迹。针对后一类场景，本研究表明：尽管所用蓝色圆珠笔墨迹的质谱差异极小，但结合先进且尤为关键的客观多元算法后，可有效解决光学上无法区分的墨迹交叠处的沉积顺序判定问题。总体而言，与光学检验方法类似，MeV SIMS对油性油墨的检验效果更佳，而在水性油墨的检验中则存在一定困难。