Ink dating, part I: Statistical distribution of selected ageing parameters in a ballpoint inks reference population
Introduction
Ink dating remains a particularly challenging forensic task. Despite almost a century of research [1], [2], [3], [4], only few methods have been reported as being used in practice [5], [6], [7], [8], [9], [10], [11]. This can mainly be explained by the complexity of the ageing processes and the amount of experiments needed to correctly interpret the results in actual caseworks. Several steps were previously described as minimum requirements to reliably estimate the age of an ink entry [12]: method development and validation, ageing parameters selection, data collection under different influencing factors and finally, data interpretation in a legal perspective. Indeed, a method must first be developed and validated in order to insure reliable and sensitive analysis of selected ink ageing parameters over time. This step generally represents the easiest task and is well described in the (forensic) literature [13]. More recently proposed approaches focused on the analysis of ink volatile compounds, and mainly phenoxyethanol (PE) and its decrease over time [5], [6], [7], [8], [9], [10], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. While gas chromatography coupled to mass spectrometry (GC/MS) was mainly used to analyse PE and other ink solvents, high performance liquid chromatography (HPLC) was also recently proposed to simultaneously analyse dyes and solvents [10], [11], [23]. Different extraction methods were also proposed such as liquid extraction [6], [8], [9], [12], [21], [22], [25], [26], [27], [29], [31], [32], thermodesorption [7], [18], [24], [33] or solid phase microextraction (SPME) ([17], [19].
Once the method is developed and validated, ageing parameters must be selected to monitor ageing according to the following criteria [12], [25], [34]:
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they must evolve as a function of time in a monotonic way (ascendant or descendent). The ageing must be measurable over a large time scale (ideally months to years for ink dating).
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they must be measurable in most ink formulations using the chosen method. It is generally known that ink composition has a significant influence on ink ageing [7], [31]. Thus, analysing representative ink reference populations is essential to insure that the selected ageing parameter can be implemented in most cases.
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they should be as reliable as possible yielding precise and accurate measurements, ideally even on different ink specimens of the same age.
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they should be influenced as little as possible by external factors typically encountered in caseworks (such as paper properties, transfer and storage conditions).
In a previous paper using liquid extraction followed by GC/MS [25], four ageing parameters were tested over time under different influence factors: the quantity of PE, different relative peak areas (RPA) and solvent loss ratios using two samples of the same ink entry, one of them artificially aged. These ratios were calculated from the quantities of PE (R%) or from the RPA values (R%*). While data was collected on a limited ink population (n = 3), obtained results were promising. RPA proved to be the most repeatable ageing parameter and the least influenced by the different tested conditions, the solvent loss ratios were the less reproducible parameters.
Finally, an adequate interpretation model must be developed to estimate the age of a questioned ink entry in a legal perspective. Several constraints are attached to this task such as the fact that the “source” ink or pen is generally unknown and thus, a representative ink population has to be considered to adequately interpret the results.
In this article, liquid extraction followed by GC/MS was used to analyse samples from 25 different inks provided by the LKA Münich. These were selected from a large ballpoint pen ink collection as representative of the ageing behaviours measured in previous studies [7], [35]. Ink lines were aged from 4 to 304 days on paper before analysis. Seven ageing parameters, including the PE quantity, the relative peak areas (RPA), three solvent loss ratios (R%, R%*, NR%) and two solvent loss parameters (Rnom and NRnom) were calculated. The first part of this paper focused on the evaluation of the collected data. The distribution of each ageing parameters over time was used to estimate their potential for ink dating purposes. The second part of this paper focused on testing different interpretation models on selected ageing parameters in order to reliably estimate the age of an ink entry in a legal perspective.
Section snippets
Material
Chloroform (99.9%) was purchased from Sigma Aldrich (Darmstadt, Germany). The reference substances phenoxyethanol (PE) 99.5%, benzyl alcohol 98% and heneicosane were purchased from Fluka (Buchs, Switzerland) and the internal standard was deuterated phenoxyethanol (PE-D2) from EGT Chemie (Tägerig, Switzerland),
For sample preparation, 1.5 ml vials, 300 μl and 50 μl inserts as well as springs were obtained from Swiss Lab (Basel, Switzerland).
Samples
The analysed samples were drawn with 25 different inks
Ink population composition
As expected, different ink formulations were observed for the 25 inks of the reference population using GC/MS (Table 1). 19 compounds were identified with the help of the NIST library in 4 days old samples. Among them, well known ballpoint pen ink volatile components were found such as benzyl alcohol (BA), hexylene glycol (HG), 2,2-ethoxyethoxyethanol (EEE), dipropylene glycol derivates (DGP 1, 2 and 3), phenoxyethanol (PE) and phenoxyethoxyethanol (PEE) [9], [25], [26], [31], [43], [44], [45].
Conclusion
This paper presented the ageing curves obtained from 25 ballpoint pen entries over one year. Seven ageing parameters were calculated, among these, four were previously proposed in the literature:
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the phenoxyethanol quantity (PE)
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the solvent loss ratio based on artificial ageing (R%)
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the solvent loss based on artificial ageing (RNORM)
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the solvent loss ratio based on natural ageing (NR%)
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the solvent loss based on natural ageing (NRNORM)
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the relative peak areas between compounds (RPA)
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the solvent loss
Acknowledgements
The authors wish to thank Dr. J. Bügler and Ms. A. Linder from the Landeskriminalamt of Munich for sharing their collection of ballpoint pen inks as well as for their precious help and availability.
The authors also wish to thank the Swiss National Foundation for its support in the frame of this research (no. PP00P1_123358 and PP00P1_150742).
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