|Year : 2019 | Volume
| Issue : 2 | Page : 95-103
Application of resistance measurements of black gel ink to identify altered documents
Department of Forensic Science, Yantai Municipal Public Security, Yantai, Shandong, China
|Date of Web Publication||26-Jun-2019|
Department of Forensic Science, Yantai Municipal Public Security, Yantai, Shandong
Source of Support: None, Conflict of Interest: None
Identification of altered documents written in black gel ink is widely recognized as problematic, especially if both the original and altered strokes contain carbon. In this work, we used a black handwriting resistance measurement instrument to measure the resistance values of ink strokes. Because black gel inks are made with different types of pigments, their resistance values could be used to differentiate between different pen models from the same brand and among brands. In this study, 30 black gel inks were classified by their resistance values. In a pair-wise comparison analysis of the results, there were 435 possible pairs and 347 (i.e., 79.8% of the total) of these were able to be effectively distinguished. This is a nondestructive, convenient, and effective method to identify altered documents.
Keywords: Altered document, black gel pen ink, document examination, electric resistance
|How to cite this article:|
Han W. Application of resistance measurements of black gel ink to identify altered documents. J Forensic Sci Med 2019;5:95-103
| Introduction|| |
Forensic document examiners often encounter cases where some strokes of ink in a document have been altered. Ink analysis is an important step in forensic investigations of these suspicious documents and is commonly used in forensic science laboratories. In addition, to observing the inherent patterns in a handwriting sample, examiners will chemically analyze the inks present to determine the document's authenticity.
Gel pens are very popular for writing throughout the world. These pens were first developed by Sakura Color Products Corporation (Osaka, Japan) in 1984. Initially, this class of writing instrument used a highly viscous, pigment-based opaque ink, which did not tend to bleed into the paper fibers as much as water-based roller ball or porous-tipped pens. The modern black gel inks incorporate both dyes and pigments for coloring, are environmentally friendly, and do not contain volatile organic components. Unfortunately, the identification of the brand of gel ink is challenging because the chemical formulations of commercial inks are unknown.
Over time, the ratio of pigments to dyes in the gel inks has changed, and this has led to changes in the techniques used to analyze these inks. Several studies have investigated the identification of gel inks using various approaches, including thin-layer chromatography (TLC), mass spectrometry, Raman spectrometry,, infrared spectrometry, and scanning electron microscopy-energy dispersive X-ray spectrometry. Each of the available techniques has various advantages and disadvantages. For forensic analysis, a nondestructive technique is always preferred over a destructive one.
The identification of altered documents written in black gel ink is widely recognized as problematic, especially when both the original and altered strokes contain carbon. The optical properties and chemical and physical characteristics are helpful for analyzing some pen strokes, but are not useful for most carbonaceous pen marks such as those of black gel ink. In this paper, electric resistance measurements were used to identify documents written in black gel ink that contained alterations or additions. This technique was selected because it is nondestructive, requires minimal sample preparation, and is effective for identifying pigments in different types of samples.
Black gel ink contains high-quality acetylene carbon black, which is close to nanometer grade. It is completely integrated with the dye and auxiliary materials in the ink. Carbon black is an amorphous carbon of microcrystalline graphite with a planar sp2 structure and stable chemical properties. After writing, the carbon content in the strokes will remain basically constant.
Resistance measurements use the conductive property of carbon in carbonaceous pen marks. The carbon atom has a sp2 hybridized structure and is conductive. The resistance is given by R = ρ × L/S, where ρ is the resistivity, L is the length, and S is the cross-sectional area. If L and S are constant, then ρ corresponds to R. Because the ink formulation varies among different models and brands of pens, the resistivity should also change. This method can be used to classify black gel inks as carbonaceous or not carbonaceous based on whether a resistance value can be measured or not. The resistance values of carbonaceous ink strokes can also be used to differentiate among gel inks and identify strokes written with the same ink.,
| Materials and Methods|| |
Thirty black gel ink samples from different brands and models of pens were selected for this study [Figure 1] and [Table 1]. Each pen was used to write its sample number and some handwriting on a piece of paper [Figure 2]. The writing samples were kept in the dark and allowed to dry.
A black handwriting resistance measurement instrument [Figure 3] for sample analysis was developed by the People's Public Security University of China and Hong-cai-mo-shi Technology Co., Ltd in a Ministry of Public Security and Beijing School Enterprise Cooperation Project.
|Figure 3: Black handwriting resistance measurement instrument: (a) workstation platform, (b) measuring head, and (c) measuring probe|
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The black handwriting resistance measurement instrument components are a main control part (host), resistance measurement head, measuring platform, display screen, and data storage module (hard disk). The core parts are the host and the head. The host contains the system and the software. Probes are installed in the measuring head and it is equipped with a high-speed video converter and infrared irradiation device. The probes are connected to the host by a USB and round connector and are made of gold-plated phosphor copper. There are two probes on the head, which are 2 mm apart, and each probe is in a U-shape for the purposes of the measurements.
Instrument operation involved opening the instrument, starting the software, and then placing the metal probe on the selected black strokes using video observations. Data were then recorded. Each pen was used to write ten strokes, which were then analyzed.
An appropriate measurement position allows for the acquisition of stable and reliable data. It is important to select pen strokes that have evenly distributed pigment with uniform thickness, density, and gloss. If the stroke width is too small, then the pigment concentration will be low and could cause errors. To achieve this, it is important, not to select points in the writing that have gaps, pauses, turns, or overlapping strokes. If the paper surface is uneven, there could be many obvious grid points or dots. The grid points change the resistance value. The same measurement position should be used on the suspect pen strokes and those that are thought to be original. To evaluate the measurement position [Figure 4], the device was used to examine the digit “1” that had been written with a black gel pen and then altered to the digit “4.”
|Figure 4: Examination of altered text written with a black gel pen (1 changed to a 4)|
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The resistance measurement workstation has two measurement modes: single measurement and continuous measurement. For the former, multiple manual measurements need to be conducted to obtain stable resistance data. For the latter, the instrument automatically conducts ten continuous measurements at a single point and produces a normalized spectrum of resistance values [Figure 5].
| Results and Discussions|| |
To ensure the validity and consistency of results, the device was calibrated daily before sample analysis. The instrument could measure up to 2000 MΩ, and the resistance values of the ink strokes were divided into five grades: <10 MΩ, 10–100 MΩ, 100–1000 MΩ, 1000–2000 MΩ., and values that could not be measured are indicated using a dash.
The resistance values of the 30 black gel ink pens are shown in [Table 2]. Among the samples, 23.3% (samples 2, 5, 6, 7, 8, 13, and 19) had resistance values in the 1000–2000 MΩ range, 23.3% (samples 3, 10, 11, 12, 18, 21, and 30) in the 100–1000 MΩ range, 10% (samples 1, 27, and 28) in the 10–100 MΩ range, and 16.7% (samples 4, 15, 20, 26, and 29) <10 MΩ. Resistance values could not be measured for 26.7% (samples 9, 14, 16, 17, 22, 23, 24, and 25) of the samples. Analysis by infrared optical examination and TLC confirmed that the inks in samples 9, 14, 16, 17, 22, 23, 24, and 25 were pure dye-based inks with no carbon, and that the rest of the inks contained carbon.
Next, the resistance values of ten strokes written by the same black carbon-containing ink were measured. As examples of the results, for sample 7, the resistance fluctuated between 1270 and 1860 MΩ. For sample 10, the resistance fluctuated between 180 and 710 MΩ. For sample 29, the resistance fluctuated between 0.5 and 4.7 MΩ. Only sample 8 was an exception, the resistance fluctuated between 300 and 1900 MΩ and distributed in the 100–1000 MΩ range and 1000–2000 MΩ range.
Criteria for determining whether two inks could be differentiated were developed using the resistance output of the corresponding normalized spectral data. Strokes written with carbonaceous inks will have measurable resistance, whereas those not written with carbonaceous inks will not have resistance. More than 70% of the black gel pen strokes in this study had measurable resistance, and a few of the black pen strokes had no resistance. Different carbon inks also had different resistance values. Because a resistance value could be measured for most of the black ink strokes, this method is viable for writing an analysis.
The experimental data are based on the resistance values measured at different positions. It is affected by many factors, among which the correlation of some experimental data is poor, which affects the analysis of experimental data. Therefore, the average value processing method is adopted in this paper to analyze the distribution range of measurement value, so as to establish judgment criteria.
Set reference value V¯: Omit a maximum and a minimum value, then calculating the data set on average. In the formula, “S-Max-Min” refers to the sum of the valid values and “n” refers to the number of measurements
Data distribution interval K: The minimum value (V min.) and the maximum value (V max.) in each set of data were compared with V¯. Analyze the data distribution interval K (V min./V¯ V max./V¯). The statistical results are as follows:
- If the data in the 1000–2000MΩ, K [0.82, 1.19]
- If the data in the 100–1000MΩ, K [0.35, 1.96]
- If the data in the 10–100MΩ, K [0.53, 1.61]
- If the data are below 10MΩ, K [0.25, 2.0].
The criterion for classing strokes as being written by different inks was that the resistance values were in different grades. If the resistance values for the different ink strokes were in the same grade, then no conclusions could be drawn.
In a pair-wise comparison analysis, there were 435 possible pairs for the 30 ink samples. Among these pairs, 347 (i.e., 79.8% of the total) were able to be distinguished effectively and 88 pairs could not be distinguished (i.e., 20.22% of the total) [Table 3].
The resistance measurement method could be used to measure the resistance values of ink strokes and identify document falsification by comparing the values of suspect strokes with other strokes.
The 30 gel pens were compared using infrared optical analysis, TLC, and Raman spectroscopy [Table 4] to evaluate the abilities of these methods to distinguish among ink samples.
Infrared optical examination
Under infrared light (λ >800 nm), the ink strokes appeared either distinct, faded, or faint [Figure 6]. In a pair-wise comparison of the color differences between the 435 possible pairs of ink samples, 249 pairs (i.e., 57.2% of the total) were able to be distinguished effectively and 186 pairs (i.e., 42.8% of the total) could not be distinguished.
|Figure 6: Infrared optical examination of ink from the 30 black gel pens. Under infrared light (λ = 856 nm), the ink strokes appear as distinct (A), faded (B), or faint (C)|
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Thin-layer chromatography analysis
Because of differences in the dye and pigment compositions of the ink samples, three types of results could be obtained after addition of chemical reagents: not unfolded, partially unfolded, and unfolded [Figure 7]. In a pair-wise comparison of the results for 435 possible pairs of the ink samples, 284 pairs (i.e., 65.3% of the total) were able to be distinguished effectively, and 151 pairs (i.e., 34.7% of the total) could not be distinguished.
|Figure 7: Thin-layer chromatography analysis of black gel ink from seven different pens. The results are not unfolded (A), partially unfolded (B), and unfolded (C)|
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The Raman spectra of 30 kinds of ink show four types of spectra: I, II, III, and IV [Figure 8]. According to the results of the analysis, 435 pairs were obtained from 30 ink samples, of which 324 pairs (i.e., 74.5% of the total) were able to distinguish effectively, and 111 pairs (i.e., 25.5% of the total) could not be distinguished.
|Figure 8: Four types of Raman spectra of 30 pieces of black gel pens ink|
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A comparison of the infrared optical, TLC, Raman spectroscopy, and resistance measurement results showed the resistance measurement method was the best for distinguishing among ink samples (79.8%), followed by Raman spectroscopy (74.5%), TLC (65.3%), and infrared optical analysis (57.2%) [Figure 9].
|Figure 9: Comparison of the rates for distinguishing between pairs of samples using the four test methods|
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Each of the available techniques has various advantages and disadvantages. For forensic analysis, a nondestructive technique is always preferred over a destructive one. Resistance measurement is a nondestructive testing method. In particular, the results show that the combination of resistance measurement and other techniques, are complementary and mutual proved, providing an accurate and handy method of black gel pen identification. It can be used to distinguish the questioned documents in forensic science, enhancing the validity of evidence.
Application of case
In a case involving a falsified document, a client asked a testing agency to identify if the black handwritten Chinese character “五” was changed from “三” [Figure 10].
Under infrared light from a strong 725-nm light source with an 856-nm longpass filter, each stroke of the character in question appeared complete and distinct. There were slight differences in color both between the three horizontal strokes and between the horizontal strokes and vertical strokes. Because color differences were observed both between what were thought to be the original strokes and those that were thought to be falsified, we could not determine if the color differences were caused by the changes in writing conditions with the same pen or by the use of two pens containing different pigments. We could conclude that all strokes of character were written with pens containing carbon black ink.
First, we conducted measurements on each stroke of the suspect character. We then obtained resistance values to prove all strokes of the character were written with pens containing carbon black ink. Next, we performed resistance measurements for each stroke in the character and found the values for the three horizontal strokes were stable at 100 MΩ and those for the two vertical strokes were stable at 1200–1400 MΩ [Figure 11]. Finally, we measured the resistance values of other characters in the target document, such as “九” “七” “十” “金” and “县” and found that all these strokes were stable at 100 MΩ [Figure 12].
|Figure 11: The normalized spectral of the resistance value and the resistance measurement points of each stroke of “五”|
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|Figure 12: The resistance measurement points of other writing samples about the inspected materials and their resistance value of the normalized spectral|
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Multispectral technology verification
We classified the strokes for spectral analysis into three categories: the three horizontal strokes of character in question, the two vertical strokes of the character in question, and strokes in all the other characters in the document (e.g., “七” and “九”). The strokes in the first and third categories gave basically the same results but were different to those in the second category in the 650–700 nm range [Figure 13].
Result of analysis
The resistance measurement examination shows that each stroke of the “五” has a stable resistance. The resistances of the three horizontal strokes and the two vertical strokes of the “五” are greatly different, which reflects the different carbon content of the two pigments. The multispectral testing as a verification shows that each stroke of the “五” has a stable spectrogram. The spectrograms of the three horizontal strokes and the two vertical strokes of the “五” display significantly differences, which shows that the two pigments are different in brightness distribution of the reflected light., The difference in resistances by the resistance measurement examination comes to the conclusion that the “五” is a changed handwriting originally from the “三”.
| Conclusions|| |
The resistance measurement method not only can distinguish the carbon in the strokes, but also can determine the amount of carbon. In questioned document examination, the resistance value of suspected stroke compared with other stroke, if the value of the difference was significant and stable, then can identify suspected stroke is altered, and can determine the file is fake.
The electric resistance measurement examination is nondestructive, fast and accurate, and suit the detection of the altered document. This method breaks through the limitation of morphology inspection in detecting changed handwriting written by black gel ink which contains carbon. The examination is based on quantitative detection, which makes the final more scientific and reliable.
This study was supported by the Program of the Major Research and Development Projects of Shandong Province (Grant No. 2017GSF20110).
Financial support and sponsorship
This study was supported by the Program of the Major Research and Development Projects of Shandong Province (2017GSF20110).
This article was accepted by the 3rd International Symposium on Sino Swiss Evidence Science (3rd ISSSES), held by China University of Political Science and Law.
Conflicts of interest
There are no conflicts of interest.
| References|| |
Wilson JD, LaPorte GM, Cantu AA. Differentiation of black gel inks using optical and chemical techniques. J Forensic Sci 2004;49:364-70.
Gerandt MN, Urlaub JJ. An Introduction to the Gel Pen. American Academy of Forensic Sciences Meeting, Seattle; 1995.
Johnson CE, Martin P, Roberts KA, Trejos T, Corzo R, Almirall JR, et al.
The capability of Raman microspectroscopy to differentiate printing inks. J Forensic Sci 2018;63:66-79.
Papson K, Stachura S, Boralsky L, Allison J. Identification of colorants in pigmented pen inks by laser desorption mass spectrometry. J Forensic Sci 2008;53:100-6.
Jones RW, Cody RB, McClelland JF. Differentiating writing inks using direct analysis in real time mass spectrometry. J Forensic Sci 2006;51:915-8.
Bell S, Stewart S, Ho YC, Craythorne B, Speers S. Comparison of the discriminating power of Raman and surface-enhanced Raman spec-troscopy with established techniques for the examination of liquid and gel inks. J Raman Spectrosc 2013;44:509-17.
Claybourn M, Ansell M. Using Raman spectroscopy to solve crime: Inks, questioned documents and fraud. Sci Justice 2000;40:261-71.
Zieba-Palus J, Kunicki M. Application of the micro-FTIR spectroscopy, Raman spectroscopy and XRF method examination of inks. Forensic Sci Int 2006;158:164-72.
Trejos T, Corzo R, Subedi K, Almirall J. Characterization of toners and inkjets by laser ablation spectrochemical methods and scanning electron microscopy-energy dispersive X-ray spectroscopy. Spectrochim Acta B 2014;92:9-22.
Jiantong H. Questioned Document Examination. 1st
ed. Beijing: Chinese People's Public Security University Press; 2003. p. 266-7.
Chuan D, Jianhui W, Shaomin S. Ink Chemical Theory and Application. Beijing: Science Press; 2007. p. 43.
Jiantong H. The Theory and New Technology of Atlered Document Examination. 1st
ed. Beijing: Chinese People's Public Security University Press; 2016. p. 67-79.
Jiantong H. Discussion on the principle of resistance measurement to examine altered handwriting. Chin Peoples Public Secur Univ J 2010;1:5-8.
Wei H, Guiqiang W. Research on the application of hyperspectral imaging technology in forensic science. J Forensic Sci Technol 2006;2:1-4.
Guiqiang W. Optical and Photographic Techniques on Fingerprints. Beijing: Masses Publishing House; 2001. p. 125-411.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]
[Table 1], [Table 2], [Table 3], [Table 4]