Hyperspectral Imaging of Gel Pen Inks: an Emerging Tool in Document Analysis
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Science and Justice 54 (2014) 71–80 Contents lists available at ScienceDirect Science and Justice journal homepage: www.elsevier.com/locate/scijus Emerging researcher article Hyperspectral imaging of gel pen inks: An emerging tool in document analysis G. Reed a,K.Savagea, D. Edwards b, N. Nic Daeid a,⁎,1 a Centre for Forensic Science, WestCHEM, Department of Pure and Applied Chemistry, The University of Strathclyde, Royal College Building, 204 George Street, Glasgow G1 1XW, UK b Foster and Freeman Ltd, Vale Business Park, Evesham, Worcestershire WR11 1TD, UK article info abstract Article history: Hyperspectral imaging (HSI) is a useful technique in the examination of writing inks, including gel pen inks, Received 30 April 2013 which combines digital imaging with % reflectance spectroscopy. This facilitates the detection of subtle differ- Received in revised form 26 August 2013 ences between chemically similar inks. This study analysed a variety of blue, red and black gel inks on white office Accepted 12 September 2013 paper using HSI. The potential of the technique for ink discrimination compared to other analytical methods of examination is highlighted. Discriminating powers of 1.00, 0.90 and 0.40 were achieved using HSI for red, blue Keywords: and black gel inks respectively. The overall discriminating power of 0.76 for the technique combined with its Gel pen ink Hyperspectral imaging non-destructive nature and minimal sampling requirements demonstrates promise for this type of application. Discrimination © 2014 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved. 1. Introduction (IR absorbance and IR luminescence) and/or ultra-violet (UV) illumina- tion conditions. Often this sufficiently distinguishes inks of similar ap- For over 60 years, the ballpoint pen has dominated the modern day pearance, and is preferred as it preserves the integrity and evidential pen market. However, this dominance is currently being challenged by value of the document. However, where inks remain physically indistin- the rapid growth in popularity of the gel ink pen [1].Thegelpen guishable, further chemical analysis involving a comparison of soluble emerged in the late 1990s following development by the Sakura Color dye components using thin layer chromatography (TLC) [10] is neces- Product Corps (Japan) as an improved alternative to the rollerball pen sary. Whilst highly discriminating and cost effective, the technique is in- [2,3]. The popularity of the pen can be attributed to an ability to deliver herently destructive. Furthermore, since gel inks predominantly use smooth, fast and consistent ink flow in an impressive array of perma- insoluble pigments to provide colour, TLC's discriminating capability is nent bright and traditional colours. The main and significant difference significantly reduced for many gel inks other than to confirm their between the formulations of gel pen inks in comparison to other writing pigmented nature. The need for a new analytical approach therefore ex- inks, including that of ballpoint pens, is in the predominant use of pig- ists if they are to be distinguished with the same high level of discrimi- ments rather than dyes as the colourant. nating power (DP) as other writing inks. General information about the type of chemicals present within gel A wide array of alternative analytical techniques have been applied pen inks is available and the inks tend to be aqueous based, with to writing inks, predominantly focussing on ballpoint inks due to their some containing as much as 80% water [1,4–9]. The colour is predomi- prevalence. Separation techniques focussing mainly on colourant com- nantly provided by microscopic sized (b0.5 μm) insoluble organic position including high performance thin layer chromatography and/or inorganic pigment particles which provide the inks with a (HPTLC) [11,12], high performance liquid chromatography (HPLC) wide array of bright colours, as well as the traditional blue and black [13,14], capillary electrophoresis (CE) [15] and gas chromatography [1,4]. More recent gel ink formulations are also known to contain dyes (GC) [16], like TLC, offer good discrimination but remain destructive instead of, or as well as, pigments (for example hybrid gel inks which by sample extraction. Spectroscopic techniques including UV–vis spec- contain both pigments and dyes [3]). Other ingredients may include troscopy [17–19], microspectrophotometry (MSP) [20],Fouriertrans- solvents, resins, lubricants, biocides, surfactants, corrosion inhibitors, form infrared spectroscopy (FTIR) [14,18,21], Raman spectroscopy sequestrants, sheer thinning agents, emulsifying agents, pH buffers [5,21–25], hyphenated mass spectrometry [26–35] and laser induced and adjusters, polymerisation agents and pseudoplasticizers [1]. breakdown spectroscopy (LIBS) [36] offer a greater potential for more Traditionally, ink analysis involves non-destructive visual examina- rapid, often in situ, analysis of minimal quantities of ink on paper mak- tion of the document by microscopy and filtered light using infrared ing discrimination based not only on colourant composition, but also additive and elemental content a possibility. UV–vis spectroscopy has shown discriminating powers of 0.79 and ⁎ Corresponding author. E-mail address: [email protected] (N. Nic Daeid). 0.96 for blue and black ballpoint inks respectively, although sample ex- 1 Tel.: +44 141 5484700. traction was required [18]. Similar inks analysed in situ on paper using % 1355-0306/$ – see front matter © 2014 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scijus.2013.09.005 72 G. Reed et al. / Science and Justice 54 (2014) 71–80 reflectance MSP achieved a satisfactory discriminating power of 0.83, grid. This provided an image width of 45.63 mm, sufficient to incorpo- benefiting from being non-destructive. However, compared to a dis- rate a sample grid in its entirety on the video screen. Gamma mode criminating power of 0.98–0.99 achieved for TLC in the same study pro- was switched off. Images of the sample grid were viewed via a continu- vided a more complimentary value rather than supplementary [20]. ously variable band pass filter placed in front of the light source, with a FTIR has also demonstrated high discriminating potential for blue (DP step width of 5 nm manually selected to generate a total of 121 images. 0.95) and black (DP 0.82) ballpoint inks, but again involved sample ex- Spectral data was acquired in reflectance mode. Images recorded for all traction [18]. More recently, blue ballpoint ink has been analysed in situ examinations were electronically stored for subsequent assessment. As- via an attenuated total reflectance (ATR) sampling interface [14].Qual- sessment was made by direct visual comparison of the spectra. itative comparison of peak absorbencies in the fingerprint region – −1 (1800 650 cm ), attributed to both colourant and resinous compo- 2.2. Samples nents of the ink, provided reasonable discrimination, supported by an – earlier study applying FTIR ATR to the discrimination of blue and In total, 42 different gel ink pens (15 blue, 13 red and 14 black) across fi black ballpoint and gel inks on paper [21].Howeverre nement of this 15 available brands were purchased from High Street and online retailers fi technique is required to overcome dif culties associated with strong within the United Kingdom. These colour groups were chosen to reflect spectral interference from the paper substrate and spectral acquisition the colours of ink most commonly encountered in case work. A variety from only limited quantities of ink likely to be typically encountered of common brands was purchased and summarised in Table 1. in forensic casework. Raman spectroscopy has shown great potential for pigmented gel inks achieving discriminating powers of 0.68 [24] and 0.76 [5] for blue gel inks using a dual excitation wavelength ap- 2.2.1. Preliminary solubility tests proach (514.5 nm and 830 nm). Whilst being rapid and non- Preliminary solubility tests were undertaken on all samples to deter- destructive, fluorescence interference arising from the sample and/or mine whether the inks were exclusively pigment only or contained dyes substrate however can be a major problem masking an already weak either on their own or in combination with pigments. ≥ Raman signal [25]. Quenching of fluorescence to increase sensitivity of The blue gel ink samples were extracted using acetic acid ( 99.8%), ≥ the Raman signal using surface enhanced resonance Raman spectrosco- red gel ink samples with acetone (puriss 99%) and black gel ink sam- ≥ py (SERRS) is possible but requires application of a colloid to the ink ples with methanol (puriss 99.7%). All extraction solvents were pur- reintroducing a destructive element [23]. Recent attention has focussed chased from Sigma-Aldrich. on the elemental content of gel inks, specifically with laser ablation– In each case, ink spots (~5 mm diameter) from each gel ink pen were fi inductively coupled mass spectrometry (LA–ICP–MS) and LIBS provid- deposited onto a piece of white A4 of ce paper. All ink samples were ing extremely high discrimination powers of 0.97–0.98 for black gel allowed to air dry for at least 15 min after which individual ink spots inks analysed in situ [36]. The application of multivariate statistical were cut out from the paper and placed into labelled small glass vials. – methodologies has also been explored to improve discrimination po- Approximately 0.5 1.0 mL of solvent was transferred to each vial, each fl tential of several analytical methods based on subtle spectral differ- vial shaken brie y by hand and allowed to stand for 5 min before noting ences with some success [13,14,17,19,34,35,37–40]. any colour change. The vials were then transferred to a pre-heated sand Hyperspectral imaging (HSI) is a technique which is beginning to bath for between 15 and 60 min depending on the colour group and sol- fl show great potential for the rapid, in situ analysis and discrimination vent combination.