Science and Justice 54 (2014) 71–80

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Science and Justice

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Emerging researcher article Hyperspectral imaging of gel : 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 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 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 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 , 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. Vials were removed and shaken brie y again, and any of gel pen inks with significant advantages over other existing methods. further change in colour was noted. The paper substrate was also HSI is a combination of two techniques, digital imaging and reflectance extracted in a similar way for each solvent. The presence of colour in or fluorescence spectroscopy and facilitates the examination of a docu- the solvent indicated the presence of a dye as a component of the ink. ment across a wide wavelength range to produce a digitally stored image cube [41,42]. The intensity of each pixel in each image can be 2.2.2. Sample preparation plotted as a function of wavelength, producing a % reflectance spectrum Three sample grids were prepared on a single sheet of plain white A4 of the ink at any given wavelength detecting subtle spectral differences office paper. Ink samples from each pen were added to a sample grid so between chemically similar inks [43]. The potential for enhanced that each grid represented a particular colour group. discrimination together with its non-destructive nature and minimal Ten spectra were acquired from different areas of un-inked (blank) sample preparation requirements makes HSI a promising analytical paper within each of the three sample grids to monitor for any spectral technique for ink discrimination, that to date has focussed primarily interference arising from the paper substrate. Ten spectra were acquired on ballpoint pen inks [7,44–47]. from the same area of a single blue gel ink (UNI UK) sample and The aim of this study was to investigate the discriminating potential of HSI for a number of gel inks on paper representing three common col- Table 1 ours encountered in casework. Summary of gel ink pens analysed by brand and colour.

2. Materials and methods Brand and model Ref. code Colour of pen Blue Red Black

2.1. Instrumentation BIC Reaction BIC UK * * * Papermate Gel 2020 PPM UK * * * HSI examinations were performed using a Foster and Freeman VSC Parker Reflex PKR UK * Hybrid Gel DX Rollerball PTL UK * * * 6000/HS containing a video camera, various light sources, optical filters G2-07 Retractable PLT UK * * * and a high resolution grating spectrometer. The instrument was operat- Stabilo Point Visco STB UK * * * ed in HSI mode, calibrated for the entire spectral range (400–1000 nm) Triplus STD UK * * * against a blank piece of A4 white office paper. Flood lighting provided by Uniball Gel Impact UM-153S UNI UK * * * a 100 W halogen spot light (capable of producing light across the 400– Jimnie ZBR UK * * * fl Great Expressions GRE UK * * * 1000 nm visible and infrared (IR) range) was used to produce re ected Inoxchrom Short Roller ICM UK * * * light for visible and IR illumination conditions. Auto exposure (integra- Partners Broad PTN UK * * * tion times 1.6–2.0 ms (dependent upon colour of ink examined), iris: Staples Sonix STP UK * * * 81%) and auto focus modes (magnification: 7.1) were switched on WH Smiths WHS UK * * * Works Essentials WKE UK * * with the default brightness value (60) used for viewing each sample G. Reed et al. / Science and Justice 54 (2014) 71–80 73

Blue Ink Group

Fig. 1. Imagery data of blue gel inks on paper under visible and IR illumination conditions in sequence at 400 nm (top left), 450 nm, 500 nm, 600 nm, 700 nm, 750 nm (top right), 800 nm, 850 nm, 900 nm and 1000 nm illustrating the discrimination of STB UK and ICM UK (450 nm), and PKR UK (700–750 nm), from all other brands. Further discrimination of the remaining brands was possible between 700 and 900 nm. Between 500 and 700 nm, all inks were strongly absorbing, and beyond 900 nm all inks were fully reflecting. 74 G. Reed et al. / Science and Justice 54 (2014) 71–80 from ten different areas of writing produced by each of the gel pen 3.3.2. Red ink group brands analysed across all three colour groups to assess measurement Discrimination of the majority of 13 red gel ink samples was possible repeatability. within the 550–650 nm spectral region. Between 400 and 550 nm, all Discriminating power (DP) was calculated according to the method red ink samples were strongly absorbing with the exception of the STB previously described by Smalldon and Moffat [48] where; UK brand. Beyond 650 nm, all ink samples were fully reflecting. The STB UK brand was the only red ink to be identified as solely pigment DP = No. of discriminated sample pairs/no. of all possible pairs. based, and was distinguishable from all others even at 400 nm. At 560 nm, three other ink brands (BIC UK; GRE UK and PTN UK) began 3. Results and discussion to show a subtle change in reflectance, followed by four other brands (PLT UK; WHS UK; PTL UK and STD UK) at 570 nm. The subtle nature 3.1. Preliminary solubility tests of the changes observed emphasises the point that a degree of subjective interpretation is involved in using hyperspectral imaging for this type of Solubility testing indicated that the blue gel pen group could be dis- analysis. By 590 nm all ink samples were reflecting to different degrees, criminated into 11 pigment and 4 dye containing inks; the red group with STB UK almost fully reflecting. By combining the various responses into 1 pigment and 12 dye containing inks; and the black group into the 13 red gel inks could be tentatively discriminated into 10 groups. 11 pigment and 3 dye containing inks. These observations are presented in the Supplementary material.

3.2. Imagery and spectral differences 3.3.3. Black ink group Discrimination of a limited number of the 14 black gel ink samples The data are described in three ways; through the examination of could be achieved between the 700 and 900 nm spectral regions. In par- the visible and infrared absorption response alone (imagery data), the ticular the STB UK; ICM UK and PLT UK brands, all dye containing, be- examination of the % reflectance spectra alone (spectral data) and the came less absorbing and fully reflecting by 900 nm. Within these combination of both imagery and spectral data. three brands subtle differences were also observed as the wavelength of illuminating IR light changed between 800 nm and 900 nm. These 3.3. Imagery data — discrimination using visible and infrared illumination observations are presented in Fig. S2 in Supplementary material. conditions In general, the interpretation of the imagery data on its own was subjective with subtle differences being observed in many cases be- 3.3.1. Blue ink group tween the behaviour of the individual inks. However, the technique Two spectral regions across all 15 of the blue inks tested (400– does provide an opportunity to investigate the changing appearance 500 nm and 700–900 nm) revealed the greatest variation under visible of generated spectra across relatively small increments. The discrimi- and IR illumination conditions. Between 500 and 700 nm, all inks were nating power of the imagery data on its own was calculated as 0.85 strongly absorbing, and beyond 900 nm all inks were fully reflecting. for the blue pens, 0.96 for the red pens and 0.40 for the black pens Three of the four dye containing inks (STB UK, ICM UK and PKR UK) with an overall discriminatory power of 0.73. could be differentiated easily from all others. In particular, the STB UK and ICM UK ink samples exhibited notable fading at 450 nm, whilst all 3.4. Spectral data — %reflectance spectra other samples did not and the PKR UK ink sample that began to fade at 700 nm and by 750 nm was the only ink fully reflecting. All other The % refl ectance data produced within the HSI mode of the instru- blue inks exhibited some degree of absorbance in this region. The imag- ment facilitated a more objective approach to sample measurement. Ini- ery data on its own facilitated the classification of the 15 blue gel pens tial repeatability and reproducibility studies demonstrated excellent into a total of 6 groups. Images of the response of all of the blue gel results indicating no instrumental variation and only very minor varia- pens across selected visible and IR wavelengths are presented in Fig. 1. tions in spectral shape.

Artefact peaks

Fig. 2. Asetof%reflectance spectra (n = 10) acquired from different areas of blank paper illustrating artefact peaks around 800 nm attributable to the cross-over points of the three RGB channels of the camera. G. Reed et al. / Science and Justice 54 (2014) 71–80 75

3.4.1. Paper background and/or curve and λmax positions. The variability in spectral shape be- Some spectral variation was observed from un-inked (blank) areas tween all 15 brands of blue gel ink is presented in Fig. 3 and areas of spe- of the paper substrate and the resultant peaks at 800 nm appeared to cific variability are between 400 and 500 nm and again primarily above be artefacts arising from spectral measurements taken using a three 700 nm. This spans the regions of variability observed with the imagery chip RGB camera, an example of which can be seen in Fig. 2. data for these samples. The artefact peaks can be noted in some samples (Groups 1, 2, 4 3.4.2. Blue ink group and 6) at 800 nm, but not in others (Groups 3 and 7). Based on the spec- The % reflectance spectra representing all of the blue gel pens reveal tral data alone, it was possible to discriminate the 15 blue group ink some brand discrimination based on differences in spectral shape, slope samples into a total of 7 groups.

Blue Ink Group

Fig. 3. A single % reflectance spectrum from each of 15 brands of blue gel ink on the same axis illustrating the similarity and differences between them (top). The greatest variation occurs within the 400–550 nm (bottom left) and 700–1000 nm (bottom right) spectral regions, with artefact peaks at ~800 nm in some ink spectra highlighted within the red circle in the latter. 76 G. Reed et al. / Science and Justice 54 (2014) 71–80

Group 1, containing the dye containing PKR UK brand was readily pen brands. Similar to the blue ink group, the artefact peaks at 800 nm distinguishable from all other brands by the shape of the spectral line can be observed in some samples (Groups 1, 2 and 5) and again not in (730–800 nm) which exhibited a rounded appearance as it emerged others (Groups 3, 4 and 6). Spectral data discriminated the red pen from a steep slope. group into a total of 6 groups. The STB UK and ICM UK brands, also dye containing, formed Group 2, The STB UK brand, identified as the only pigment based ink, exhibit- and were distinguishable by the point at which the spectral line began ed a squared rather than curved spectral shape (550–600 nm) which to rise up into a steep curve, 630–670 nm compared to N690 nm for distinguished it from most other brands to form Group 1. most other brands. Similar to Group 1 spectral shape, Group 2 consisting of only the ICM Group 3 included the last remaining dye containing ink, PLT UK, UK brand, also displayed a squared spectral appearance (550–600 nm), which was indistinguishable from the pigment based PTN UK brand. but could be distinguished by a trough with a flat rounded bottom curv- The spectral line for these two brands displayed a shallow gradient ing up into a steep slope (500–550 nm) in the latter, compared to the from ~600 nm before curving up into a steep slope (~700 nm) eventu- presence of a shoulder in the former. ally returning to a shallow gradient (~800 nm) and flattening out A similar feature was also observed in Group 3 (PLT UK) spectra, but N850 nm. was distinguishable from Group 2 by a gradually rounded curvature of Group 4 (BIC UK and STD UK) revealed a more curved appearance to the spectral line at the top of a steep slope (550–650 nm). the spectral line. Group 3–6 spectra inclusive were all considered to be very similar to The overall spectral shape of Group 5 (GRE UK; WKE UK; WHS UK; one another at first glance, but were distinguishable on closer inspec- and STP UK) ink samples distinguished them easily from all other tion. The presence of a small peak (~780 nm) and the absence of a shal- brands by the presence of a shallow gradient to the slope (700– low trough (900–1000 nm) distinguished Group 4 (PTL UK; PTN UK; 900 nm), rounding off at ~800 nm into a steadily rising % reflectance in- STP UK; WHS UK; and GRE UK) from Groups 3, 5 (ZBR UK; BIC UK; tensity (800–1000 nm). and STD UK) and 6 (PPM UK and UNI UK). Furthermore, the apparent Group 6 (PTL UK and ZBR UK) exhibited a small downward shift to steepness to the rounded curve to the spectral line above a steep slope

λmax in the 400–500 nm region, occurring at ~440 nm compared to (600–650 nm) provided further discrimination between these four 450–460 nm for the majority of other brands, and an upward shift to groups when considered in combination with the aforementioned ~470 nm for Group 5 samples. distinguishing features. Finally, the appearance of the spectral line The final group, Group 7 (PPM UK and UNI UK), was easily discrim- (400–500 nm) aided further discrimination, specifically between inated from Groups 1, 2, 4 and 6 by a generally more rounded spectral Group 3 and Group 6, where in the latter, % reflectance intensity was appearance (600–1000 nm). Likewise, Group 3 and Group 7 could be minimal giving a flatter impression to the spectral line. It is worth not- readily distinguished by spectral shape N700 nm. Group 3 exhibited ing that this last feature of discrimination was subtle, and could argu- an apparent greater steepness of slope (700–800 nm) and generally ably be considered a subjective interpretation. As with the blue ink flatter appearance N800 nm, whilst the steep slope in Group 7 spectra group, the findings from the imagery data provided further support extended from 700 to 880 nm before rounding off into another shallow for the brand discriminations based on subtle spectral differences, in slope. Group 5 and Group 7 displayed a highly similar spectral shape this case, that the two brands represented by Group 6 were distinguish- that was arguably indistinguishable. However, the former exhibited a able from those in Group 3. subtle more, smoother rounded spectral shape N750 nm. Table 3 provides a summary of the key distinguishing spectral fea- Table 2 provides a summary of the key distinguishing spectral fea- tures that discriminated the red ink group. tures for the 7 blue ink groups.

3.4.4. Black ink group 3.4.3. Red ink group Eleven of the 14 black pen brands exhibited flat and featureless The red ink group spectra revealed differences in spectral shape and spectra unsuitable for discrimination. The three dye containing brands slope with Fig. 4 illustrating the variability in spectral shape of all 13 red exhibited distinctly different spectra with spectral features above

Table 2 Summary of distinguishing spectral features for the blue ink group samples.

Group Brand Artefact peaks Distinguishing features no. present at 800 nm

1STBUKYes 630–670 nm: spectral lines start to curve up into a steep slope distinguishing it from most other brands whose spectral lines generally start ICM UK to curve up beyond ~690 nm 2 PKR UK Yes 600–800 nm: spectral line curves up into a steep slope with a rounded top (~730–800 nm) distinguishing it readily from all other brands 3PLTUKNo 600–800 nm: spectral line sloped with small gradient before curving up into a steep slope (~700 nm) PTN UK 800–850 nm: spectral line returns to a slope of shallow gradient 850–1000 nm: spectral line exhibits relatively flat appearance 4BICUKYes 600–800 nm: spectral line starts to curve up into a steep slope (~700 nm) distinguishing it from Group 1 (~630–670 nm) which otherwise STD UK has similar overall spectral appearance 800–850 nm: spectral line exhibits curved appearance compared to a sloping appearance of other brands 5GREUKNO 400–500 nm: small shift in peak maximum (~470 nm) compared to majority of other brands (~450–460 nm) WKE UK 600–1000 nm: spectral line exhibits a lesser gradient to slope (~700–900 nm) compared to other brands, rounding off into steadily WHS UK increasing % reflectance intensity (N800 nm) providing further discrimination STP UK 6PTLUKYes 400–500 nm: lower % reflectance intensity with small shift in peak maximum (~440 nm) compared to majority of other brands ZBR UK (~450–460 nm) giving an overall flatter appearance to spectral line 7PPMUKNo 600–1000 nm: spectral line exhibits a generally more rounded appearance with increasing % reflectance intensity distinguishing it readily UNI UK from Groups 1, 2, 4 and 6 700–1000 nm: spectral shape and lesser gradient to steepness of spectral line (~700–800 nm) and continuation of slope (~800–900 nm) before curving off (~900–1000 nm) distinguishes it from Group 3 750–1000 nm: highly similar overall spectral shape to Group 5, but distinguishable by a subtle smoother rounded appearance to shape of spectral line in the latter G. Reed et al. / Science and Justice 54 (2014) 71–80 77

Red Ink Group

Fig. 4. A single % reflectance spectrum from each of 13 brands of red gel ink illustrating the similarity and differences between them (top), with specific areas of variation occurring between 500–650 nm (bottom left) and 900–1000 nm (bottom right).

600 nm permitting discrimination. Fig. 5 illustrates the variability in 3.5. Combined imagery and spectral data spectral shape between the three dye containing brands and the 11 pig- ment based samples. The discrimination of each brand of gel pen ink within each colour The discriminating power of the spectral data alone was calculated based on visible and IR absorption response and/or spectral differences as 0.90 for the blue pens, 0.82 for the red pens and 0.38 for the black in their % reflectance spectra was assessed and the results are presented pens with an overall discriminatory power of 0.70. in Table 4. 78 G. Reed et al. / Science and Justice 54 (2014) 71–80

Table 3 Summary of distinguishing spectral features for the red ink group samples.

Group no. Brand Artefact peaks Distinguishing features present at 800 nm

1 STB UK Yes 550–600 nm: shoulder in spectral line before curving up into a steep slope 550 –600 nm: squared edge to curve followed by a flat appearance (~650 nm) distinguishes it from majority of other brands 2ICMUKYes500–550 nm: a flat rounded curve into a steep slope distinguishes it from Group 1 550–600 nm: squared edge to curve followed by flat appearance (~650 nm) similar to Group 1, but distinguishes it from majority of other brands 3 PLT UK No 500–550 nm: flat rounded curve before entering a steep slope similar to Group 2 550–600 nm: gradual rounded curve to spectral line distinguishes it readily from Group 2 4 PTL UK Yes 600–650 nm: smooth rounded curvature at the top of a steep slope to, highly similar to Group 3, but distinguishable by spectral PTN UK features N 750 nm, i.e. presence of a peak (~780 nm) and absence of a shallow trough (~900–1000 nm) giving a flatter appearance STP UK to spectral line WHS UK GRE UK 5 ZBR UK Yes 600–650 nm: lesser gradient to steepness of curve distinguishes from Group 4 BIC UK 750 nm–1000 nm: the absence of a peak (~780 nm) and the presence of a trough (~900–1000 nm) provides further discrimination STD UK 6 PPM UK No 600–650 nm: subtle difference in steepness of curve gradient, i.e. greater gradient gives impression of greater drop to curve, similar UNI UK for both Group 4 and Group 6, but the latter distinguishable from the former by spectral features N 750 nm, i.e. absence and presence of peak (~780 nm) and trough (~900–1000 nm) respectively 400–500 nm: distinguishable from Group 3 by a flat sloping appearance to spectral line with minimal % reflectance intensity, otherwise highly similar overall spectral shape

When the imagery and spectral data are used in combination, dis- data. Table 5 summarises the discriminating powers calculated by each crimination of the blue pens into 8 groups (from 15 different brands) individual colour group and all three combined for each of the imagery and the black pens into 4 groups (from 14 different brands) was data, spectral data and both imagery and spectral data combined. achieved. The red pens were completely discriminated with all 13 When comparing these discriminating powers against other analyt- pens distinguished from each other. The discriminating power was ical techniques used for gel ink analysis it is clear that for blue gel inks in the order of red (1.00) N blue (0.90) N black (0.40) for the combined in particular HSI offers superior discriminating potential. Traditional

Black Ink Group

PLT UK

ICM UK

STB UK

Fig. 5. A single spectrum from each of 14 brands of black gel ink illustrating the similarity and differences between them. The spectra of the dye containing STB UK, ICM UK and PLT UK brands can be clearly distinguished from that of the pigment based gel inks which appear flat and featureless. G. Reed et al. / Science and Justice 54 (2014) 71–80 79

Table 4 Summary of classification groupings by colour group based on imagery data, spectral data and combined imagery and spectral data * PLT UK contained dye components, whilst the other inks in the same grouping contained only pigments.

Colour Imagery data Spectral data Combined HSI data group Sample Group Colourant Sample Group Colourant Sample Group Colourant

Blue PKR UK 1 Dye containing PKR UK 1 Dye containing PKR UK 1 Dye containing STB UK; ICM UK 2 STB UK; ICM UK 2 STB UK; ICM UK 2 PLT UK*; PTN UK; BIC UK 3 *Dye containing PLT UK*; PTN UK 3 *Dye containing PLT UK*; PTN UK 3 *Dye containing 7pigment pigment Pigment BIC UK; STD UK 4 Pigment BIC UK 4 Pigment GRE UK; WKE UK; WHS UK; 4 Pigment GRE UK; WKE UK; 5 STD UK 5 STP UK; STD UK WHS UK; STP UK GRE UK; WKE UK; 6 WHS UK; STP UK PTL UK; ZBR UK 5 PTL UK; ZBR UK 6 PTL UK; ZBR UK 7 PPM UK; UNI UK 6 PPM UK; UNI UK 7 PPM UK; UNI UK 8 Red STB UK 1 Pigment STB UK 1 Pigment STB UK 1 Pigment BIC UK; PTN UK 2 Dye containing ICM UK 2 Dye containing ICM UK 2 Dye containing PLT UK 3 PLT UK 3 GRE UK 3 PTL UK; PTN UK; STP UK; 4 BIC UK 4 PLT UK; WHS UK 4 WHS UK; GRE UK STD UK 5 PTN UK 6 PTL UK 5 ZBR UK; BIC UK; STD UK 5 GRE UK 7 STD UK 6 WHS UK 8 ICM UK 7 STP UK 9 PPM UK 8 PPM UK: UNI UK 6 ZBR UK 10 UNI UK; STP UK 9 STD UK 11 UNI UK 12 ZBR UK 10 PPM UK 13 Black WKE UK; BIC UK; PPM UK; 1 Pigment WKE UK; BIC UK; PPM UK; 1 Pigment WKE UK; BIC UK; PPM UK; 1Pigment PTL UK; PTN UK; STP UK; PTL UK; PTN UK; STP UK; PTL UK; PTN UK; STP UK; WHS UK; GRE UK; UNI UK; WHS UK; GRE UK; UNI UK; WHS UK; GRE UK; UNI UK; ZBR UK; STD UK ZBR UK; STD UK ZBR UK; STD UK STB UK 2 Dye containing STB UK 2 Dye containing ICM UK 3 STB UK; ICM UK 2 Dye containing ICM UK 3 PLT UK 4 PLT UK 3 PLT UK 4

filtered light examination has been shown to offer a discriminating The potential of HSI for the analysis and discrimination of red gel ink power of 0.72 for blue gel inks, whilst using Raman spectroscopy offers is clear to see, although arguably limited by the inherent subjectivity of only a marginal improvement of 0.76. In contrast scanning electron mi- the technique. Application of a multivariate statistical methodology croscopy (SEM) used to distinguish gel inks on the basis of ink line mor- would greatly assist in providing a more objective interpretation of phology has a significantly higher discriminating potential (DP 0.88) [5], HSI examinations, confirming the discriminating potential of this prom- but requires the use of large expensive instrumentation, which in com- ising technique. parison to HSI, may limit its viability for routine questioned document applications. Development of a multivariate statistical methodology may improve 4. Conclusion the discrimination potential of HSI for blue gel inks even further. Such a methodology has recently been developed for data generated from the The discriminating potential of HSI over other analytical techniques laser desorption/ionization mass spectrometry (LDI–MS) analysis of for gel ink analysis has been investigated. Discrimination by HSI relies blue [34] and black [35] gel inks on paper. Based on Pearson correlation upon the opinion of the examiner however some objectivity is provided coefficients, maximum discriminating powers of 0.92 and 0.85 respec- by the % reflectance measurements. The ability to observe subtle tively were achieved. For the blue gel inks analysed as part of this changes in the responses of the ink under visible and IR illumination study, HSI compares well to the combined discriminating potential of at specificwelldefined wavelengths in combination with comparison LDI–MS and multivariate profiling. For black gel inks however, HSI ap- of spectral differences between inks, greatly assists discrimination. pears to offer only limited discrimination potential against even the HSI provided a complete discrimination of all red gel inks examined, more traditional analytical techniques of filtered light examination and was also highly discriminating for all blue gel inks examined, (FLE) (DP 0.49) and MSP (DP 0.74) [35]. For the black gel inks used in achieving a discriminating power of 0.90. However, HSI performed this study it seems unlikely that multivariate profiling would add fur- poorly in discrimination of the black gel inks. The interpretation of HSI ther value since the majority did not provide a % reflectance spectrum data from gel pen inks, specifically for blue and red colours, could be fur- suitable for discrimination. ther developed by using suitable multivariate statistical methods simi- lar to those applied in combination with other instrumental analytical techniques.

Table 5 Summary of discriminating powers by colour group for imagery, spectral and combined Acknowledgments imagery and spectral data.

Colour group Imagery Spectral Combined The authors would like to acknowledge Foster and Freeman Ltd. and Blue 0.85 0.90 0.90 the Engineering and Physical Science Research Council (EPSRC) for Red 0.96 0.82 1.00 jointly funding this project through a Royal Society of Chemistry Analyt- Black 0.40 0.38 0.40 ical Chemistry Trust Fund (RSC/ACTF) Studentship awarded to Graham Combined 0.73 0.70 0.76 Reed. 80 G. Reed et al. / Science and Justice 54 (2014) 71–80

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