- Email: [email protected]
Application of laser induced breakdown spectroscopy to examination of writing inks for forensic purposes
Science and Justice 54 (2014) 118–125
Contents lists available at ScienceDirect
Science and Justice journal homepage: www.elsevier.com/locate/scijus
Application of laser induced breakdown spectroscopy to examination of writing inks for forensic purposes Agnieszka Kula a, Renata Wietecha-Posłuszny a,⁎, Katarzyna Pasionek a, Małgorzata Król a, Michał Woźniakiewicz a, Paweł Kościelniak a,b a b
Jagiellonian University, Department of Analytical Chemistry, Laboratory for Forensic Chemistry, 3 Ingardena St., 30-060 Krakow, Poland Institute of Forensic Research, 9 Westerplatte St., 31-033 Krakow, Poland
a r t i c l e
i n f o
Article history: Received 20 July 2013 Received in revised form 24 September 2013 Accepted 27 September 2013 Keywords: Forensic science Questioned document examination Writing inks Laser induced breakdown spectroscopy Ink analysis Elemental analysis
a b s t r a c t The research was focused on the analysis of writing inks using the laser induced breakdown spectroscopy (LIBS) technique. 34 samples of blue, 30 of black, and 21 of red writing inks were analyzed under optimized conditions to determine the variation of chemical composition of inks between different colours, brands and types. Nine elements (Ba, Cr, Cu, Fe, Li, Mo, Mn, Ni and W) were taken into account during comparative analysis of inks. Because of the strong effect of the paper spectrum, elements often found in inks (Ca, Al, Mg, Na, Ti, and Si) were eliminated from LIBS analysis of inks. It was determined that the LIBS method is capable of revealing qualitative elemental differences between ink samples. The discrimination power of this method was found to be 83, 82 and 61% for blue, black and red inks, respectively. Inks produced by the same producer were able to be differentiated in some cases. The results showed the potential of LIBS for forensic purposes as an effective and robust technique, requiring a small amount of sample and giving analytical information in a very short time. © 2013 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction In the modern world of computers, scanners and printers, the number of handwritten documents has been drastically reduced. However, handwritten notes, wills and prescriptions continue to be widely used by people and a signature is still required as proof of consent in many cases. Therefore (alleged) forgeries of documents written by hand are the focus of many civil and criminal investigations. One of the most important aspects of questioned document examination is comparison and/or identification of inks on the basis of their chemical composition. Writing inks are composed of colourants (dye and/or pigment), a solvent known as a “vehicle” and additives (responsible for the physicochemical properties of inks). All ingredients in the ink mixture can yield important analytical information to the forensic document examiner. However, the wide variety of dyes (natural or synthetic, organic or inorganic) and pigments (metallic, organometallic or organic) that can be present in different combinations in inks make this group of substances the most useful in comparative analysis. A number of analytical techniques have been proposed to distinguish between different writing ink formulations. Those allowing the non-invasive examination of ink components are the techniques of
⁎ Corresponding author. Tel.: +48 12 6632084; fax: +48 12 6340515. E-mail addresses: [email protected] (A. Kula), [email protected] (R. Wietecha-Posłuszny), [email protected] (K. Pasionek), [email protected] (M. Król), [email protected] (M. Woźniakiewicz), [email protected] (P. Kościelniak).
choice. Microscopic observation, UV and IR radiation as well as Raman spectroscopy (RS) are commonly applied for this purpose [1–4]. However, in some cases further investigations employing methods having a (destructive) effect on the structure of the document are needed. Separation techniques such as thin layer chromatography (TLC) or high performance thin layer chromatography (HPTLC) [1,5–9] and capillary electrophoresis (CE) [10–15] are widely used discriminatory tools that are applied to a variety of inks. They are based on the screening of ink sample dye composition. In recent years, mass spectrometry (MS) coupled with various ionization techniques has more often been employed for comparison and identification of ink components [8,16–18]. Despite the constant development of new techniques for the chemical analysis of inks, TLC and its high performance version are still recognized as the standard methodology in this field. Weyermann et al. [8] reported the application of two techniques: laser desorption ionization mass spectrometry (LDI-MS) and HPTLC for differentiation of 31 blue ballpoint pen inks. Both methods were characterized by high efficiency (92% of pairs of inks discriminated by HPTLC and 99% for LDI-MS). In some cases complementary results were obtained. However, the drawbacks of the HPTLC technique (complex sample preparation, long analysis time) made LDI-MS the preferred technique by the authors for differentiating inks. There are only a few techniques allowing analysis of the elemental composition of inks. One such technique is particle induced X-ray emission (PIXE), which, together with CE, was applied to determine the elements present in fountain pen inks [10] and ballpoint pen inks [11]. In
1355-0306/$ – see front matter © 2013 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scijus.2013.09.008
A. Kula et al. / Science and Justice 54 (2014) 118–125
the investigated fountain pen inks, both non-metals (S, Br, Cl, I) and metals (K, Ca, Cu, Fe, Cr, Zn) were detected. The elemental profile for each ink only allowed 3 out of 12 samples to be distinguished. Most of the analyzed ballpoint pen inks were characterized by the presence of copper, zinc, nickel and lead, and in some cases iron, titanium, chromium and cobalt were identified. A comparison of the samples was conducted on the basis of the elemental characteristics of those inks and the Cu/Zn signal ratio. It was found that PIXE yielded results that were complementary to the CE results. Zięba-Palus and Kunicki [4] reported the use of the X-ray fluorescence technique (XRF) in combination with micro-Fourier transform infrared spectroscopy (micro-FTIR) and RS for examination of ballpoint pen and gel pen inks. The XRF examination revealed the presence of sulphur, copper, silica, and phosphorus in all the studied ballpoint pen inks, and zinc, chlorine, bromine, calcium, chromium and lead in some of the samples, but on the whole quantitative rather than qualitative differences between inks were observed. The best discrimination power of the applied XRF method was revealed in relation to the group of gel pen inks. It should be stressed that both utilized methods, PIXE and XRF, provide results that are insufficient for differentiation of analyzed writing inks, and so sequential analysis together with other techniques (CE, micro-FTIR, RS) is required. Comprehensive analysis of molecular structure and elemental composition of writing inks could be performed using time of flight secondary ion mass spectrometry (TOF-SIMS). Coumbaros et al. were able to differentiate between blue ballpoint pen ink samples from 6 different manufacturers, using the results of molecular and elemental characteristics for each sample [19]. Discrimination of inks within a manufacturer was possible with the use of only elemental composition analysis. Denman et al. developed the TOF-SIMS method (supported by statistical analysis of results) for the study of 24 blue ballpoint pen inks [20]. Separated molecular or elemental characteristics made it possible to distinguish ink samples in ~60%. Combined results allowed an increase in the percentage of differentiated samples to 91%. The laser induced breakdown spectrometry technique (LIBS), in which a short laser pulse is focused onto a sample surface, is noteworthy. As a result, a small sample mass is ablated and high-temperature plasma, containing excited atoms and ions (from the analyzed sample), is generated. Electrons of excited species falling down to a ground state cause light emissions with discreet spectral wavelengths (lines). Each element is associated with unique spectral lines, allowing its identification. In recent years, there have been major developments in LIBS, but it is not yet well established as a technique for the forensic examination of the elemental composition of inks. However, LIBS fulfils the basic requirements of questioned document examinations: firstly, simultaneous multi-elemental analysis of inks can be performed without any preparation of samples (directly on paper), and secondly, the surface area interrogated by the laser pulse involves a very small amount of tested material, making LIBS a semi-destructive technique. The literature reports numerous applications of the LIBS technique to analysis of colourants in works of art, archaeological remains and illuminated manuscripts [21–24]. The LIBS technique has also been employed to study pigments and binders present in paper [25–28]. There are only two reports demonstrating the possibilities of LIBS in the analysis of contemporary writing inks [28,29]. In these procedures, LIBS was only a supplementary technique to laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or RS. Furthermore, examinations were only conducted for a limited number of inks, 11 black gel inks and 10 black and blue inks, respectively. Trejos et al. were successful in discrimination of 96.4% of all possible pairs of samples with the use of emission lines of only 4 elements. On the other hand, Hoehse et al. concluded that separate LIBS data (revealing the presence of Cu, Ti, K, Ca, Na, Li, and Al in analyzed inks) were insufficient to discriminate the sample set. Nevertheless, these investigations showed the potential of utilizing the LIBS method in the field of document examinations for forensic purposes. The aim of this paper was to perform more systematic studies with the use of the LIBS method and to verify the reliability of this analytical
119
tool in the examination of writing inks for forensic purposes. Several dozen inks of three colours (blue, black, and red) and of different brands and types were analyzed for the presence of nine elements (Ba, Cr, Cu, Fe, Li, Mo, Mn, Ni and W). Such writing tools as ballpoint pens, gel pens, porous point pens and rollerball pens were included in the study. The discrimination power of the developed LIBS method for writing inks was defined. 2. Materials and methods 2.1. Samples 34 blue, 30 black and 21 red writing instruments of various types (40 ballpoint pens, 29 gel pens, 6 porous point pens and 10 rollerball pens) produced by 17 different manufacturers from France, Japan, Korea, Poland and the USA were randomly collected from stationery shops in Poland during a 4 month period. A complete list of pens is presented in Table 1. A4 sheets of commercially available standard white office paper of five different brands made by three different producers: Polspeed (80 g/m2), Pollux (80 g/m2) and Poljet (90 g/m2) by International Paper (Poland), Opti Grat (80 g/m2) by Papyrus (Sweden), and Presentation (100 g/m2) by Navigator (USA) were investigated. Copper(II) phthalocyanine was obtained from Aldrich Chem Co. (USA). The ink depositions on paper were made in the form of straight lines, applying normal hand pressure. All ink samples placed on paper were stored in plastic bags, in the dark at room temperature. 2.2. Instrumentation An LIBS-6 system manufactured by Applied Photonics (United Kingdom) equipped with a Q-switched Quantel Ultra Nd:YAG laser operating at 1064nm (Quantel, France) was used. The laser emitted pulses of about 6ns duration with energy per pulse 150mJ and caused ablation spots with an average diameter of 1mm. An Avaspec-2048-2-USB2 fibre optic Czerny-Turner spectrometer with CCD detector (Avantes, The Netherlands) was used, which was able to record emission spectra across wavelength ranges 255–416 and 496–718 nm, with spectral resolution of 0.1 nm. Under normal conditions, the spectrometer did not need wavelength calibration because it had no moving elements inside. The system was equipped with a camera that enabled observation of the analyzed object and aiming of the laser beam. The analyzed objects were placed at the focal point of the focusing lens (approximately 70mm from the optical head). Experiments were carried out in ambient air. The LIBS-6 system was supported by LIBSoft V6.O software. Plasus SpecLine 2.13 software was used for spectral analysis and line identification of recorded data. 3. Results and discussion 3.1. Optimization of LIBS method Optimization of the developed method was performed using writing inks applied directly on paper. The aim was to obtain a reasonable signal intensity for the elements originating from components of writing inks and low signal intensity for the elements originating from paper. This was an important issue because discriminatory elements present in the analyzed samples mainly occurred in trace amounts. The amount of a sample consumed in the analysis and data quality was also taken into account. The power of laser shots (expressed by the Q-switch delay [μs]) was optimized using blue ballpoint pen ink (B7). To this end, a series of measurements in the range of Q-switch delay from 135 to 255 μs. The value 165 μs was found to be the best in terms of the signal intensity and amount of ablated paper. A study of detector parameters, including gate delays (1.27–5.00 μs) and integration time (1.1–2.0 ms), was also performed. In both cases,
120
A. Kula et al. / Science and Justice 54 (2014) 118–125
Table 1 List of pens examined in the study. Type of pen
Ballpoint pens
Gel pens
Porous point pens Roller-ball pens
Blue inks (B)
Black inks (K)
Red inks (R)
Company/model
Sample ID
Company/model
Sample ID
Company/model
Sample ID
Bic/Atlantis Bic/Cristal Medium Bic/ReAction Cresco/Exclusive Easy/Galaxy Easy/Oval Easy/Silver Easy/Style Empen/WZ-2011D Karin/113 BNP Lecce/Pen Lexi/5 Mattel/Hot Wheels Patio/Erase It Pentel/BK-77 Superb Pilot/Rexgrip Toma/Superfine 069 Uni-ball/Jetstream Bic/Cristal Gel Easy/Follow Easy/Massy Easy/Vestis Handy/Gel Patio/Nano Pilot/G-1 Pilot/G-1 grip Tetis/KZ107-N Uni-ball/Signo Pilot/Frixion point Pilot/G-TEC-C4 MonAmi/VuRiter Pentel/Bln75-C Pilot/V ball grip Uni-ball/UB-150
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23 B24 B25 B26 B27 B28 B29 B30 B31 B32 B33 B34
Bic/ReAction Cresco-Exclusive Easy/Silver Easy/Style Paper Mate/Eraser.Max Handy/Salsa ball Karin/113 BNP Lexi/5 Patio/Erase It Patio/Vigo Pentel/BK-77 Superb Pilot/Rexgrip Toma/Superfine 069 Uni-ball/Jetstream
K1 K2 K3 K4 K5 K6 K7 K8 K9 K10 K11 K12 K13 K14
Easy/Retro Easy/Silver Karin/113 BNP Patio/Vigo Pentel/BK-77 Superb Pilot/Rexgrip Toma/Superfine 069 Uni-ball/Jetstream
R1 R2 R3 R4 R5 R6 R7 R8
Bic/Cristal Gel Easy/Follow Easy/Massy Handy/Gel Handy/Intense Gel Pilot/Alphagel Pilot/Frixion ball Pilot/G-1 Pilot/G-1 grip Tetis/KZ107-V Pilot/G-TEC-C4 Tetis/KC030-V MonAmi/VuRiter Pentel/Bln75-A Pilot/V ball grip Uni-ball/UB-150
K15 K16 K17 K18 K19 K20 K21 K22 K23 K24 K25 K26 K27 K28 K29 K30
Easy/Follow Easy/Twist Easy/Vestis Handy/Salsa gel Pilot/Frixion ball Pilot/G-1 Pilot/G-1 grip Tetis/KZ107-C Uni-ball/Signo
R9 R10 R11 R12 R13 R14 R15 R16 R17
Pilot/G-TEC-C4 Pilot/V5 Hi-tecpoint MonAmi/VuRiter Pentel/Bln75-B
R18 R19 R20 R21
extension of time resulted in decreasing signal intensities for elements present in the ink. Therefore, measurements were carried out with a fixed gate delay of 1.27 μs and integration time of 1.2 ms. It was observed that a writing ink created such a thin layer on the paper substrate that a single laser shot was powerful enough to penetrate through it into the paper. Consequently, the acquired emission spectra had signals characteristic for both the ink components and the paper substrate. For this reason, the scan mode was applied during measurements, i.e. the ablation point was changed after each laser shot, and an average spectrum was created. To ascertain the optimum number of laser shots needed per ink, an investigation was performed using a blue gel pen sample (B25). It was observed that a spectrum based on 1 to 5 shots caused significant enhancement of the signal to noise ratio. Additional shots resulted in too much destroyed analyzed sample without any improvements in intensity of signals originating from the ink depositions. Assuming a compromise between data quality, analysis time and sample destruction, data were collected from 5 different positions along the ink line with only one laser pulse delivered at each point. The repeatability of intensity values was studied for exemplary ink samples B11, K3 and R16. For this purpose, intensities of lines at 327.4 (Cu), 403.1 (Mn) and 520.8 (Cr) for samples B11, K3 and R16, respectively, were used. The experimental intensities were obtained from 5 measurements each at a different position, with a single laser shot at each point. The results showed that the intensities of the chromium lines strongly varied from one measurement point to another, with a CV value of approximately 18, 24 and 35% for selected lines for Cu, Mn and Cr, respectively. The heterogeneity of the sample composition deposited on the paper and the laser shot-to-shot fluctuation caused poor reproducibility of the signal intensities. In order to capture the qualitative variability of the results obtained for different analytical processes, the developed measurement procedure
was carried out three times for 10 ink samples (randomly selected: B6, B16, B20, B22, K4, K13, K25, R7, R14, and R15), by two analysts every four months. The elements detected for analyzed samples were reproducible. 3.2. LIBS analysis of paper Emission spectra for 5 analyzed papers were recorded under optimized conditions of the LIBS technique. The spectrum of Polspeed paper is shown in Fig. 1 as an example. Spectra obtained for all papers were very rich, containing lines from neutral species (I), singly charged ions (II) and molecules (see Fig. 1). Spectra for all brands of paper revealed the presence of calcium, aluminium, magnesium, sodium, titanium and silicon. Emissions of diatomic species CN, Ca2 and C2 formed by collisions in the plasma were also observed in ranges 382–389 nm, 545–558 nm and 592–613 nm, respectively. The characteristic lines for Ca and Ti were related to calcium carbonate (extender used in paper) and titanium dioxide (white pigment), respectively. It was noticed that no qualitative, but only quantitative differences in elemental composition between analyzed paper brands were observed. However, the presence of titanium (lines in the windows in Fig. 1) in a given sample was detected only at selected measurement points. In effect, two kinds of spectra characteristic for paper components could be observed — with and without lines belonging to titanium. This shows that titanium (in the form of white pigment titanium dioxide) is not evenly distributed throughout paper sheets and special caution should be taken when lines originating from this element are absent in a spectrum. Because no differences in elemental profiles of the examined papers were observed, only the Polspeed paper was chosen for further study.
A. Kula et al. / Science and Justice 54 (2014) 118–125
121
Fig. 1. Spectrum of Polspeed paper.
The variation of elemental composition within reams of Polspeed paper was also studied. No distinctions between 5 sheets belonging to the same ream of this paper were found. Thus, emission spectra for writing ink samples could be collected from lines deposited on different sheets of the Polspeed paper. In order to minimize the effect of the paper spectrum, it was decided to search for lines derived from the ink components in some selected ranges that were relatively free of paper components, i.e. 320–420, 505–530, 560–600 and 660–715nm. In some cases, the rich paper spectrum could cause masking of lines originating from elements which may be present in ink samples (e.g. zinc or lead). Moreover, elements identified in paper were eliminated from qualitative analysis of the studied inks. Despite this, distinctive lines were detected for almost all inks examined. 3.3. LIBS analysis of inks The variation in elemental composition of inks taken from the same package (the same model and brand) was evaluated. The investigation was conducted with the use of 8 B11 pens (blue ballpoint pens), 3 K22 pens (black gel pens) and 3 R17 pens (red gel pens). No significant differences were observed between spectra of inks originating from pens of a given model. Hence, a single pen of each model was taken for further examinations. A total of 85 considered writing instruments (noted in Table 1) were used to identify elements present in the investigated inks and to test the ability of the LIBS method to discriminate between ink samples. In order to determine the threshold of significant line intensity, the instrumental noise was established. To this end, the biggest value of threefold standard deviation for intensities of signals at noise level, from three wavelength ranges (337–340, 508–515 and 660–666 nm) for 5 paper samples was calculated (684 V). To find emission lines characteristic for each ink composition, the spectrum of paper was subtracted from each sample spectrum. Disclosed lines of elements present in ink samples, with a signal height of at least 684 V, were taken into account during the identification process. Significant lines observed in the emission spectrum of each sample were identified using the National Institute of Standards and Technology (NIST) spectral database in wavelength ranges ±0.1 nm and standard dye — copper(II) phthalocyanine (in the case of Cu). The presence of two different persistent lines allowed positive identification of a given element
in the analyzed sample. The spectrum of water solution of copper(II) phthalocyanine deposited on paper and the spectra of exemplary samples B18 (without any element identified) and B24 (containing copper) are shown in Fig. 2. Table 2 summarizes the elemental profiles of the inks under investigation (listed in Table 1) obtained from LIBS analysis. The analysis of persistent lines of the set of 85 inks from pens of various brands and models disclosed 8 elements in the examined samples: Ba I (at 705.9 nm), Cr I (at 359.4, 360.5, 520.5 and 520.8 nm), Cu I (at 324.7, 327.4, 510.6, 515.3 and 521.8 nm), Fe I (at 344.1, 373.5, 374.9 and 404.6 nm), Li I (at 670.7 nm), Mn I (at 403.1, 403.3 and 403.4 nm), Mo I (at 317.0, 319.4, 390.3 and 407.0 nm), Ni I (at 341.5 and 352.4 nm) and W I (at 361.7, 400.9 and 407.4 nm). The presence of barium and lithium in the analyzed samples was verified by a single persistent line (705.9 and 670.7 nm, respectively). However, the line at 670.7 nm was assigned only to the persistent line of Li I, and neutral species of barium were also confirmed by a non-persistent line at 577.7 nm. As an example, the spectra obtained for samples B6, K5 and R12 are shown in Fig. 3. In the case of blue inks, various elements identified allowed the samples to be divided into 10 groups. One group included five inks which did not reveal any extra element in comparison with the paper spectrum. 15 out of 34 samples had in their composition only one examined element amongst copper, chromium and lithium. Sample B14 revealed a unique composition, with almost all the examined elements; barium, molybdenum and tungsten were detected exclusively in this ink. The most numerous group, characterized by copper, contained 11 inks (Fig. 2, spectrum c). Copper was the most commonly identified element in blue inks. It was detected in 24 blue inks of three types: ballpoint pen, gel pen and roller-ball pen inks. This confirms that the copper-containing colourants (such as Pigment Blue 15 or Solvent Blue 38) are currently frequently used to impart a blue colour to inks. Lithium occurred in 13 blue ink compositions of all types except gel inks (Fig. 3, spectrum a). For black inks, 11 different elemental profiles were obtained (see Table 2). A group with no distinctive elements comprised 12 pens. In turn, there were six one-ink groups amongst the black inks. There was no element that was characteristic for black inks (such as copper in the case of blue inks); however, there were three elements frequently detected: chromium (8 inks), manganese (8 inks) and copper (6 inks). Molybdenum found in sample K5 gave this ink composition individual properties (see Fig. 3, spectrum b).
122
A. Kula et al. / Science and Justice 54 (2014) 118–125
Fig. 2. Spectra of a) a water solution of blue pigment copper(II) phthalocyanine, b) ballpoint pen ink from a blue Uni-ball Jetstream pen (B18), c) gel pen ink from a blue Patio Nano pen (B24).
The red inks were divided into 5 groups, taking into account their elemental profiles (see Table 2). In the case of this colour, the number of inks indistinguishable from the paper substrate was revealed as the greatest. For these inks, the red ink colour could be the result of using cationic dyes like Rhodamine B. For the rest of the red inks, at least chromium was detected.
Table 2 Elemental profiles obtained by LIBS analysis for blue (B), black (K) and red (R) inks. Samples
Detected elements
B3, B8, B11, B13, B16, B19, B22, B24, B27, B28, B34 B25, B26 B29, B31 B2, B4, B5, B9, B10, B12, B15, B17 B7, B21 B32 B1 B6 B14 B18, B20, B23, B30, B33 K3, K19, K24, K29 K2, K10, K13 K12, K20, K30 K27 K6, K17 K4 K15 K26 K5 K9 K1, K7, K8, K11, K14, K16, K18, K21, K22, K23, K25, K28 R6, R16, R20 R1, R4 R10 R15 R12 R2, R3, R5, R7, R8, R9, R11, R13, R14, R17, R18, R19, R21
Cu Cr Li Cu, Li Cu, Mn Cr, Mn Cu, Fe, Li Cu, Fe, Li, Ni Ba, Cr, Cu, Fe, Li, Mn, Mo, W No element Mn Cu Cr Li Cr, Mn Cr, Cu Cu, Li Cr, Li Cu, Mn, Mo Cr, Fe, Mn No element Cr Cr, Cu Cr, Fe Cr, Li Cr, Fe, Mn No element
It can be seen in Table 2 (compared with Table 1) that most blue ballpoint pen inks (except sample B18) had copper in their composition. Looking now at gel inks, only three elements (Cu, Mn, and Cr) were present in different combinations in the blue gel inks (B19–B28). Apart from these three elements, one additional – lithium – was detected in black gel inks (K15–K24). No further links between the composition of inks and their type were observed. Inks of the same colour and type do not exhibit any characteristic elemental composition. However, there are cases of groups of inks (e.g. the group of 8 blue ballpoint pens containing Cu and Li, or the group of black ballpoint pens containing Cu) that can be easily distinguished from inks of the three other types. A similar situation, with no links between elemental profile and type of ink, is observed when comparing inks of different colours but the same type. Only blue and black roller-ball pen inks and black and red ballpoint pen inks had common elemental profiles, which did not occur in a group of inks of the third colour (profiles: Li for roller-ball pens and Cr with Cu for ballpoint pens, respectively). This confirmed that black inks may contain components typical for both blue and red inks. The d-block elements such as Cr, Fe, Mn, Mo and Ni detected in examined inks may be contained in metallic or organometallic compounds responsible for ink colours. Lithium is more often used instead of sodium and potassium as a counterion for anionic dyes. Based on qualitative elemental analysis, ink samples were differentiated from each other within groups of inks of the same colour. Inks within each group within a given colour group could not be distinguished from each other, but they could be differentiated from inks from the other groups. Multi-elemental samples of pen inks of all colours (B1, B6, B14, K5, K6, K9 and R12), containing from 3 to 8 elements, were easily distinguishable from the other samples. The discrimination power (DP) was calculated as the ratio of the number of discriminated pairs to the number of all possible pairs of inks of a given colour. The DP values were found to be 83, 82 and 61% for blue, black and red inks, respectively. Concerning ballpoint and gel pen inks of blue, black and red colours, the DP values were 75, 86 and 61% and 73, 80 and 69%, respectively. The variation of the elemental composition of inks produced by different companies was also investigated. The analytical results obtained for inks of Bic, Pilot and Easy companies are presented in Table 3.
A. Kula et al. / Science and Justice 54 (2014) 118–125
123
Fig. 3. Spectra of inks from: a) a blue Easy Oval ballpoint pen (B6), b) a black Paper Mate Eraser.Max ballpoint pen (K5) and c) a red Handy Salsa gel pen (R12).
For each producer, indistinguishable inks of the same colour (except black and red Bic inks and black Easy inks) were found. The same elemental profiles, within each colour of ink, were expected for inks from two models of Pilot gel pens: G-1 and G-1 grip, which were compatible with the same refill. In the case of the blue (B25 and B26) and black (K22 and K23) inks, identical elemental profiles were observed. In contrast, the red inks (R14 and R15) revealed distinguishable LIBS spectra. The refills found in these writing instruments were from different batches and presumably the chemical composition of red inks had been changed. In cases of inks from different pen models of the same (e.g. B25 and B26) or different (e.g. K12 and K20) types, made by the same manufacturer, identical LIBS spectra were recorded. In effect, differentiation of inks within a producer is possible only to some extent. In exceptional cases, the spectra examined can be enriched by some additional information, which facilitates comparative analysis. An example is the spectra obtained for three indistinguishable black Pilot gel inks: K21, K22 and K23 (see Table 3), which are presented in Fig. 4. For all three inks no elements were detected. However, ink K21 yielded a very characteristic pattern in the range of 560–590 nm with two non-identified lines, most likely derived from diatomic species. Thanks to that, ink K21 can in fact be differentiated from inks K22 and K23. In the case of two other inks, from blue and red Pilot Frixion pens (samples B29 and R13, respectively), these additional emission lines were also observed. The Pilot Frixion inks are able (in contrast to the other inks considered) to be erased from paper with an included
eraser and presumably they contain some specific additives giving a characteristic pattern to their spectra.
3.4. Inter-laboratory test A commercial inter-laboratory blind test was purchased from LGC Standards (United Kingdom) to evaluate the discrimination capability of the developed LIBS method. The test consisted of 3 black writing inks deposited on different sheets of paper of the same brand and ream (samples: “suspect 1”, “suspect 2” and “reference”). The brands and types of these ink samples and paper were unknown. The primary objective of this test was to answer the following questions: Was “suspect 1” written with the same ink as the “reference” sample? and Was “suspect 2” written with the same ink as the “reference” sample? Writing lines were first observed under a microscope and using IR radiation. These non-destructive studies did not reveal any differences in the composition of the examined inks. The same results were obtained even when the samples were examined using the CE method [12]. The LIBS spectra obtained for all three samples collected from lines deposited on paper are shown in Fig. 5. Analysis of the spectra revealed the presence of persistent emission lines belonging to copper for samples “suspect 1” and “suspect 2”. For the spectrum of the “reference” sample no such lines were detected, thus copper was not present in this sample. Based on these results, negative answers to both above mentioned questions were given. The results were verified as correct.
Table 3 Elemental profiles obtained by LIBS analysis for Bic, Pilot and Easy companies. Company
Blue inks Sample
Detected elements
Sample
Detected elements
Bic
B3, B19 B2 B1 B16 B25, B26 B29 B30, B33 B8, B22 B5 B7, B21 B6 B20
Cu Cu, Li Cu, Fe, Li Cu Cr Li No element Cu Cu, Li Cu, Mn Cu, Fe, Li, Ni No element
K15 K1
Cu, Li No element
K12, K20 K29 K21, K22, K23, K25 K3 K4 K17 K16
Pilot
Easy
Black inks
Red inks Sample
Detected elements
Cr Mn No element
R6 R15 R13, R14, R18, R19
Cr Cr, Li No element
Mn Cr, Cu Cr, Mn No element
R1 R10 R2, R9, R11
Cr, Cu Cr, Fe No element
124
A. Kula et al. / Science and Justice 54 (2014) 118–125
Fig. 4. Spectra of black Pilot gel pen inks from models: a) Frixion ball (K21), b) G-1 (K22) and c) G-1 grip (K23), (* — non-identified lines).
4. Conclusions The usefulness of the LIBS method in the analytical examination of writing inks for forensic purposes was verified. The elemental compositions of commonly available blue, black and red writing inks were established. For a great number of the studied inks it was possible to identify characteristic atomic emissions from one or more elements. The detected elements indicated that the LIBS method is especially suited to the study of pigments with d-block elements and in some cases counterions of anionic dyes. Furthermore, pigments create a layer on top of a paper surface, hence the signals for elements originating from paper components are observed to be lower for pigment-based inks than for dye-based inks.
The advantage of the LIBS method is that the doubtful presence of an element in a sample ascertained on the basis of a weak line can usually be verified by other lines of the same element. On the other hand, some difficulties are caused by a rich paper spectrum. Atomic and diatomic emissions of paper components masked lines from ink components making possible detection of only single or non-persistent lines. Three other aspects of the LIBS method should also be stressed: simplicity, semi-destructivity and rapidity. The analytical results obtained by this method are influenced by three instrumental parameters only: power of laser shots, gate delay, and integration time. The ink sampling process can be minimized to 5 laser shots and traces of these shots at ink lines are almost unnoticed. The fact that there are no visible changes in the questioned document means that additional studies can be carried
Fig. 5. Spectra of inks from samples: a) “suspect 1”, b) “suspect 2”, c) “reference”.
A. Kula et al. / Science and Justice 54 (2014) 118–125
out using the same or another analytical technique (the possibility of replication of the analysis in the comparison process is very important, especially in the case of inhomogeneous ink samples). In addition, analysis of a single sample only takes several dozen seconds. All the advantages are very important from the practical point of view. In most cases, it was possible to discriminate between ink samples of different types, producers and models with the use of the developed LIBS method. It was suitable for differentiating within blue, black and red ink samples with a discrimination power of 83, 82 and 61%, respectively, based only on qualitative elemental analysis. Writing inks of the same manufacturer were discriminated to some extent. Furthermore, within the scope of the created database, some pen models have unique ink formulas, containing barium, molybdenum, nickel or tungsten. This enables those samples to be easily differentiated from others in the comparison process. In order to identify a given pen (define it as a particular model), a bigger and – what is more important – statistically representative pen database is needed. However, linking a particular writing instrument model with ink composition is a difficult task because of possible batch to batch variation. The study confirmed the occurrence of this practice on the market. In one case of red gel inks, it was possible to distinguish inks from different batches. The proposed method was not useful in differentiating inks from the same package. Apart from the practical advantages listed above, it is expected that elemental analysis performed by the LIBS technique is more resistant (than e.g. TLC) to changes in ink composition induced by environmental factors, like degradation of dyes of inks deposited on paper. The inter-laboratory test revealed the usefulness of the LIBS method in questioned document examinations. The combination of the great effectiveness of the proposed method and its practical advantages makes it a good analytical tool for analysis of the elemental composition of samples. The LIBS method can be used as a standalone method for the comparative analysis of inks (especially for inks containing organometallic pigments as colourants) or as a screening method in combination with methods allowing for analysis of the dye composition of inks (e.g. TLC and CE). Acknowledgements The authors gratefully acknowledge the National Science Center and the Ministry of Science and Higher Education (Poland) for its financial support (grant no. ON204045738). References [1] J.D. Wilson, G.M. LaPorte, A.A. Cantu, Differentiation of black gel inks using optical and chemical techniques, J. Forensic Sci. 49 (2004) 364–370. [2] W.D. Mazzella, A. Khanmy-Vital, A study to investigate the evidential value of blue gel pen inks, J. Forensic Sci. 48 (2003) 419–424. [3] W.D. Mazzella, P. Buzzini, Raman spectroscopy of blue gel pen inks, Forensic Sci. Int. 152 (2005) 241–247. [4] J. Zięba-Palus, M. Kunicki, Application of the micro-FTIR spectroscopy, Raman spectroscopy and XRF method examination of inks, Forensic Sci. Int. 158 (2006) 164–172. [5] V.N. Aginsky, Forensic examination of ‘slightly soluble’ ink pigments using thin-layer chromatography, J. Forensic Sci. 38 (1993) 1131–1133.
125
[6] G.M. LaPorte, M.D. Arredondo, T.S. McConnell, J.C. Stephens, A.A. Cantu, D.K. Shaffer, An evaluation of matching unknown writing inks with United States International Ink Library, J. Forensic Sci. 51 (2006) 689–692. [7] S. Houlgrave, G.L. LaPorte, J.C. Stephens, The use of filtered light for the evaluation of writing inks analyzed using thin layer chromatography, J. Forensic Sci. 56 (2011) 778–782. [8] C. Weyermann, R. Marquis, W. Mazzella, B. Spengler, Differentiation of blue ballpoint pen inks by laser desorption mass spectrometry and high-performance thin-layer chromatography, J. Forensic Sci. 52 (2007) 216–220. [9] C. Neumann, R. Ramotowski, T. Genessay, Forensic examination of ink by highperformance thin layer chromatography — the United States Secrete Service Digital Ink Library, J. Chromatogr. A 1218 (2011) 2793–2811. [10] C. Vogt, J. Vogt, A. Becker, E. Rohde, Separation, comparison and identification of fountain pen inks by capillary electrophoresis with UV–visible and fluorescence detection and by proton-induced X-ray emission, J. Chromatogr. A 781 (1997) 391–405. [11] C. Vogt, A. Becker, J. Vogt, Investigation of ball point pen inks by capillary electrophoresis (CE) with UV/Vis absorbance and laser induced fluorescence detection and particle induced X-ray emission (PIXE), J. Forensic Sci. 44 (1999) 819–831. [12] J. Mania, K. Madej, P. Kościelniak, Inks analysis by capillary electrophoresis — analytical conditions optimisation, Chem. Anal. 47 (2002) 585–593. [13] M. Szafarska, R. Wietecha-Posłuszny, M. Woźniakiewicz, P. Kościelniak, Examination of colour inkjet printing inks by capillary electrophoresis, Talanta 84 (2011) 1234–1243. [14] M. Szafarska, R. Wietecha-Posłuszny, M. Woźniakiewicz, P. Kościelniak, Application of capillary electrophoresis to examination of colour inkjet printing inks for forensic purposes, Forensic Sci. Int. 212 (2011) 78–85. [15] M. Król, A. Kula, R. Wietecha-Posłuszny, M. Woźniakiewicz, P. Kościelniak, Examination of black inkjet printing inks by capillary electrophoresis, Talanta 96 (2012) 236–242. [16] J. Siegel, J. Allison, D. Mohr, J. Dunn, The use of laser desorption/ionization mass spectrometry in the analysis of inks in questioned documents, Talanta 67 (2005) 425–429. [17] M.R. Williams, C. Moody, L. Arceneaux, C. Rinke, K. White, M.E. Sigman, Analysis of black writing ink by electrospray ionization mass spectrometry, Forensic Sci. Int. 191 (2009) 97–103. [18] M. Gallidabino, C. Weyermann, R. Marquis, Differentiation of blue ballpoint pen inks by positive and negative mode LDI-MS, Forensic Sci. Int. 204 (2011) 169–178. [19] J. Coumbaros, K.P. Kirkbride, G. Klass, W. Skinner, Application of time of flight secondary ion mass spectrometry to the in situ analysis of ballpoint pen inks on paper, Forensic Sci. Int. 193 (2009) 42–46. [20] J.A. Denman, W.M. Skinner, K.P. Kirkbride, I.M. Kempson, Organic and inorganic discrimination of ballpoint pen inks by ToF-SIMS and multivariate statistics, Appl. Surf. Sci. 256 (2010) 2155–2163. [21] M. Oujja, A. Vila, E. Rebollar, J.F. Garcia, M. Castillejo, Identification of inks and structural characterization of contemporary artistic prints by laser-induced breakdown spectroscopy, Spectrochim. Acta B 60 (2005) 1140–1148. [22] R. Ponterio, S. Trusso, C. Vasi, G.F. La Torre, A. Toscano Raffa, Laser induced breakdown spectroscopy for the analysis of archaeological dyes from Licata (Sicily), Radiat. Eff. Defects Solids 163 (2008) 535–543. [23] M.P. Mateo, T. Ctvrtnickova, G. Nicolas, Characterization of pigments used in painting by means of laser-induced plasma and attenuated total reflectance FTIR spectroscopy, Appl. Surf. Sci. 255 (2009) 5172–5176. [24] K. Melessanaki, V. Papadakis, C. Balas, D. Anglos, Laser induced breakdown spectroscopy and hyper-spectral imaging analysis of pigments on an illuminated manuscript, Spectrochim. Acta B 56 (2001) 2337–2346. [25] H.J. Häkkänen, J.E.I. Korppi-Tommola, Laser-induced plasma emission spectrometric study of pigments and binders in paper coatings: matrix effects, Anal. Chem. 70 (1998) 4724–4729. [26] H. Häkkänen, J. Houni, S. Kaski, J.E.I. Korppi-Tommola, Analysis of paper by laser-induced plasma spectroscopy, Spectrochim. Acta B 56 (2001) 737–742. [27] A. Sarkar, S.K. Aggarwal, D. Alamelu, Laser induced breakdown spectroscopy for rapid identification of different types of paper for forensic application, Anal. Methods 2 (2010) 32–36. [28] T. Trejos, A. Flores, J.R. Almirall, Micro-spectrochemical analysis of document paper and gel inks by laser ablation inductively coupled plasma mass spectrometry and laser induced breakdown spectroscopy, Spectrochim. Acta B 65 (2010) 884–895. [29] M. Hoehse, A. Paul, I. Gornushkin, U. Panne, Multivariate classification of pigments and inks using combined Raman spectroscopy and LIBS, Anal. Bioanal. Chem. 402 (2012) 1443–1450.
Contents lists available at ScienceDirect
Science and Justice journal homepage: www.elsevier.com/locate/scijus
Application of laser induced breakdown spectroscopy to examination of writing inks for forensic purposes Agnieszka Kula a, Renata Wietecha-Posłuszny a,⁎, Katarzyna Pasionek a, Małgorzata Król a, Michał Woźniakiewicz a, Paweł Kościelniak a,b a b
Jagiellonian University, Department of Analytical Chemistry, Laboratory for Forensic Chemistry, 3 Ingardena St., 30-060 Krakow, Poland Institute of Forensic Research, 9 Westerplatte St., 31-033 Krakow, Poland
a r t i c l e
i n f o
Article history: Received 20 July 2013 Received in revised form 24 September 2013 Accepted 27 September 2013 Keywords: Forensic science Questioned document examination Writing inks Laser induced breakdown spectroscopy Ink analysis Elemental analysis
a b s t r a c t The research was focused on the analysis of writing inks using the laser induced breakdown spectroscopy (LIBS) technique. 34 samples of blue, 30 of black, and 21 of red writing inks were analyzed under optimized conditions to determine the variation of chemical composition of inks between different colours, brands and types. Nine elements (Ba, Cr, Cu, Fe, Li, Mo, Mn, Ni and W) were taken into account during comparative analysis of inks. Because of the strong effect of the paper spectrum, elements often found in inks (Ca, Al, Mg, Na, Ti, and Si) were eliminated from LIBS analysis of inks. It was determined that the LIBS method is capable of revealing qualitative elemental differences between ink samples. The discrimination power of this method was found to be 83, 82 and 61% for blue, black and red inks, respectively. Inks produced by the same producer were able to be differentiated in some cases. The results showed the potential of LIBS for forensic purposes as an effective and robust technique, requiring a small amount of sample and giving analytical information in a very short time. © 2013 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction In the modern world of computers, scanners and printers, the number of handwritten documents has been drastically reduced. However, handwritten notes, wills and prescriptions continue to be widely used by people and a signature is still required as proof of consent in many cases. Therefore (alleged) forgeries of documents written by hand are the focus of many civil and criminal investigations. One of the most important aspects of questioned document examination is comparison and/or identification of inks on the basis of their chemical composition. Writing inks are composed of colourants (dye and/or pigment), a solvent known as a “vehicle” and additives (responsible for the physicochemical properties of inks). All ingredients in the ink mixture can yield important analytical information to the forensic document examiner. However, the wide variety of dyes (natural or synthetic, organic or inorganic) and pigments (metallic, organometallic or organic) that can be present in different combinations in inks make this group of substances the most useful in comparative analysis. A number of analytical techniques have been proposed to distinguish between different writing ink formulations. Those allowing the non-invasive examination of ink components are the techniques of
⁎ Corresponding author. Tel.: +48 12 6632084; fax: +48 12 6340515. E-mail addresses: [email protected] (A. Kula), [email protected] (R. Wietecha-Posłuszny), [email protected] (K. Pasionek), [email protected] (M. Król), [email protected] (M. Woźniakiewicz), [email protected] (P. Kościelniak).
choice. Microscopic observation, UV and IR radiation as well as Raman spectroscopy (RS) are commonly applied for this purpose [1–4]. However, in some cases further investigations employing methods having a (destructive) effect on the structure of the document are needed. Separation techniques such as thin layer chromatography (TLC) or high performance thin layer chromatography (HPTLC) [1,5–9] and capillary electrophoresis (CE) [10–15] are widely used discriminatory tools that are applied to a variety of inks. They are based on the screening of ink sample dye composition. In recent years, mass spectrometry (MS) coupled with various ionization techniques has more often been employed for comparison and identification of ink components [8,16–18]. Despite the constant development of new techniques for the chemical analysis of inks, TLC and its high performance version are still recognized as the standard methodology in this field. Weyermann et al. [8] reported the application of two techniques: laser desorption ionization mass spectrometry (LDI-MS) and HPTLC for differentiation of 31 blue ballpoint pen inks. Both methods were characterized by high efficiency (92% of pairs of inks discriminated by HPTLC and 99% for LDI-MS). In some cases complementary results were obtained. However, the drawbacks of the HPTLC technique (complex sample preparation, long analysis time) made LDI-MS the preferred technique by the authors for differentiating inks. There are only a few techniques allowing analysis of the elemental composition of inks. One such technique is particle induced X-ray emission (PIXE), which, together with CE, was applied to determine the elements present in fountain pen inks [10] and ballpoint pen inks [11]. In
1355-0306/$ – see front matter © 2013 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scijus.2013.09.008
A. Kula et al. / Science and Justice 54 (2014) 118–125
the investigated fountain pen inks, both non-metals (S, Br, Cl, I) and metals (K, Ca, Cu, Fe, Cr, Zn) were detected. The elemental profile for each ink only allowed 3 out of 12 samples to be distinguished. Most of the analyzed ballpoint pen inks were characterized by the presence of copper, zinc, nickel and lead, and in some cases iron, titanium, chromium and cobalt were identified. A comparison of the samples was conducted on the basis of the elemental characteristics of those inks and the Cu/Zn signal ratio. It was found that PIXE yielded results that were complementary to the CE results. Zięba-Palus and Kunicki [4] reported the use of the X-ray fluorescence technique (XRF) in combination with micro-Fourier transform infrared spectroscopy (micro-FTIR) and RS for examination of ballpoint pen and gel pen inks. The XRF examination revealed the presence of sulphur, copper, silica, and phosphorus in all the studied ballpoint pen inks, and zinc, chlorine, bromine, calcium, chromium and lead in some of the samples, but on the whole quantitative rather than qualitative differences between inks were observed. The best discrimination power of the applied XRF method was revealed in relation to the group of gel pen inks. It should be stressed that both utilized methods, PIXE and XRF, provide results that are insufficient for differentiation of analyzed writing inks, and so sequential analysis together with other techniques (CE, micro-FTIR, RS) is required. Comprehensive analysis of molecular structure and elemental composition of writing inks could be performed using time of flight secondary ion mass spectrometry (TOF-SIMS). Coumbaros et al. were able to differentiate between blue ballpoint pen ink samples from 6 different manufacturers, using the results of molecular and elemental characteristics for each sample [19]. Discrimination of inks within a manufacturer was possible with the use of only elemental composition analysis. Denman et al. developed the TOF-SIMS method (supported by statistical analysis of results) for the study of 24 blue ballpoint pen inks [20]. Separated molecular or elemental characteristics made it possible to distinguish ink samples in ~60%. Combined results allowed an increase in the percentage of differentiated samples to 91%. The laser induced breakdown spectrometry technique (LIBS), in which a short laser pulse is focused onto a sample surface, is noteworthy. As a result, a small sample mass is ablated and high-temperature plasma, containing excited atoms and ions (from the analyzed sample), is generated. Electrons of excited species falling down to a ground state cause light emissions with discreet spectral wavelengths (lines). Each element is associated with unique spectral lines, allowing its identification. In recent years, there have been major developments in LIBS, but it is not yet well established as a technique for the forensic examination of the elemental composition of inks. However, LIBS fulfils the basic requirements of questioned document examinations: firstly, simultaneous multi-elemental analysis of inks can be performed without any preparation of samples (directly on paper), and secondly, the surface area interrogated by the laser pulse involves a very small amount of tested material, making LIBS a semi-destructive technique. The literature reports numerous applications of the LIBS technique to analysis of colourants in works of art, archaeological remains and illuminated manuscripts [21–24]. The LIBS technique has also been employed to study pigments and binders present in paper [25–28]. There are only two reports demonstrating the possibilities of LIBS in the analysis of contemporary writing inks [28,29]. In these procedures, LIBS was only a supplementary technique to laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or RS. Furthermore, examinations were only conducted for a limited number of inks, 11 black gel inks and 10 black and blue inks, respectively. Trejos et al. were successful in discrimination of 96.4% of all possible pairs of samples with the use of emission lines of only 4 elements. On the other hand, Hoehse et al. concluded that separate LIBS data (revealing the presence of Cu, Ti, K, Ca, Na, Li, and Al in analyzed inks) were insufficient to discriminate the sample set. Nevertheless, these investigations showed the potential of utilizing the LIBS method in the field of document examinations for forensic purposes. The aim of this paper was to perform more systematic studies with the use of the LIBS method and to verify the reliability of this analytical
119
tool in the examination of writing inks for forensic purposes. Several dozen inks of three colours (blue, black, and red) and of different brands and types were analyzed for the presence of nine elements (Ba, Cr, Cu, Fe, Li, Mo, Mn, Ni and W). Such writing tools as ballpoint pens, gel pens, porous point pens and rollerball pens were included in the study. The discrimination power of the developed LIBS method for writing inks was defined. 2. Materials and methods 2.1. Samples 34 blue, 30 black and 21 red writing instruments of various types (40 ballpoint pens, 29 gel pens, 6 porous point pens and 10 rollerball pens) produced by 17 different manufacturers from France, Japan, Korea, Poland and the USA were randomly collected from stationery shops in Poland during a 4 month period. A complete list of pens is presented in Table 1. A4 sheets of commercially available standard white office paper of five different brands made by three different producers: Polspeed (80 g/m2), Pollux (80 g/m2) and Poljet (90 g/m2) by International Paper (Poland), Opti Grat (80 g/m2) by Papyrus (Sweden), and Presentation (100 g/m2) by Navigator (USA) were investigated. Copper(II) phthalocyanine was obtained from Aldrich Chem Co. (USA). The ink depositions on paper were made in the form of straight lines, applying normal hand pressure. All ink samples placed on paper were stored in plastic bags, in the dark at room temperature. 2.2. Instrumentation An LIBS-6 system manufactured by Applied Photonics (United Kingdom) equipped with a Q-switched Quantel Ultra Nd:YAG laser operating at 1064nm (Quantel, France) was used. The laser emitted pulses of about 6ns duration with energy per pulse 150mJ and caused ablation spots with an average diameter of 1mm. An Avaspec-2048-2-USB2 fibre optic Czerny-Turner spectrometer with CCD detector (Avantes, The Netherlands) was used, which was able to record emission spectra across wavelength ranges 255–416 and 496–718 nm, with spectral resolution of 0.1 nm. Under normal conditions, the spectrometer did not need wavelength calibration because it had no moving elements inside. The system was equipped with a camera that enabled observation of the analyzed object and aiming of the laser beam. The analyzed objects were placed at the focal point of the focusing lens (approximately 70mm from the optical head). Experiments were carried out in ambient air. The LIBS-6 system was supported by LIBSoft V6.O software. Plasus SpecLine 2.13 software was used for spectral analysis and line identification of recorded data. 3. Results and discussion 3.1. Optimization of LIBS method Optimization of the developed method was performed using writing inks applied directly on paper. The aim was to obtain a reasonable signal intensity for the elements originating from components of writing inks and low signal intensity for the elements originating from paper. This was an important issue because discriminatory elements present in the analyzed samples mainly occurred in trace amounts. The amount of a sample consumed in the analysis and data quality was also taken into account. The power of laser shots (expressed by the Q-switch delay [μs]) was optimized using blue ballpoint pen ink (B7). To this end, a series of measurements in the range of Q-switch delay from 135 to 255 μs. The value 165 μs was found to be the best in terms of the signal intensity and amount of ablated paper. A study of detector parameters, including gate delays (1.27–5.00 μs) and integration time (1.1–2.0 ms), was also performed. In both cases,
120
A. Kula et al. / Science and Justice 54 (2014) 118–125
Table 1 List of pens examined in the study. Type of pen
Ballpoint pens
Gel pens
Porous point pens Roller-ball pens
Blue inks (B)
Black inks (K)
Red inks (R)
Company/model
Sample ID
Company/model
Sample ID
Company/model
Sample ID
Bic/Atlantis Bic/Cristal Medium Bic/ReAction Cresco/Exclusive Easy/Galaxy Easy/Oval Easy/Silver Easy/Style Empen/WZ-2011D Karin/113 BNP Lecce/Pen Lexi/5 Mattel/Hot Wheels Patio/Erase It Pentel/BK-77 Superb Pilot/Rexgrip Toma/Superfine 069 Uni-ball/Jetstream Bic/Cristal Gel Easy/Follow Easy/Massy Easy/Vestis Handy/Gel Patio/Nano Pilot/G-1 Pilot/G-1 grip Tetis/KZ107-N Uni-ball/Signo Pilot/Frixion point Pilot/G-TEC-C4 MonAmi/VuRiter Pentel/Bln75-C Pilot/V ball grip Uni-ball/UB-150
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23 B24 B25 B26 B27 B28 B29 B30 B31 B32 B33 B34
Bic/ReAction Cresco-Exclusive Easy/Silver Easy/Style Paper Mate/Eraser.Max Handy/Salsa ball Karin/113 BNP Lexi/5 Patio/Erase It Patio/Vigo Pentel/BK-77 Superb Pilot/Rexgrip Toma/Superfine 069 Uni-ball/Jetstream
K1 K2 K3 K4 K5 K6 K7 K8 K9 K10 K11 K12 K13 K14
Easy/Retro Easy/Silver Karin/113 BNP Patio/Vigo Pentel/BK-77 Superb Pilot/Rexgrip Toma/Superfine 069 Uni-ball/Jetstream
R1 R2 R3 R4 R5 R6 R7 R8
Bic/Cristal Gel Easy/Follow Easy/Massy Handy/Gel Handy/Intense Gel Pilot/Alphagel Pilot/Frixion ball Pilot/G-1 Pilot/G-1 grip Tetis/KZ107-V Pilot/G-TEC-C4 Tetis/KC030-V MonAmi/VuRiter Pentel/Bln75-A Pilot/V ball grip Uni-ball/UB-150
K15 K16 K17 K18 K19 K20 K21 K22 K23 K24 K25 K26 K27 K28 K29 K30
Easy/Follow Easy/Twist Easy/Vestis Handy/Salsa gel Pilot/Frixion ball Pilot/G-1 Pilot/G-1 grip Tetis/KZ107-C Uni-ball/Signo
R9 R10 R11 R12 R13 R14 R15 R16 R17
Pilot/G-TEC-C4 Pilot/V5 Hi-tecpoint MonAmi/VuRiter Pentel/Bln75-B
R18 R19 R20 R21
extension of time resulted in decreasing signal intensities for elements present in the ink. Therefore, measurements were carried out with a fixed gate delay of 1.27 μs and integration time of 1.2 ms. It was observed that a writing ink created such a thin layer on the paper substrate that a single laser shot was powerful enough to penetrate through it into the paper. Consequently, the acquired emission spectra had signals characteristic for both the ink components and the paper substrate. For this reason, the scan mode was applied during measurements, i.e. the ablation point was changed after each laser shot, and an average spectrum was created. To ascertain the optimum number of laser shots needed per ink, an investigation was performed using a blue gel pen sample (B25). It was observed that a spectrum based on 1 to 5 shots caused significant enhancement of the signal to noise ratio. Additional shots resulted in too much destroyed analyzed sample without any improvements in intensity of signals originating from the ink depositions. Assuming a compromise between data quality, analysis time and sample destruction, data were collected from 5 different positions along the ink line with only one laser pulse delivered at each point. The repeatability of intensity values was studied for exemplary ink samples B11, K3 and R16. For this purpose, intensities of lines at 327.4 (Cu), 403.1 (Mn) and 520.8 (Cr) for samples B11, K3 and R16, respectively, were used. The experimental intensities were obtained from 5 measurements each at a different position, with a single laser shot at each point. The results showed that the intensities of the chromium lines strongly varied from one measurement point to another, with a CV value of approximately 18, 24 and 35% for selected lines for Cu, Mn and Cr, respectively. The heterogeneity of the sample composition deposited on the paper and the laser shot-to-shot fluctuation caused poor reproducibility of the signal intensities. In order to capture the qualitative variability of the results obtained for different analytical processes, the developed measurement procedure
was carried out three times for 10 ink samples (randomly selected: B6, B16, B20, B22, K4, K13, K25, R7, R14, and R15), by two analysts every four months. The elements detected for analyzed samples were reproducible. 3.2. LIBS analysis of paper Emission spectra for 5 analyzed papers were recorded under optimized conditions of the LIBS technique. The spectrum of Polspeed paper is shown in Fig. 1 as an example. Spectra obtained for all papers were very rich, containing lines from neutral species (I), singly charged ions (II) and molecules (see Fig. 1). Spectra for all brands of paper revealed the presence of calcium, aluminium, magnesium, sodium, titanium and silicon. Emissions of diatomic species CN, Ca2 and C2 formed by collisions in the plasma were also observed in ranges 382–389 nm, 545–558 nm and 592–613 nm, respectively. The characteristic lines for Ca and Ti were related to calcium carbonate (extender used in paper) and titanium dioxide (white pigment), respectively. It was noticed that no qualitative, but only quantitative differences in elemental composition between analyzed paper brands were observed. However, the presence of titanium (lines in the windows in Fig. 1) in a given sample was detected only at selected measurement points. In effect, two kinds of spectra characteristic for paper components could be observed — with and without lines belonging to titanium. This shows that titanium (in the form of white pigment titanium dioxide) is not evenly distributed throughout paper sheets and special caution should be taken when lines originating from this element are absent in a spectrum. Because no differences in elemental profiles of the examined papers were observed, only the Polspeed paper was chosen for further study.
A. Kula et al. / Science and Justice 54 (2014) 118–125
121
Fig. 1. Spectrum of Polspeed paper.
The variation of elemental composition within reams of Polspeed paper was also studied. No distinctions between 5 sheets belonging to the same ream of this paper were found. Thus, emission spectra for writing ink samples could be collected from lines deposited on different sheets of the Polspeed paper. In order to minimize the effect of the paper spectrum, it was decided to search for lines derived from the ink components in some selected ranges that were relatively free of paper components, i.e. 320–420, 505–530, 560–600 and 660–715nm. In some cases, the rich paper spectrum could cause masking of lines originating from elements which may be present in ink samples (e.g. zinc or lead). Moreover, elements identified in paper were eliminated from qualitative analysis of the studied inks. Despite this, distinctive lines were detected for almost all inks examined. 3.3. LIBS analysis of inks The variation in elemental composition of inks taken from the same package (the same model and brand) was evaluated. The investigation was conducted with the use of 8 B11 pens (blue ballpoint pens), 3 K22 pens (black gel pens) and 3 R17 pens (red gel pens). No significant differences were observed between spectra of inks originating from pens of a given model. Hence, a single pen of each model was taken for further examinations. A total of 85 considered writing instruments (noted in Table 1) were used to identify elements present in the investigated inks and to test the ability of the LIBS method to discriminate between ink samples. In order to determine the threshold of significant line intensity, the instrumental noise was established. To this end, the biggest value of threefold standard deviation for intensities of signals at noise level, from three wavelength ranges (337–340, 508–515 and 660–666 nm) for 5 paper samples was calculated (684 V). To find emission lines characteristic for each ink composition, the spectrum of paper was subtracted from each sample spectrum. Disclosed lines of elements present in ink samples, with a signal height of at least 684 V, were taken into account during the identification process. Significant lines observed in the emission spectrum of each sample were identified using the National Institute of Standards and Technology (NIST) spectral database in wavelength ranges ±0.1 nm and standard dye — copper(II) phthalocyanine (in the case of Cu). The presence of two different persistent lines allowed positive identification of a given element
in the analyzed sample. The spectrum of water solution of copper(II) phthalocyanine deposited on paper and the spectra of exemplary samples B18 (without any element identified) and B24 (containing copper) are shown in Fig. 2. Table 2 summarizes the elemental profiles of the inks under investigation (listed in Table 1) obtained from LIBS analysis. The analysis of persistent lines of the set of 85 inks from pens of various brands and models disclosed 8 elements in the examined samples: Ba I (at 705.9 nm), Cr I (at 359.4, 360.5, 520.5 and 520.8 nm), Cu I (at 324.7, 327.4, 510.6, 515.3 and 521.8 nm), Fe I (at 344.1, 373.5, 374.9 and 404.6 nm), Li I (at 670.7 nm), Mn I (at 403.1, 403.3 and 403.4 nm), Mo I (at 317.0, 319.4, 390.3 and 407.0 nm), Ni I (at 341.5 and 352.4 nm) and W I (at 361.7, 400.9 and 407.4 nm). The presence of barium and lithium in the analyzed samples was verified by a single persistent line (705.9 and 670.7 nm, respectively). However, the line at 670.7 nm was assigned only to the persistent line of Li I, and neutral species of barium were also confirmed by a non-persistent line at 577.7 nm. As an example, the spectra obtained for samples B6, K5 and R12 are shown in Fig. 3. In the case of blue inks, various elements identified allowed the samples to be divided into 10 groups. One group included five inks which did not reveal any extra element in comparison with the paper spectrum. 15 out of 34 samples had in their composition only one examined element amongst copper, chromium and lithium. Sample B14 revealed a unique composition, with almost all the examined elements; barium, molybdenum and tungsten were detected exclusively in this ink. The most numerous group, characterized by copper, contained 11 inks (Fig. 2, spectrum c). Copper was the most commonly identified element in blue inks. It was detected in 24 blue inks of three types: ballpoint pen, gel pen and roller-ball pen inks. This confirms that the copper-containing colourants (such as Pigment Blue 15 or Solvent Blue 38) are currently frequently used to impart a blue colour to inks. Lithium occurred in 13 blue ink compositions of all types except gel inks (Fig. 3, spectrum a). For black inks, 11 different elemental profiles were obtained (see Table 2). A group with no distinctive elements comprised 12 pens. In turn, there were six one-ink groups amongst the black inks. There was no element that was characteristic for black inks (such as copper in the case of blue inks); however, there were three elements frequently detected: chromium (8 inks), manganese (8 inks) and copper (6 inks). Molybdenum found in sample K5 gave this ink composition individual properties (see Fig. 3, spectrum b).
122
A. Kula et al. / Science and Justice 54 (2014) 118–125
Fig. 2. Spectra of a) a water solution of blue pigment copper(II) phthalocyanine, b) ballpoint pen ink from a blue Uni-ball Jetstream pen (B18), c) gel pen ink from a blue Patio Nano pen (B24).
The red inks were divided into 5 groups, taking into account their elemental profiles (see Table 2). In the case of this colour, the number of inks indistinguishable from the paper substrate was revealed as the greatest. For these inks, the red ink colour could be the result of using cationic dyes like Rhodamine B. For the rest of the red inks, at least chromium was detected.
Table 2 Elemental profiles obtained by LIBS analysis for blue (B), black (K) and red (R) inks. Samples
Detected elements
B3, B8, B11, B13, B16, B19, B22, B24, B27, B28, B34 B25, B26 B29, B31 B2, B4, B5, B9, B10, B12, B15, B17 B7, B21 B32 B1 B6 B14 B18, B20, B23, B30, B33 K3, K19, K24, K29 K2, K10, K13 K12, K20, K30 K27 K6, K17 K4 K15 K26 K5 K9 K1, K7, K8, K11, K14, K16, K18, K21, K22, K23, K25, K28 R6, R16, R20 R1, R4 R10 R15 R12 R2, R3, R5, R7, R8, R9, R11, R13, R14, R17, R18, R19, R21
Cu Cr Li Cu, Li Cu, Mn Cr, Mn Cu, Fe, Li Cu, Fe, Li, Ni Ba, Cr, Cu, Fe, Li, Mn, Mo, W No element Mn Cu Cr Li Cr, Mn Cr, Cu Cu, Li Cr, Li Cu, Mn, Mo Cr, Fe, Mn No element Cr Cr, Cu Cr, Fe Cr, Li Cr, Fe, Mn No element
It can be seen in Table 2 (compared with Table 1) that most blue ballpoint pen inks (except sample B18) had copper in their composition. Looking now at gel inks, only three elements (Cu, Mn, and Cr) were present in different combinations in the blue gel inks (B19–B28). Apart from these three elements, one additional – lithium – was detected in black gel inks (K15–K24). No further links between the composition of inks and their type were observed. Inks of the same colour and type do not exhibit any characteristic elemental composition. However, there are cases of groups of inks (e.g. the group of 8 blue ballpoint pens containing Cu and Li, or the group of black ballpoint pens containing Cu) that can be easily distinguished from inks of the three other types. A similar situation, with no links between elemental profile and type of ink, is observed when comparing inks of different colours but the same type. Only blue and black roller-ball pen inks and black and red ballpoint pen inks had common elemental profiles, which did not occur in a group of inks of the third colour (profiles: Li for roller-ball pens and Cr with Cu for ballpoint pens, respectively). This confirmed that black inks may contain components typical for both blue and red inks. The d-block elements such as Cr, Fe, Mn, Mo and Ni detected in examined inks may be contained in metallic or organometallic compounds responsible for ink colours. Lithium is more often used instead of sodium and potassium as a counterion for anionic dyes. Based on qualitative elemental analysis, ink samples were differentiated from each other within groups of inks of the same colour. Inks within each group within a given colour group could not be distinguished from each other, but they could be differentiated from inks from the other groups. Multi-elemental samples of pen inks of all colours (B1, B6, B14, K5, K6, K9 and R12), containing from 3 to 8 elements, were easily distinguishable from the other samples. The discrimination power (DP) was calculated as the ratio of the number of discriminated pairs to the number of all possible pairs of inks of a given colour. The DP values were found to be 83, 82 and 61% for blue, black and red inks, respectively. Concerning ballpoint and gel pen inks of blue, black and red colours, the DP values were 75, 86 and 61% and 73, 80 and 69%, respectively. The variation of the elemental composition of inks produced by different companies was also investigated. The analytical results obtained for inks of Bic, Pilot and Easy companies are presented in Table 3.
A. Kula et al. / Science and Justice 54 (2014) 118–125
123
Fig. 3. Spectra of inks from: a) a blue Easy Oval ballpoint pen (B6), b) a black Paper Mate Eraser.Max ballpoint pen (K5) and c) a red Handy Salsa gel pen (R12).
For each producer, indistinguishable inks of the same colour (except black and red Bic inks and black Easy inks) were found. The same elemental profiles, within each colour of ink, were expected for inks from two models of Pilot gel pens: G-1 and G-1 grip, which were compatible with the same refill. In the case of the blue (B25 and B26) and black (K22 and K23) inks, identical elemental profiles were observed. In contrast, the red inks (R14 and R15) revealed distinguishable LIBS spectra. The refills found in these writing instruments were from different batches and presumably the chemical composition of red inks had been changed. In cases of inks from different pen models of the same (e.g. B25 and B26) or different (e.g. K12 and K20) types, made by the same manufacturer, identical LIBS spectra were recorded. In effect, differentiation of inks within a producer is possible only to some extent. In exceptional cases, the spectra examined can be enriched by some additional information, which facilitates comparative analysis. An example is the spectra obtained for three indistinguishable black Pilot gel inks: K21, K22 and K23 (see Table 3), which are presented in Fig. 4. For all three inks no elements were detected. However, ink K21 yielded a very characteristic pattern in the range of 560–590 nm with two non-identified lines, most likely derived from diatomic species. Thanks to that, ink K21 can in fact be differentiated from inks K22 and K23. In the case of two other inks, from blue and red Pilot Frixion pens (samples B29 and R13, respectively), these additional emission lines were also observed. The Pilot Frixion inks are able (in contrast to the other inks considered) to be erased from paper with an included
eraser and presumably they contain some specific additives giving a characteristic pattern to their spectra.
3.4. Inter-laboratory test A commercial inter-laboratory blind test was purchased from LGC Standards (United Kingdom) to evaluate the discrimination capability of the developed LIBS method. The test consisted of 3 black writing inks deposited on different sheets of paper of the same brand and ream (samples: “suspect 1”, “suspect 2” and “reference”). The brands and types of these ink samples and paper were unknown. The primary objective of this test was to answer the following questions: Was “suspect 1” written with the same ink as the “reference” sample? and Was “suspect 2” written with the same ink as the “reference” sample? Writing lines were first observed under a microscope and using IR radiation. These non-destructive studies did not reveal any differences in the composition of the examined inks. The same results were obtained even when the samples were examined using the CE method [12]. The LIBS spectra obtained for all three samples collected from lines deposited on paper are shown in Fig. 5. Analysis of the spectra revealed the presence of persistent emission lines belonging to copper for samples “suspect 1” and “suspect 2”. For the spectrum of the “reference” sample no such lines were detected, thus copper was not present in this sample. Based on these results, negative answers to both above mentioned questions were given. The results were verified as correct.
Table 3 Elemental profiles obtained by LIBS analysis for Bic, Pilot and Easy companies. Company
Blue inks Sample
Detected elements
Sample
Detected elements
Bic
B3, B19 B2 B1 B16 B25, B26 B29 B30, B33 B8, B22 B5 B7, B21 B6 B20
Cu Cu, Li Cu, Fe, Li Cu Cr Li No element Cu Cu, Li Cu, Mn Cu, Fe, Li, Ni No element
K15 K1
Cu, Li No element
K12, K20 K29 K21, K22, K23, K25 K3 K4 K17 K16
Pilot
Easy
Black inks
Red inks Sample
Detected elements
Cr Mn No element
R6 R15 R13, R14, R18, R19
Cr Cr, Li No element
Mn Cr, Cu Cr, Mn No element
R1 R10 R2, R9, R11
Cr, Cu Cr, Fe No element
124
A. Kula et al. / Science and Justice 54 (2014) 118–125
Fig. 4. Spectra of black Pilot gel pen inks from models: a) Frixion ball (K21), b) G-1 (K22) and c) G-1 grip (K23), (* — non-identified lines).
4. Conclusions The usefulness of the LIBS method in the analytical examination of writing inks for forensic purposes was verified. The elemental compositions of commonly available blue, black and red writing inks were established. For a great number of the studied inks it was possible to identify characteristic atomic emissions from one or more elements. The detected elements indicated that the LIBS method is especially suited to the study of pigments with d-block elements and in some cases counterions of anionic dyes. Furthermore, pigments create a layer on top of a paper surface, hence the signals for elements originating from paper components are observed to be lower for pigment-based inks than for dye-based inks.
The advantage of the LIBS method is that the doubtful presence of an element in a sample ascertained on the basis of a weak line can usually be verified by other lines of the same element. On the other hand, some difficulties are caused by a rich paper spectrum. Atomic and diatomic emissions of paper components masked lines from ink components making possible detection of only single or non-persistent lines. Three other aspects of the LIBS method should also be stressed: simplicity, semi-destructivity and rapidity. The analytical results obtained by this method are influenced by three instrumental parameters only: power of laser shots, gate delay, and integration time. The ink sampling process can be minimized to 5 laser shots and traces of these shots at ink lines are almost unnoticed. The fact that there are no visible changes in the questioned document means that additional studies can be carried
Fig. 5. Spectra of inks from samples: a) “suspect 1”, b) “suspect 2”, c) “reference”.
A. Kula et al. / Science and Justice 54 (2014) 118–125
out using the same or another analytical technique (the possibility of replication of the analysis in the comparison process is very important, especially in the case of inhomogeneous ink samples). In addition, analysis of a single sample only takes several dozen seconds. All the advantages are very important from the practical point of view. In most cases, it was possible to discriminate between ink samples of different types, producers and models with the use of the developed LIBS method. It was suitable for differentiating within blue, black and red ink samples with a discrimination power of 83, 82 and 61%, respectively, based only on qualitative elemental analysis. Writing inks of the same manufacturer were discriminated to some extent. Furthermore, within the scope of the created database, some pen models have unique ink formulas, containing barium, molybdenum, nickel or tungsten. This enables those samples to be easily differentiated from others in the comparison process. In order to identify a given pen (define it as a particular model), a bigger and – what is more important – statistically representative pen database is needed. However, linking a particular writing instrument model with ink composition is a difficult task because of possible batch to batch variation. The study confirmed the occurrence of this practice on the market. In one case of red gel inks, it was possible to distinguish inks from different batches. The proposed method was not useful in differentiating inks from the same package. Apart from the practical advantages listed above, it is expected that elemental analysis performed by the LIBS technique is more resistant (than e.g. TLC) to changes in ink composition induced by environmental factors, like degradation of dyes of inks deposited on paper. The inter-laboratory test revealed the usefulness of the LIBS method in questioned document examinations. The combination of the great effectiveness of the proposed method and its practical advantages makes it a good analytical tool for analysis of the elemental composition of samples. The LIBS method can be used as a standalone method for the comparative analysis of inks (especially for inks containing organometallic pigments as colourants) or as a screening method in combination with methods allowing for analysis of the dye composition of inks (e.g. TLC and CE). Acknowledgements The authors gratefully acknowledge the National Science Center and the Ministry of Science and Higher Education (Poland) for its financial support (grant no. ON204045738). References [1] J.D. Wilson, G.M. LaPorte, A.A. Cantu, Differentiation of black gel inks using optical and chemical techniques, J. Forensic Sci. 49 (2004) 364–370. [2] W.D. Mazzella, A. Khanmy-Vital, A study to investigate the evidential value of blue gel pen inks, J. Forensic Sci. 48 (2003) 419–424. [3] W.D. Mazzella, P. Buzzini, Raman spectroscopy of blue gel pen inks, Forensic Sci. Int. 152 (2005) 241–247. [4] J. Zięba-Palus, M. Kunicki, Application of the micro-FTIR spectroscopy, Raman spectroscopy and XRF method examination of inks, Forensic Sci. Int. 158 (2006) 164–172. [5] V.N. Aginsky, Forensic examination of ‘slightly soluble’ ink pigments using thin-layer chromatography, J. Forensic Sci. 38 (1993) 1131–1133.
125
[6] G.M. LaPorte, M.D. Arredondo, T.S. McConnell, J.C. Stephens, A.A. Cantu, D.K. Shaffer, An evaluation of matching unknown writing inks with United States International Ink Library, J. Forensic Sci. 51 (2006) 689–692. [7] S. Houlgrave, G.L. LaPorte, J.C. Stephens, The use of filtered light for the evaluation of writing inks analyzed using thin layer chromatography, J. Forensic Sci. 56 (2011) 778–782. [8] C. Weyermann, R. Marquis, W. Mazzella, B. Spengler, Differentiation of blue ballpoint pen inks by laser desorption mass spectrometry and high-performance thin-layer chromatography, J. Forensic Sci. 52 (2007) 216–220. [9] C. Neumann, R. Ramotowski, T. Genessay, Forensic examination of ink by highperformance thin layer chromatography — the United States Secrete Service Digital Ink Library, J. Chromatogr. A 1218 (2011) 2793–2811. [10] C. Vogt, J. Vogt, A. Becker, E. Rohde, Separation, comparison and identification of fountain pen inks by capillary electrophoresis with UV–visible and fluorescence detection and by proton-induced X-ray emission, J. Chromatogr. A 781 (1997) 391–405. [11] C. Vogt, A. Becker, J. Vogt, Investigation of ball point pen inks by capillary electrophoresis (CE) with UV/Vis absorbance and laser induced fluorescence detection and particle induced X-ray emission (PIXE), J. Forensic Sci. 44 (1999) 819–831. [12] J. Mania, K. Madej, P. Kościelniak, Inks analysis by capillary electrophoresis — analytical conditions optimisation, Chem. Anal. 47 (2002) 585–593. [13] M. Szafarska, R. Wietecha-Posłuszny, M. Woźniakiewicz, P. Kościelniak, Examination of colour inkjet printing inks by capillary electrophoresis, Talanta 84 (2011) 1234–1243. [14] M. Szafarska, R. Wietecha-Posłuszny, M. Woźniakiewicz, P. Kościelniak, Application of capillary electrophoresis to examination of colour inkjet printing inks for forensic purposes, Forensic Sci. Int. 212 (2011) 78–85. [15] M. Król, A. Kula, R. Wietecha-Posłuszny, M. Woźniakiewicz, P. Kościelniak, Examination of black inkjet printing inks by capillary electrophoresis, Talanta 96 (2012) 236–242. [16] J. Siegel, J. Allison, D. Mohr, J. Dunn, The use of laser desorption/ionization mass spectrometry in the analysis of inks in questioned documents, Talanta 67 (2005) 425–429. [17] M.R. Williams, C. Moody, L. Arceneaux, C. Rinke, K. White, M.E. Sigman, Analysis of black writing ink by electrospray ionization mass spectrometry, Forensic Sci. Int. 191 (2009) 97–103. [18] M. Gallidabino, C. Weyermann, R. Marquis, Differentiation of blue ballpoint pen inks by positive and negative mode LDI-MS, Forensic Sci. Int. 204 (2011) 169–178. [19] J. Coumbaros, K.P. Kirkbride, G. Klass, W. Skinner, Application of time of flight secondary ion mass spectrometry to the in situ analysis of ballpoint pen inks on paper, Forensic Sci. Int. 193 (2009) 42–46. [20] J.A. Denman, W.M. Skinner, K.P. Kirkbride, I.M. Kempson, Organic and inorganic discrimination of ballpoint pen inks by ToF-SIMS and multivariate statistics, Appl. Surf. Sci. 256 (2010) 2155–2163. [21] M. Oujja, A. Vila, E. Rebollar, J.F. Garcia, M. Castillejo, Identification of inks and structural characterization of contemporary artistic prints by laser-induced breakdown spectroscopy, Spectrochim. Acta B 60 (2005) 1140–1148. [22] R. Ponterio, S. Trusso, C. Vasi, G.F. La Torre, A. Toscano Raffa, Laser induced breakdown spectroscopy for the analysis of archaeological dyes from Licata (Sicily), Radiat. Eff. Defects Solids 163 (2008) 535–543. [23] M.P. Mateo, T. Ctvrtnickova, G. Nicolas, Characterization of pigments used in painting by means of laser-induced plasma and attenuated total reflectance FTIR spectroscopy, Appl. Surf. Sci. 255 (2009) 5172–5176. [24] K. Melessanaki, V. Papadakis, C. Balas, D. Anglos, Laser induced breakdown spectroscopy and hyper-spectral imaging analysis of pigments on an illuminated manuscript, Spectrochim. Acta B 56 (2001) 2337–2346. [25] H.J. Häkkänen, J.E.I. Korppi-Tommola, Laser-induced plasma emission spectrometric study of pigments and binders in paper coatings: matrix effects, Anal. Chem. 70 (1998) 4724–4729. [26] H. Häkkänen, J. Houni, S. Kaski, J.E.I. Korppi-Tommola, Analysis of paper by laser-induced plasma spectroscopy, Spectrochim. Acta B 56 (2001) 737–742. [27] A. Sarkar, S.K. Aggarwal, D. Alamelu, Laser induced breakdown spectroscopy for rapid identification of different types of paper for forensic application, Anal. Methods 2 (2010) 32–36. [28] T. Trejos, A. Flores, J.R. Almirall, Micro-spectrochemical analysis of document paper and gel inks by laser ablation inductively coupled plasma mass spectrometry and laser induced breakdown spectroscopy, Spectrochim. Acta B 65 (2010) 884–895. [29] M. Hoehse, A. Paul, I. Gornushkin, U. Panne, Multivariate classification of pigments and inks using combined Raman spectroscopy and LIBS, Anal. Bioanal. Chem. 402 (2012) 1443–1450.