SCIJUS-00509; No of Pages 10 Science and Justice xxx (2015) xxx–xxx

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Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation Nany Elsherbiny a, O. Aied Nassef b,⁎ a b

Medico-Legal Department, Ministry of Justice, Cairo, Egypt National Institute of Laser Enhanced Science (NILES), Cairo University, Egypt

a r t i c l e

i n f o

Article history: Received 25 November 2014 Received in revised form 29 January 2015 Accepted 12 February 2015 Available online xxxx Keywords: Laser induced breakdown spectroscopy (LIBS) Forensic Document investigation Black ink

a b s t r a c t The fast and nearly non-destructive criteria of laser induced breakdown spectroscopy (LIBS) technique has been exploited for forensic purposes, specifically, document investigation. The dependence of the optical emission spectra of different black gel ink samples on the excitation laser wavelength, namely the visible wavelength at λ = 532 nm and the IR wavelength at λ = 1064 nm, was studied. The inks of thirty black gel-ink pens comprising ten brands were analyzed to determine the variation of the chemical composition of ink and to discriminate among them with minimum mass removal and minimum damage to the document's paper. Under the adopted experimental conditions, the ability of the visible LIBS to differentiate among the different ink samples was successful compared to IR LIBS at the same laser pulse energy (~25 mJ/pulse, laser fluence is ~1400 J·cm-2 for visible laser and ~1100 J·cm−2 for IR laser) which could be attributed to the IR absorption effects by the black ink. However, the visible LIBS produces deeper crater with respect to that produced by IR LIBS. Applying IR LIBS with higher pulse energy of ~87 mJ (laser fluence is ~4100 J·cm−2), identification and differentiation of the adopted samples was performed with producing a larger-diameter but superficial crater. The plasma parameters are discussed at the adopted experimental conditions. The results support the potential of LIBS technique using both the visible and IR lasers to be commercially developed for forensic document examination. © 2015 Published by Elsevier Ireland Ltd. on behalf of Forensic Science Society.

1. Introduction Document examination, considered as one of the practical branches of forensic science, mainly focuses on the investigation of authenticity of a suspected document that is used in the human society, specifically in economic activities. Generally speaking, questioned document examination goes beyond the traditional identification of questioned handwriting; it extends to the determination of signature authenticity, examining questioned typewriting, paper, document alterations, erasures or other obliterations, and also the relative dating of a particular writing [1–6]. In general, a document is composed of three major parts: writing, ink, and paper; however the analysis of ink represents an efficient forensic procedure that can provide useful information about questioned documents. The ink used in writing pens is composed of two basic components: colorant and vehicle [6,7]. The coloring material can be either dyes or pigments [8,9]; dyes are soluble while pigments are insoluble finely ground multi-molecular granules. In order to achieve the desired color, the producer may mix two or more pure colorants. The composition of the vehicle, which can be oils, solvents or resins, affects the flowing and drying characteristics of the ink and specify the ink properties according to the manufacturer's needs. It ⁎ Corresponding author. E-mail address: [email protected] (O. Aied Nassef).

also allows the ink mixture to be deposited and flow on the paper surface in a relatively predictable manner. Other ingredient substances include driers, plasticizers, waxes, greases, soaps and detergents [6]. The various components present in inks, added to the contribution from the writing surface cause a complex challenge for the forensic examiner. However, most forensic analyses aim at comparing different writing inks on a document and determine whether or not they originate from the same ink. Distinguishing between different writing ink formulations has been extensively studied using conventional analytical techniques such as UV–VIS spectrometry [9–11], Fourier transform infra-red spectroscopy (FTIR) [11–13], thin layer chromatography (TLC mass spectrometry) and high performance liquid chromatography (HPLC) [14]. Despite the development of such techniques for the chemical analysis of inks, TLC and its high performance version are recognized as the standard methodology in the forensic field [6]. However, the destructive criterion of such techniques (some ink is extracted chemically leading to some kind of visible damage to the original document) is not favorable in some forensic cases. Moreover, some limitations to separate some dye-based gel inks from pigment-based gel inks, when carrying out spectral comparison, were reported by Wilson et al. [15]. They explained that pigment-based inks, particularly the black gel inks, do not migrate on the TLC plate and remain inseparable by spectral comparisons.

http://dx.doi.org/10.1016/j.scijus.2015.02.002 1355-0306/© 2015 Published by Elsevier Ireland Ltd. on behalf of Forensic Science Society.

Please cite this article as: N. Elsherbiny, O. Aied Nassef, Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation, Sci. Justice (2015), http://dx.doi.org/10.1016/j.scijus.2015.02.002

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N. Elsherbiny, O. Aied Nassef / Science and Justice xxx (2015) xxx–xxx

On the other hand, non-destructive methods that are routinely applied in forensic laboratories for the characterization and authentication of writing inks on questioned documents initially involve visual examination by a microscope. This can be followed by optical examinations utilizing the infrared light absorbance and luminescence and/or ultraviolet illumination carried out in a video spectral comparator (Foster– Freeman VSC) or any similar instruments [16,17]. The main principle of such techniques depends on the comparison of IR absorption or luminescent images using filtered light. The emitted luminescent light is usually integrated using a near-IR sensitive camera to produce an image taking into consideration the wavelength threshold of the highpass filter [18,19]. Research mentioned the difficulty of having two sources that may luminesce; the ink because of the mixture of dyes, in particular crystal violet, which contribute to the black color, and the paper due to both to chemical brighteners added during manufacture and the natural luminescence of lignin in the wood pulp [19]. This can be overcome by differentiating the contrast in the image arising from the luminescence from the ink and the paper by a suitable choice of excitation wavelength and high-pass filter settings [19]. Such techniques are very basic forms of spectroscopic imaging and provide a qualitative comparison that is highly dependent on the examiner judgment of whether the two samples show the same or different degrees of luminescence under the same experimental conditions [19]. It is also subject to interference due to the interactions of ink, paper, and other chemicals. Moreover, sensitivity of the method is limited for inks that show similar luminescence [20]; besides, there is no numerical measure of the degree of discrimination between inks [19]. The capabilities and degree of certainty of conventional methods, to discriminate among different sources of gel inks for document examinations, are lacked thus the tendency of applying and evaluating new techniques is worthy of consideration. Laser induced breakdown spectroscopy (LIBS) as one of the well established laser-based techniques has been extensively reported for the analysis of colors in art works, archeological remains and historical manuscripts [21–23]. An example is a study conducted by Oujja et al. [24] in which they identified the inks used in artistic prints and the order in which different ink layers were applied on a paper substrate as important factors for the authentication of this type of objects. They applied LIBS to determine the chemical composition and structural distribution of the constituent materials of model prints made by applying one or two layers of several blue and black inks on an Arches paper substrate. By using suitable laser excitation conditions, they identified the inks by virtue of emissions from key elements present in the inks' composition. They succeeded in the identification of the order in which the inks were applied on the paper by analyzing successive spectra on the same spot. Their results showed the potential of LIBS for the chemical and structural characterization of artistic prints. The suitability of LIBS in many applications is numerous: rapidity, ease of sample handling with no preparation required, capability of in-situ and multi-elemental analysis and more importantly, the nearly non-destructive criteria. To the best of our knowledge, literature reporting the use of LIBS as an analytical technique applied to forensic purposes on questioned documents specifically the gel inks is limited. There are only few reports demonstrating the application of LIBS in the analysis of contemporary writing inks for forensic examination, however coupled to another spectroscopic technique such as laser ablation inductively coupled plasma (LA-ICP) mass spectrometry (LA-ICP-MS) by Trejos et al. [25–27] or to Raman spectroscopy (RS) by Hoehse et al. [28]. Application of LIBS to improve the forensic comparisons of different materials such as gel inks, ballpoint inks and document papers based on similarities in elemental composition. The authors were successful in the discrimination of 96– 99% of all possible pairs of samples with the use of emission lines of only four elements. On the other hand, Hoehse et al. concluded that separate LIBS data which revealed 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. Another study by Kula et al. [6] focused on the analysis of writing inks using LIBS technique. They analyzed samples of different colors, brands and types of writing inks under optimized conditions to determine the variation of chemical composition of inks. They adopted nine elements (Ba, Cr, Cu, Fe, Li, Mo, Mn, Ni and W) during their comparative analysis of inks. They concluded that the LIBS method is capable of revealing qualitative elemental differences between ink samples and its discrimination power was 83, 82 and 61% for blue, black and red inks, respectively. However inks produced by the same producer were only differentiated in some cases. The aim of this study is taking advantage of this laser-based and sample-friendly technique, LIBS for the forensic analytical diagnosis of black gel ink writings on regular office document paper. The effect of applying two different wavelengths; the visible and IR-Nd:YAG laser at wavelengths 532 nm and 1064 nm as irradiation sources is examined. Also, differentiation among thirty pens comprising ten brands available in the market at each wavelength is performed. To sum it up, the reliability of LIBS as an analytical tool using two different laser wavelengths at the same pulse energy is studied for the examination of writing gel inks for forensic purposes. Additionally, a comparison of our results with a conventional analytical technique that is commonly used in analyzing ink samples, such as SEM–EDX, is presented. 2. Experiment 2.1. Instrumentation In the present measurements, a typical single pulse LIBS setup has been used. A Q-switched Nd:YAG laser (BRIO, Quantel, France), operating at both the fundamental wavelengths; 1064 nm and the visible wavelength; 532 nm with a pulse duration of 5 ns (FWHM), acted as our excitation source. The measurements were carried out using pulse energies of ~25 mJ for the visible wavelength and ~ 25 and ~87 mJ for the IR laser wavelength. The pulse energy was adjusted using glass slides and monitored by a power meter (ScienTech AC5001-USA). A 10-cm focal length plano-convex fused silica lens was used to focus the laser beam onto the target. The samples were mounted on an X–Y translational stage for controlling the irradiated position and introducing a fresh spot in each acquisition. The emitted light from the laserinduced plasma is collected by a 1.5 m optical fiber (600 μm diameter) which is connected to an echelle spectrometer (Mechelle 7500, Multichannel instruments, Sweden) coupled with an ICCD camera, DiCAMPRO (PCO Computer Optics, Germany) for the detection of the produced plasma emission. The obtained atomic emission spectra are displayed on a PC where data analysis and processing are carried out using the commercial 2D- and 3D-Gram/32, software programs (National Instruments, USA). Additionally, peak identification is performed using LIBS++ software the main task of which is to compare the measured center wavelengths and intensities to those of each element as listed in a large emission spectra database based on the National Institute of Standards and Technology (NIST) [29]. A scanning electron microscope SEM (Model JEOL-JXA-A840, Japan) and an electron probe micro-analyzer with an accelerating voltage of 30 kV were used as a morphological imaging tool while the attached EDX Unit (model INCA X-sight, Oxford instruments, England) provided the analytical elemental analysis. The samples were mounted on a holder and were investigated directly with gold surface coating. SEM–EDX analysis was applied to the samples to examine the samples' composition, elements' concentration and the surface morphology as a result of the interaction of both visible and IR laser with the same laser pulse energy. 2.2. Sample preparation and collection Gel pen inks are commonly pigment-based inks, and recently some are dye-based manufactured. These inks have become the prominent

Please cite this article as: N. Elsherbiny, O. Aied Nassef, Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation, Sci. Justice (2015), http://dx.doi.org/10.1016/j.scijus.2015.02.002

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3. Results and discussion The laser wavelength is an important factor for the initiation of plasma and its choice is commonly dependent on the analytical task. Our discussion will be divided into four sections starting with optimization of the experimental conditions followed by the results of applying two different laser wavelengths; the visible second harmonic of Nd-YAG laser (λ = 532 nm) and the fundamental IR wavelength (λ = 1064 nm) on the investigated black gel inks. This includes the qualitative analyses of the different samples by identifying atomic and ionic emission lines in the spectral range between 200 nm and 950 nm. The effectiveness of both visible and IR LIBS in differentiating the black gel inks is examined. The fourth section discusses the plasma parameters which explain the characteristic transient entity of the laser produced plasma via its electron density and the excitation temperature parameters. 3.1. Method of optimization Upon the irradiation of the samples under study (the above mentioned inks on standard regular office papers) with both the visible and IR laser in air at atmospheric pressure, an optimization of the experimental parameters was studied. Firstly, the laser single-pulse mode was preferably applied bearing in mind the sample damage effects, minimum removal of paper, maximum removal of ink and additionally, the economic consideration for in-situ applications. Secondly, the examination of the detection parameters such as the acquisition time of the spectral data relative to the laser pulse (the delay time) and the detection window for the data acquisition (the gate time) was performed. This was carried out by inspecting the emission spectra, over a wavelength range between 240 and 950 nm, at different delay times ranging

Table 1 List of the investigated black gel pens.

from 0.5 to 5 μs and different gate times ranging from 0.9 ns to 4 μs. Under the adopted experimental conditions, the choice of the optimum detection parameters that provide the best signal-to-background ratio (S/B) of the spectral emission lines are 1500 ns delay time and 2 μs gate time. A general observation is that at the early stage of the interaction of the laser pulse with the sample, an initial intense continuum is emitted close to the target surface, corresponding to the emission of blackbody radiation from the dense plasma. As the plasma cools and expands away from the target, the process of recombination of electrons and ions plays an important role in the production of line emission which tends to dominate the radiation processes with first the highly ionized lines being emitted close to the target (superimposed on the continuum) and, then, the atomic lines appear at later time of the plume lifetime. Line emission lasts for a few microseconds compared to that of the continuum emission. In our analysis, we considered that the emission spectrum resulting from the interaction of a single laser shot with an investigated sample represents a combination of elements comprising both the ink and the paper. In other words, the laser pulse could penetrate the ink layer reaching the paper so the obtained emission spectra had mixed signals of ink and paper. To overcome this, subtraction of the paper spectrum from each resulting spectrum was done throughout the whole analysis. Due to the inherent LIBS shot-to-shot signal fluctuation that usually affects the improvement of the S/B ratio of the spectral intensity, the acquisition of an individual spectrum was an accumulation of twenty laser shots incident on twenty fresh spots both for the inked and plain office papers. In such single pulse mode, three replicates of such explained acquisition for each specific pen and also for the plain office paper were performed. Thus, for each type of ink, nine LIBS spectra were stored corresponding to 180 laser-sample interaction spots. The net spectrum to be analyzed for each specific ink was an average of the mentioned replicates. Also, the emission of some spectral lines was chosen as markers to differentiate between different inks in a pair-wise method. The intensity of the spectral line emission was analyzed either as it was measured under the specified experimental conditions (laser pulse energy, laser wavelength, delay time and gate time) or normalized with respect to an internal reference selected among those elements characteristic of the ink matrix considered as it will be mentioned. The reproducibility of the peak intensity of some distinguishing emission lines in terms of relative standard deviation (%RSD) was studied for ink samples. As an example, the peak intensity of CuI 327.4 nm, CrI 359.4 nm MnII 250.6 nm (for samples Exrastyle, Zebra and Smooth, respectively)

CaII

50000

Exam

40000

CaII 30000

Intensty

type in forensic document examinations as a result of its being widespread in the market and low cost of manufacturing. They represent a challenge to the document examiner because most of the gel inks are difficult to analyze by conventional techniques such as TLC and capillary electrophoresis [15]. Research showed that the pigment-based inks will not migrate on a TLC plate and also some gel inks, particularly, the black gel inks are inseparable with spectral techniques [15,30,31]. In our study, black gel pens were chosen as the investigated samples to be exposed to the visible and IR laser based analytical technique; LIBS. Ink samples were collected by writing directly on regular office paper. Sheets of commercially available standard document paper of a specific brand, are used in all measurements. The investigated set includes writing inks from 10 different sources (different brand, manufacturer, and batch). Each ink collection consisted of three straight lines of ~30 mm length and ~5 mm width then they were placed directly in the respective ablation holder. The ink from 30 pens was analyzed to determine the variation of the chemical composition of ink within a single pen, and between brands of black gel inks. All ink samples placed on document paper were kept in standard envelopes at room temperature. Table 1 provides a complete list of black gel pens tested in this study.

3

CN

20000

CaI

Type

Brand

Made in

Purchase location

1 2 3 4 5 6 7 8 9 10

El-safa Exam Extrastyle Smooth Today's Teckjob Zebra sarasa Faber-Castell Uniball Signo Uniball gel

China China China China India China Japan Malaysia Japan Japan

Local market Local market Local market Local market Local market Local market Local market Local market Local market Local market

10000

FeI

FeI MgII,MgI CI

FeI

NaI

CaICaI

CrI

0 250

300

350

400

450

500

550

600

Wavelength Fig. 1. The net spectrum for one example of ink pen using the visible LIBS with laser wavelength of 532 nm and pulse energy of ~25 mJ (laser fluence ~1700 J·cm−2). This spectrum is as a result of subtracting the acquired spectrum for the inked office paper minus the one for the plain paper.

Please cite this article as: N. Elsherbiny, O. Aied Nassef, Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation, Sci. Justice (2015), http://dx.doi.org/10.1016/j.scijus.2015.02.002

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N. Elsherbiny, O. Aied Nassef / Science and Justice xxx (2015) xxx–xxx

NiI

240

250

260

270

280

290

Smooth

Normalized intensty

Elsafa

NiI

b

Teckjop

Normalized intensty

a

MnII

300

Extrastyle

320

330

340

350

Wavelength

360

370

380

Wavelength

CrI CrI

Uniball gel

c

Zebra

Exam

Normalized intensty

Normalized intensty

d

Fabercastle

350

355

360

365

Wavelength

370

375

380

350

355

360

365

370

375

380

Wavelength

Fig. 2. Part of the visible LIBS (λ = 532 nm) spectra for eight different black gel inks at different spectral ranges that enables the differentiation process of these eight gel ink pens at the wavelength range of 240–300 nm for (a), 320–380 nm for (b), and 350–380 nm for (c) and (d).

Please cite this article as: N. Elsherbiny, O. Aied Nassef, Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation, Sci. Justice (2015), http://dx.doi.org/10.1016/j.scijus.2015.02.002

N. Elsherbiny, O. Aied Nassef / Science and Justice xxx (2015) xxx–xxx

provided %RSD of 8.44, 7.88 and 8.67%, respectively. These are reasonable values to consider such emission lines for differentiation purposes.

5

(DP) was calculated for visible LIBS as the ratio of the number of discriminated pairs to the number of all possible pairs of ink samples and were found to be 88% for black gel pen inks.

3.2. LIBS on black ink using the visible laser 3.3. LIBS on black ink using IR laser Short laser wavelength is known to aid the ablation process because of the relatively high photon energy in comparison with the IR wavelength. Additionally, the use of a short wavelength laser (the second harmonics of Nd-YAG; λ = 532 nm) facilitates the multi-photon ionization seeding at the very beginning of the breakdown followed by the fast initiation and formation of the plasma. The LIBS spectra were acquired using the 532 nm-laser of pulse energy of ~25 mJ (laser fluence ~1700 J·cm−2) when interacting with the ten brands of ink pens. Fig. 1 displays the net spectrum for one specific ink pen by using 532 nm-LIBS. The ink net spectrum is a result of subtracting the acquired spectrum for the ink minus the one for the plain document paper. The acquired spectrum for each type of black gel-ink is obtained as mentioned above by averaging nine spectra of three pens resulting from the interaction of laser beam with 180 fresh spots of ink while the acquired spectrum for plain document paper is a result of averaging three spectra resulting from the interaction of laser beam with 60 fresh spots of ink. It is observed that the spectral lines have shown satisfactory S/B which can be attributed to the increase of the amount of the sample removed which also improves the emission line intensity. The variability within same ink samples is investigated for three brands of black gel-ink by comparing the net spectra belonging to each type of pen. The spectra showed no noticeable differences. Using the 532-nm wavelength as the irradiation beam, significant and detectable differences were observed between black inks of different sources (listed in Table 1). The line emission of Cu I 324.3 nm, Cu I 327.4 nm, Mg II 280.2 nm, Mg II 279.4 nm, Mg I 285.2 nm, Mn II 255.8 nm, Mn II 257.7 nm, Mn II 259.3 nm, Si I 288.2, Cr I 359.5 nm, Cr I 360.5 nm, Ni I 341.47 nm, and Ni I 349.29 nm were chosen as markers to differentiate between different inks in a pairwise method. Fig. 2 shows some examples of qualitative discrimination between black gel inks, for example, Mn II signals shown in spectra A (Fig. 2a) or Ni I signals shown in spectra B (Fig. 2b), Cr I spectral line shown in spectra C and D (Fig. 2c and d) qualitatively differentiated among the eight types of black gel ink which correspond to twenty four different pens. This represents only one pair-wise comparison for each ink out of all the possible comparisons of the adopted samples (Table 1). Under the application of laser fluence of ~1700 J·cm−2, the detectable elemental profiles of the investigated ink samples are summarized in Table 2, taking into account that only the spectral emission lines that showed a minimum of three fold S/B ratio were taken as a differentiating element. Two types of samples were easily distinguished by the presence of copper; another one yielded no distinguishing spectral data which comprised a characteristic pattern by itself. The overall emission of the examined samples does not show important differences in the high wavelength region of the spectra. The discrimination power

In case of IR wavelength and using a nanosecond laser, the sequence of its interaction with the sample surface starts with the ablation process as the laser pulse reaches the surface followed by the interaction of the following part of the laser beam with the evaporated species in the locality of the target surface leading to a strong heating and ionization of the vapor and consequently, the plasma formation. This is mainly attributed to the Inverse Bremsstrahlung (IB) absorption in which free electrons gain kinetic energy from the laser beam, thus promoting plume ionization and excitation through collisions with the excited and ground-state neutrals [32]. The plasma then acts as a shield for the laser radiation, when high energy laser pulses are used, which limits the efficiency of the IB photon absorption as the last part of the pulse does not reach the target surface. The IB mechanism is more efficient with IR wavelength than with shorter ones because of the λ3 dependence of IB on the laser wavelength. This has been extensively studied throughout literature [33,34]. Starting with the same pulse energy equal to the energy used in the visible LIBS (~25 mJ, laser fluence ~ 1100 J·cm−2), we applied the IRLIBS (λ = 1064 nm) focused on the adopted samples under the same experimental conditions. Some of the produced spectra did not show any characteristic emission lines (after subtracting the spectrum of the plain document paper) that could help differentiate elements among the different gel inks. This could be attributed to the good absorption of the IR laser by the black ink leading to insufficient laser energy to produce distinguished atomic or ionic lines enabling the differentiation process. Other spectra showed no spectral emission with the IR wavelength when the laser fluence is reduced to a value below ~1100 J·cm−2. This can be attributed to the fact that at the infrared region, the photon energy is too low, compared with the ionization energy of the target material, to produce plasma. In this case more photons with higher irradiance are needed for the plasma production. Applying higher pulse energy of ~87 mJ (laser fluence ~4100 J·cm−2) and insuring that no resulting sample damage occurs, we obtained IR-LIBS spectra as shown in Fig. 3 which displays the net spectrum for one ink (Type no. 2; Table 1) after subtracting the spectrum for the plain document paper. The variability of IR LIBS spectra for the different pens

CaII

50000

Exam

40000

Table 2 Elemental profiles of black gel inks obtained by visible LIBS analysis with laser wavelength of 532 nm and pulse energy of ~25 mJ (laser fluence ~1700 J·cm−2). Samples

Detected elements

El-safa Exam Extrastyle Smooth Today's Teckjob Zebra sarasa Faber-Castell Uniball Signo Uniball gel

Mg, Si, Fe Mg, Si, Fe, Cr Cu, Mg, Si, Ni, Fe Cu, Mn, Mg, Si, Fe No element Mn, Mg, Si, Al, Fe Mn, Cr, Al, Mg, Si, Fe Mn, Mg, Si, Fe, Al Mn, Mg, Si, Fe, Al Mn, Mg, Si, Fe

intensty

30000

20000

CaI 10000

CI

CN

MgI,II

0 250

NaI

FeI 300

350

400

450

500

550

600

wavelength Fig. 3. The net spectrum for one example of ink pen using IR LIBS with laser wavelength of 1064 nm and pulse energy of ~87 mJ (laser fluence ~4100 J·cm−2). This spectrum is as a result of subtracting the acquired spectrum for the inked office paper minus the one for the plain document paper.

Please cite this article as: N. Elsherbiny, O. Aied Nassef, Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation, Sci. Justice (2015), http://dx.doi.org/10.1016/j.scijus.2015.02.002

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Today`s

250

260

270

280

290

Normalized intensty

Extrastyle

240

SiI

MnII

b

Normalized intensty

a

Smooth

MnII

Exam

240

300

250

260

Wavelength

MgI, MgII SiI

280

290

CrI

Zebra

Elsafa

260

270

Wavelength

280

290

300

Normalized intensty

Normalized intensty

d

250

300

Teckjop

c

240

270

Wavelength

Fabercastle

350

355

360

365

370

375

380

Wavelength

Fig. 4. Part of the IR-LIBS (λ = 1064 nm) spectra of eight different black gel inks at different spectral ranges that enables the differentiation process of these eight black gel inks at 240– 300 nm (a, b and c) and at 350–380 nm for (d). The laser pulse energy is ~87 mJ (laser fluence ~4100 J·cm−2).

Please cite this article as: N. Elsherbiny, O. Aied Nassef, Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation, Sci. Justice (2015), http://dx.doi.org/10.1016/j.scijus.2015.02.002

N. Elsherbiny, O. Aied Nassef / Science and Justice xxx (2015) xxx–xxx 14

Table 3 Elemental profiles of black gel inks obtained by IR-LIBS analysis with laser wavelength of 1064 nm and pulse energy of ~87 mJ (Laser fluence ~4100 J·cm−2).

El-safa Exam Extrastyle Smooth Today's Teckjob Zebra sarasa Faber-Castell Uniball Signo Uniball gel

Na Mg, Na Cu, Mg Cu, Mg, Si, Mn Mn, Si, Fe, Mg Mg, Si, Fe Mn, Mg, Ni, Cr Mn, Mg, Si Mg, Na Mg, Na

17

Detected elements

λ= 532 nm λ = 1064 nm

12

Electron density*10 cm-3

Samples

7

10 8 6 4 2 0

belonging to the same ink type was checked for three different ink samples and no significant differences were observed. The higher laser energy allows the differentiation of the spectral lines among eight types of ink as shown in Fig. 4. Part of the IR-LIBS spectra of eight different black gel inks are displayed in Fig. 4 at different spectral ranges that enable the differentiation process of such gel ink pens. The wavelength of Fig. 4 (a, b and c) ranges at 240–300 nm for spectra A, B and C while Fig. 4(d) ranges at wavelength of 350–380 nm for spectrum D. The laser pulse energy is ~87 mJ (laser fluence ~4100 J·cm−2). It is noticeable that eight black gel inks are differentiated qualitatively which represents only one example of the possible comparisons among the samples under investigation listed in Table 1. For example, Mn II signals shown in spectrum A (Fig. 4a), Mn II and Si I signals shown in spectrum B (Fig. 4b), Mg I, II and Si I shown in spectrum C (Fig. 4c) and Cr I shown in spectrum D (Fig. 4d), provide elemental differences among the eight types of black gel ink which also correspond to twenty four different pens. The elemental profiles of the samples showing the differentiating detected elements for each sample is listed in Table 3 in case of applying IR LIBS. Under the mentioned experimental conditions, six black pens belonging to two different brands of gel ink were not differentiated from each other which can be attributed to their low concentration levels of inorganic components. The discrimination power (DP) for IR-LIBS was calculated to be 91% for the samples under study listed in Table 1. 3.4. Plasma parameters The determination of plasma parameters (the electron density and the plasma excitation temperature) is essential for the comprehension of the mechanisms underlying LIBS technique. The observed change in emission intensity is related to the change in mass removal and vaporization [35] and to the change in plasma parameters such as temperature and electron number density [33]. The approximation of local thermodynamic equilibrium (LTE) is often used for modeling the plasma [36]. When the collisional processes especially those involving electrons are the dominating mechanism in plasma, such plasma is referred to as collisional-dominant plasma (CDP). A crucial parameter for the establishment of LTE conditions in the CD plasma is its electron density (ne), since a necessary conditions for LTE is given by McWhirter [37] criterion: ne (cm−3) ≥ 1.4 × 1014 (kT)1/2 ΔE3, where T (in eV) is the plasma excitation temperature and ΔE (in eV) is the energy difference between the upper and lower levels of the transitions. In our study, we calculated the electron density using the most common spectroscopic techniques; the Stark-broadened line profile of isolated atoms or singly ionized ions which gives reasonable accuracy of calculation [38]. Applying this method, the Stark-broadened profile of the Mg I transition (3s3p 1P°–2p63s2 1S) at 285.2 nm, was adopted. In our calculation, the calculated line shape was corrected by simply subtracting the contribution of the instrumental line broadening through the relation Δλture = Δλobserved − Δλinstrument, where Δλinstrument is found to be

Elsafa

Smooth Teckjop Todays

Zebra Fabercastle Exam ExtrastyleUniball GUniball S

Sample Fig. 5. The electron density for the different types of ink a s calculated using the Stark broadening of Mg I (285.2 nm) spectral emission line. The average electron density is ~ 6 × 1017·cm−3 in both cases. Laser fluence ~ 4100 J·cm−2 in case IR LIBS while the laser fluence ~1700 J·cm−2 in case of visible LIBS.

0.06 nm. Fig. 5 illustrates the calculated electron density for each type of black gel ink for both visible and IR-LIBS plasmas. It shows that the average electron density for the black gel ink plasma is ~6 × 1017·cm−3 to 7 × 1017·cm−3 which confirms the LTE assumption. Although the IR LIBS used a laser pulse with about triple the laser pulse energy in case of the visible LIBS (laser fluence ~ 4100 J·cm− 2 in case IR LIBS while the laser fluence ~ 1700 J·cm− 2 in case of visible LIBS), we notice no prominent change in the plasma electron density. This can be attributed to the partial absorption effects that can occur by the black ink samples (Table 4). On the other hand, the plasma temperature plays a significant role in determining the elemental composition. The population density of atomic states is described well by a Boltzmann distribution and the ionization states are populated according to Saha–Boltzmann equilibrium equation in case LTE is verified in the plasma [36]. In our calculations, the Boltzmann plot method is used for determining the plasma temperature. In LTE approximation, the emission intensity of the emitting line is related to the total density by the Boltzmann law: ln [λ i / gi Ai] = Ln (N / ZT)i − Ei / kTex, where the spectroscopic constants: gi, λ and Ai are the ith level degeneracy, the wavelength and the transition probability, respectively. These relevant spectroscopic constants are tabulated in Table 2; i is the emission intensity of the emitting line; N is the number of atoms or ions; Z(T) is the partition function of the emitting species, which depends on plasma temperature Tex and k is the Boltzmann constant. By plotting the left hand side versus Ei the slope of the obtained line is (−1/kT), therefore, the plasma temperature can be obtained without knowing the values of N, the total number density, and Z(T), the partition function. The special conditions required for thermometric purposes (lines in close spectral proximity, reasonably intense, of known transition probability and with different upper

Table 4 Spectroscopic data for the wavelengths that was used in the estimation of plasma temperature using Mg I emission lines. λ (nm)

Ai

gk

E(eV)

429.899 430.253 430.774 431.865 443.496

4.66e + 07 1.36e + 08 1.99e + 08 7.40e + 07 6.70e + 07

3 5 1 3 5

4.7690282 4.7797838 4.7631681 4.7690282 4.6806348

Please cite this article as: N. Elsherbiny, O. Aied Nassef, Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation, Sci. Justice (2015), http://dx.doi.org/10.1016/j.scijus.2015.02.002

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14000

λ= 532 nm λ = 1064 nm

Plasma Temperature (K)

12000 10000

excitation temperature is ~ 6000 K when using IR LIBS (laser fluence ~4100 J·cm−2) while it is ~4000 K in case of visible LIBS (laser fluence ~1700 J·cm−2). 4. Scanning electron microscope SEM–EDX

8000 6000 4000 2000 0 Extrastyle ELsafa

Uniball G UnibaIl S Fabercastle Zebra

Teckjop

Smooth

Exam

Sample Fig. 6. The plasma excitation temperature for the different types of ink as calculated using the Boltzmann plot method of five Ca I spectral emission lines. The average plasma excitation temperature when using IR LIBS is ~6000 K (laser fluence ~4100 J·cm−2) while it is ~4000 K in case of visible LIBS (laser fluence ~1700 J·cm−2).

energy levels) have been fulfilled by the emission lines of Ca I to determine the plasma excitation temperature under the condition of LTE. Table 2 states the adopted spectral lines considered in our calculations of the plasma temperature and their spectroscopic data obtained from NIST. As it is inferred from Fig. 6, the average plasma

The scanning electron microscope (SEM) images provide a characteristic surface morphology and are useful for judging the surface structure of the investigated sample, its fibers quality as well as its damage aspects. The morphology of the surface of the fibers was investigated using a JEOL Scanning Electron Microscope where different spots of the plain office document paper (Fig. 7a) and the inked one (Fig. 7b) were imaged and investigated with different magnification power, while their corresponding elemental identification is provided by an EDX unit. It is noticeable that the ink is not forming a deposited layer on top of the paper but it is penetrating or absorbed in the fibers of the paper. Due to this fiber-ink absorption, it is not possible to remove only the ink without removing some paper components, therefore the method of correction by subtracting the paper's spectrum was surely required. It is also important to notice that SEM elemental results of paper and ink agree with the ones provided by both visible and IR-LIBS spectra, but not the chlorine which is a common difficulty in LIBS analysis. However, LIBS allows discriminating the presence of additional elements that SEM failed to identify like, Ni and Fe. Moreover, SEM imaging was conducted to compare the craters' morphology produced by the visible and the IR lasers with the different laser pulse energies used in this study. Craters produced by the 532 nm laser of pulse energy ~ 25 mJ (Fig. 8a) are larger in diameter, deeper and

Fig. 7. SEM–EDX analysis of the plain document office paper and the inked paper provides both their morphological images along with their corresponding elemental analysis.

Please cite this article as: N. Elsherbiny, O. Aied Nassef, Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation, Sci. Justice (2015), http://dx.doi.org/10.1016/j.scijus.2015.02.002

N. Elsherbiny, O. Aied Nassef / Science and Justice xxx (2015) xxx–xxx

a

9

energy of ~ 25 mJ is ~ 350 μm and ~ 590 μm when IR pulse energy of ~ 87 mJ is applied. As a general observation, the total mass of ink removed by both laser wavelengths is not recognized by the naked eye and the document is almost unchanged. 5. Conclusion

b

c

Fig. 8. SEM images of the craters produced as a result of the interaction of a) visible laser of pulse energy of ~25 mJ (laser fluence ~1700 J·cm−2), b) IR laser of pulse energy of ~25 mJ (laser fluence ~1100 J·cm−2) and c) IR laser of pulse energy of ~87 mJ (laser fluence ~4100 J·cm−2) with black gel inks.

penetrate more into the paper substrate than the ones produced by the 1064 nm laser of pulse energy of ~25 mJ (Fig. 8b). However, increasing the IR-laser pulse energy to ~87 mJ produced a crater (Fig. 8c) whose diameter is comparable with the one produced by the IR wavelength but it is still superficial and didn't penetrate deep in the paper substrate in comparison with the one produced by the visible laser. The diameters of the craters (as shown in Fig. 8) produced by visible laser of pulse energy of ~25 mJ is ~ 570 μm while that produced by IR laser of pulse

In this study, black gel inks which introduce a major problem in the forensic field, specifically, the branch of document investigations was examined using the LIBS technique. The effect of both wavelengths; the second harmonic (λ = 532 nm) and the fundamental wavelength (λ = 1064 nm) of Nd-YAG laser on LIBS analysis was studied. This was conducted on ten brands of black gel inks (listed in Table 1); with each three pens belong to a specific brand. This means that a total of 30 writings were used to identify elements present in the investigated inks and test the ability of the LIBS method to discriminate between ink samples using both wavelengths. First, the LIBS experiment was carried out using a wavelength of 532 nm at pulse energy of ~ 25 mJ. Analysis of the acquired spectra on fresh spots initially allows the identification of the structure of each type of black gel ink and then allows the differentiation of the different adopted types of inks applied on regular document papers. Comparisons were done qualitatively and quantitatively following pair-wise manner. Applying the same pulse energy but at a wavelength of 1064 nm, the spectra showed high noise and there were no specific differentiating spectral lines and the spectra appeared identical in the range from 400 nm to 600 nm. The low energy of 1064 nm laser was not enough to produce characteristic emission lines for ink samples due to the probable high absorption effects of the IR laser by the black ink. Applying higher energy IR laser of ~87 mJ, provided a compositional identification and differentiation among the adopted black gel ink samples. Scanning Electron Microscope images display the morphology of the produced craters for each experimental condition. It is observed (Fig. 8a and c) that the visible wavelength of pulse energy of ~25 mJ produces a crater of a diameter that is comparable to that produced by the IR wavelength with approximately triple the pulse energy (~87 mJ) (~570 μm and ~590 μm for visible and IR, respectively). However, the crater produced by the visible wavelength indicates more depth when compared to that formed by the IR laser. Practically speaking, an advantage of using the visible radiation at 532 nm in the present investigation is that the commercial optical elements required by the system are of lower cost compared to other harmonics of the fundamental wavelength. Besides the beam trajectory is easily observed, thus decreasing the safety problems. The high calculated discrimination power for both visible and IR LIBS confirms the effective applicability of the technique under the adopted experimental conditions for differentiation among black gel inks. The potential of both visible and IR LIBS results suggest the development of the method for field and on-line analysis of gel inks for document investigation. References [1] Y. Liu, J. Yu, M. Xie, Y. Liu, J. Han, T. Jing, Classification and dating of black gel pen ink by ion-pairing high-performance liquid chromatography, J. Chromatogr. A 1135 (2006) 57–64. [2] Y. Xu, J. Wang, L. Yao, Dating the writing age of black roller and gel inks by gas chromatography and UV–vis spectrophotometer, Forensic Sci. Int. 162 (2006) 140–143. [3] C. Weyermann, J. Almog, J. Bugler, A.A. Cantu, Minimum requirements for application of ink dating methods based on solvent analysis in casework, Forensic Sci. Int. 210 (2011) 52–62. [4] S.N. Srihari, K. Singer, Role of automation in the examination of handwritten items, Pattern Recogn. 47 (2014) 1083–1095. [5] Y. Wu, Chun-Xi Zhou, Yu. Jing, H. Liu, M. Xie, Differentiation and dating of gel pen ink entries on paper by laser desorption ionization- and quadruple-time of flight mass spectrometry, Dyes Pigments 94 (2012) 525–532. [6] A. Kula, R. Wietecha-Posłuszny, K. Pasionek, M. Król, M. Woźniakiewicz, P. Kościelniak, Application of laser induced breakdown spectroscopy to examination of writing inks for forensic purposes, Sci. Justice 54 (2013) 118–125.

Please cite this article as: N. Elsherbiny, O. Aied Nassef, Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation, Sci. Justice (2015), http://dx.doi.org/10.1016/j.scijus.2015.02.002

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Please cite this article as: N. Elsherbiny, O. Aied Nassef, Wavelength dependence of laser induced breakdown spectroscopy (LIBS) on questioned document investigation, Sci. Justice (2015), http://dx.doi.org/10.1016/j.scijus.2015.02.002