Yong Liu

The analysis of seals, mostly via the ink types, can provide evidence to authenticate the related documents and can play a key role in legal cases. In the present study, a method to discriminate red seal inks using Raman microscopy and Fourier transform infrared (FT-IR) spectroscopy was developed. The results showed that the developed Raman method was powerful, sensitive, and rapid, and can be used to directly differentiate the red inks with paper substrates; thus, this method would be helpful for ink analysis in the forensic, drawing, and cultural heritage fields.

	Hand signatures are popularly used to prove identity in the west and seals are used in the east, especially in eastern Asia, such as China, Japan, and Korea. The seal is a cultural heritage in Asian counties that has represented identity and credentials since a long time ago. The first record of a seal in China can be traced to 554 B.C. (1). Almost all of the official documents, formal commercial contracts, and other items requiring acknowledgement or authority must be certificated by seals in China and many other Asian countries. Seals symbolize statutory powers of the governments, organizations, enterprises, and institutes. Nowadays, seals are still widely applied to various aspects of social life, such as contracts, checks, wills, and artworks. Forgery of seals will be punished by imprisonment for several years in China, but there are still numerous cases relating to the forgery of seals every year. Therefore, it is necessary to identify genuine and fake seals and seal ink analysis can play a role therein. There are three significant problems in seal ink analysis: the characterization and differentiation of inks, the sequencing of intersecting lines (2,3), and dating of inks in documents (4). Among these three problems, the most basic problem is the first one. Solving this problem can provide important information to confirm the authenticity of documents and archaeological artworks.

	Many techniques including thin-layer chromatography (TLC) (5), gas chromatography (GC), liquid chromatography (LC), and mass spectrometry (MS) (6–10) have been used to classify the samples. These methods proved to be accurate and effective. However, most of these commonly used methods are time-consuming and destructive to samples. In addition, the seal ink contains binders, fillings, and pigments; the binders are polymers and the fillings and some pigments are inorganic compounds, which are difficult to analyze using the chromatographic methods. Wang and colleagues (11) developed a nondestructive laser desorption ionization mass spectrometry method that successfully classified 38 red ink entries into seven groups. The method can effectively identify the pigments used in the inks, but it cannot analyze the polymers, additives, and inorganic pigments. Because of the limitation in sample quantity in their study, the distribution of different types was not discussed further.

	Spectral methods, especially infrared (IR) and Raman spectroscopy, can cover the shortages of the chromatographic and MS methods. They are fast, accurate, nondestructive, and suitable for both organic and inorganic substance analysis. The binders, fillings, and pigments can be easily analyzed using IR spectroscopy. There have been many studies on seal inks using IR methods (12–16), and the results showed that IR can be a valuable nondestructive tool. The biggest drawback of Fourier transform infrared (FT-IR) spectroscopy in ink sample analysis is the interference from paper. Most FT-IR studies have focused on pure inks on slides, which are not available in real cases.

	Raman spectroscopy can supply extra molecule vibration information. It has been successfully used in pigment analysis (17–19) and can effectively avoid the paper interference (20), which makes it more applicable than FT-IR in real cases. Thus, in this study Raman microscopy was tentatively used to examine the ink samples. The indicative peaks were specifically assigned, classified, and summarized to discriminate them. FT-IR on-slide analysis was also used to verify the Raman results.

	A total of 110 red seal ink samples from 37 brands were randomly collected from 10 Chinese provinces. Sample information is listed in Table I. Reagents and standards were analytical grade. A Renishaw inVia confocal Raman microscope system containing a charge-coupled device (CCD) detector was used to collect Raman spectra with the laser emitting at 785 nm. The laser wavelength was chosen for emission at 1–100% power, depending on the proper energy to get signals strong enough and protect the samples from burning. Crystal silicon with a fixed peak position at 520 cm

 was used to calibrate the Raman shifts before sample analysis. The spectra were collected in WIRE3 workstation software (Renishaw) in extensive mode ranging from 100 cm

. Every spectrum was obtained by three accumulations to enhance the signal-to-noise ratio. Multiple measurements (

 ≥ 3) were conducted in different regions of the same sample to assess the variation.

	A Spectrum Nicolet iN10 system from Thermo Fisher Scientific with a diamond anvil cell and an MCT detector was used for IR observation. Spectra 5.01 workstation software from PerkinElmer was used for data processing. Ink samples were placed in the diamond anvil cells, pressed into thin film, and subjected to IR analysis. The background was subtracted for every measurement. Spectra were collected in high resolution in transmission mode (scan times 22 s, resolution 4 cm

/s, and number of scans 64). Triplicate tests were performed at different sites for every sample.

	All samples were analyzed with Raman microscopy and their spectra were compared to obtain the differences therein. The 110 samples were thus classified into 18 groups. The typical spectrum of each group is illustrated in Figure 1. Each spectrum was characteristic of the peak positions and intensities, which can be used to discriminate one group from the others. The identified components and the detected rates of the 18 groups as well as the samples included are listed in Table I. Scarlet powder and bronze red C were the pigments most frequently detected in the seal inks. The former was detected in groups 3, 4, 15, and 17 with the total detected rate of 36.4% in 110 samples and the latter was detected in groups 2 and 12 with the rate of 31.8%. PbCrO

 existed in groups 15, 17, and 18 (10.0%); here, the PbCrO

 that we identified could possibly be PbMoO

 since these two substances have similar peak positions and they were not further discriminated in the present study. HgS was detected in groups 15 and 18 (8.2%), Fe

 was detected in groups 12 and 16 (5.5%), rhodamine B was detected in group 13 (2.7%), and fast red was detected in group 10 (3.6%). There were still some groups with no typical components identified (groups 1, 5, 6, 7, 8, 9, 11, and 14). The Raman analysis results of bronze red C, scarlet powder, and the fast red coincided well with the recently published studies (11). PbCrO

, HgS, and rhodamine B in ink samples were identified using the Raman method for the first time, to our best knowledge.


	Figure 1: Comparison of Raman spectra in different groups: (a) 1–6, (b) 7–12, (c) 13–18.

	The most frequently detected pigments in the present study, bronze red C and pigment scarlet powder, are widely used in daily life. Bronze red C is a kind of light-resistant, heat-resistant, and colorfast golden powder. It is insoluble in water, alcohol, or paraffin, and is used in printing inks, stationeries, and coloring PVC or rubber products. Pigment scarlet powder (P.R.21) is a red azo pigment, which is usually used in paints, inks, leather, plastic, rubber, latex products, building materials, stationeries, and cosmetics. The chemical structures and the standard Raman spectra of the two compounds are shown in Figure 2.


	Figure 2: Two pigments detected with the highest frequency: (a) Bronze red C, (b) scarlet powder.

	The main peaks of the Raman spectrum of the seal ink containing bronze red C were observed at 1599 cm

 represented the benzene stretching; 1482 cm

 represented the CCH bending in the benzene ring and C=O stretching; 1401 cm

 represented the stretching of the naphthalene ring; 1334 cm

 represented the C-C stretching, N-N, or C-N stretching; and 624 cm

represented the C-Cl stretching. The absorption at 986 cm

 in-plane bending. The peak of the symmetric stretching of SO

in many sulfates appeared at about 990 cm

 and can be used to identify the pigments containing BaSO

 (m) were characteristic and can be used in the identification of bronze red C.

	Pigment scarlet powder main peaks appeared at 1595 cm

 should be assigned to benzene ring vibration; 1546 cm

 should be assigned to the frequency of N-H bending and C-N stretching; 1160 cm

 should be assigned to C-C stretching; 998 cm

 should be assigned to benzene skeleton stretching; 958 cm

 should be assigned to the C-C=O stretching; 519 cm

should be assigned to C-C=O bending and N-C=O bending. Overall, the indicative peaks at 1595 cm

 (s) can be used to discriminate pigment scarlet powder from other the pigments.

, and HgS were detected in some ink samples. The peaks at 824–838 cm

 came from the Cr-O stretching vibration; the peak at 338 cm

 came from the O-Cr-O bending; and the peak at 130 cm

 came from the ion transition in the crystal. There were six indicative peaks in the Raman spectrum of Fe

	FT-IR can hardly be used to analyze the ink samples with paper substrates because of the intense absorption of cellulose in the paper, thus in the present study FT-IR was used to analyze the ink samples on slide only to verify the Raman classification results. The typical samples from each group were further analyzed with FT-IR. The results are illustrated in Figure 3 and Table III. Significant differences were observed in the IR spectra among different groups, which further supported the availability of Raman method in seal inks differentiation.


	Figure 3: IR spectra of inks in different groups.

	The main IR peaks of the ink sample in group 2 with bronze red C as the typical component coincided well with its Raman result and they were observed at 736 cm

 represented the stretching vibration of C=O. The intense peak at 1458 cm

 should be assigned as the skeleton vibration of benzene ring. These two peaks and the peak at 1273 cm

 were indicative and can be used to differentiate the group from the others.

	In the spectrum of group 3 with scarlet powder as the identified component, the main IR peaks appeared at 756 cm

 (s). The IR result was in agreement with the Raman result. The peak at 3184 cm

 represented the absorption of OH stretching vibration; the peak at 3068 cm

 represented the C-H in benzene ring stretching; the peak at 1677 cm

 represented the C=O stretching; the peaks at 1594 cm

 represent the skeleton vibration of benzene ring; the peaks at 1533 cm

 represent the combination of N-H bending and C-N stretching vibration; the peaks at 1014 cm

 represent C-H in ortho-substituents of benzene in-plane bending vibration; and the peaks at 759 cm

represent the out-plane bending vibration. The peaks at 1677 cm

 can be easily found in the spectra of the paints containing scarlet powder with less interference in real cases.

 range in the FT-IR spectrum can help to identify PbCrO

 and HgS, IR cannot provide useful information in their identification.

	A total of 110 seal ink samples from 37 brands collected from 10 Chinese provinces were investigated. They were successfully discriminated into 18 groups with Raman microscopy and the discrimination rate was greatly improved compared to the recent studies. No sample preparation was needed in the analysis, and high quality spectra can be obtained in only 1–2 min. Scarlet powder and bronze red C were the pigments most frequently detected in the seal inks, with detected rates of 36.4% and 31.8%, respectively; PbCrO

 (5.5%), rhodamine B (2.7%), and fast red (3.6%) were also detected in some samples. FT-IR on-slide analysis was used to verify the availability of the Raman method in seal inks differentiation. The indicative peak positions and intensities of all the groups in Raman and FT-IR spectra were summarized. The results showed that Raman analysis can discriminate seal inks with paper substrates as well as FT-IR on-slide analysis. The developed approach was proved to be a powerful, sensitive, and rapid method for the differentiation of ink entries of seals on documents.

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Journal of Chinese People's Public Security University 

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	(12) Q. Zhang, "Water-Based Ink Blots and Stability of Principal Component Analysis," thesis of masters degree, Beijing University of Chemical Technology (2011).

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	(16) W. Dirwono, J.S. Park, A. Camacho, J. Kim, H.M. Park, Y.H. Lee, and K.B. Lee, 

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	(20) S. Liu, J.M. Feng, W. Zhang, and J.G. Lv, 

Jungang Lv, Shuo Liu, Yuanyuan Li Jimin Feng, Yong Liu, and Songdong Zhou

 are with the Procuratoral Technology and Information Research Center at the Supreme People's Procuratorate in Beijing, China. Direct correspondence to: 

Articles

The analysis of seals, mostly via the ink types, can provide evidence to authenticate the related documents and can play a key role in legal cases. In the present study, a method to discriminate red seal inks using Raman microscopy and Fourier transform infrared (FT-IR) spectroscopy was developed.

Differentiation of Inks Used for Seals by Confocal Raman Microscopy and Fourier Transform Infrared Microscopy

Carbonate-containing and carbonate-free polyvinyl chloride (PVC) are polymers widely used in daily life. Many parts of bicycles and automotives are made of different kinds of PVC and they may be important trace evidence, especially in hit-and-run cases. The analysis of different PVC types is important to forensic scientists. In this study, we used a Fourier transform infrared (IR) microscope and confocal Raman spectroscopy to investigate these polymers. IR and Raman spectra were tentatively interpreted. The results indicated that IR and Raman microscopes are both effective tools to identify PVC and carbonate. Raman spectroscopy was found to be particularly sensitive to a low amount of carbonate in PVC that can be very difficult to identify using IR. The data help forensic scientists to trace suspects and avoid wrong certificates of authenticity. It also offers PVC producers and researchers an effective and fast method to characterize and analyze the carbonate components in PVC products.

Polyvinyl chloride (PVC, Figure 1), as a kind of inexpensive plastic, has become completely pervasive in modern society. It is the third most widely produced plastic, after polyethylene and polypropylene. The list of products made from PVC is exhaustive in daily life, ranging from drainage and potable piping to cards, cling film, toys, and water bottles. It is expected that PVC production will exceed 40 million tons by 2016. PVC can be used in many other ways, including window frames, doors, wallpaper, flooring, furniture, binders, pens, and pressure-sensitive tapes. Even many of the accessories for bicycles and automotives are products of PVC, such as pedals, covers for gear handles, and bumpers. Because these accessories are often found at hit-and-run scenes, it is important for forensic scientists to characterize these PVC products to trace the suspect vehicles. 

Figure 1: Chemical structure and synthesis of PVC.			

Carbonates are usually added into PVC to improve mechanical properties (tensile strength and elongation), thermal stability, and electrical properties. Calcium carbonate (CaCO

) is an important carbonate additive and its three mineral polymorphs are aragonite, calcite, and vaterite (1). Infrared (IR) spectroscopy, scanning electron microscopy–energy dispersive spectrometer (SEM-EDS), and thermogravimetric analysis (TGA) were used to discriminate carbonate-containing and carbonate-free PVC (2–14). It is easy to discriminate both forms of PVC by IR when carbonate content is high. However, when carbonate content is low, the IR spectrum is not available because many indicative peaks of carbonate are similar to those of PVC and may be overlapped by the latter. The interoperation of the IR spectrum is not detailed enough yet. SEM-EDS can provide information about the metal element content, but it cannot tell whether the metal is from carbonates or other mineral compounds (such as clay, kaolin, or bentonite) on the surface or in the interior of PVC. TGA also can be used to discriminate the two polymers, but it requires a larger sample amount than the spectroscopy approaches and destroys the samples in the analysis.  

In this study, we used Fourier transform infrared (FT-IR) microscopy and Raman microscopy to investigate carbonate-containing and carbonate-free PVC. IR and Raman spectra were collected and tentatively interpreted. The indicative peaks to distinguish the two PVC in IR and Raman spectra are summarized. Considering that dioctyl phthalate (DOP) is often added into PVC products, we also used the IR and Raman microscopes to discriminate PVC containing DOP and PVC containing both DOP and CaCO

. During the analysis, fewer samples were needed and high-quality spectra and good discrimination could be obtained in only a few seconds. The method proved to be fast, accurate, and effective. To the best of our knowledge, there have been few studies to discriminate carbonate-containing and carbonate-free PVC using FT-IR and confocal Raman microscopy. 

, PVC containing DOP, and PVC containing DOP, CaCO

, and other carbonate samples were provided by the National Technical Committee on Plastic Products of Standardization Administration of China. A PerkinElmer Spectrum GX 2000 system with a diamond anvil cell, a deuterated triglycine sulfate (DTGS) detector, and Spectra 5.01 workstation software were used to collect the IR spectra. The background was subtracted for every measurement and triplicate tests were performed at different sites for each sample. 

Figure 2: IR spectrum of (a) CaCO3, (b) PVC, and (c) PVC with CaCO3.			

A Renishaw inVia confocal Raman microscope system with two lasers emitting at 532 nm and 633 nm, respectively, and a charge-coupled device (CCD) detector was used to collect the Raman spectrum. The 532-nm laser was chosen for emission at 10–100% power, depending on the robustness of the samples. Crystal silicon with a fixed peak position (520 cm

) was used to calibrate the Raman shifts before measurement. The spectra were collected using WIRE 3.0 workstation software in extensive mode ranging from 100 cm

. Each spectrum was obtained by 3–10 accumulations to enhance the signal-to-noise ratio. Multiple measurements (

 = 3) were made in different regions of the same sample to avoid variation. 

Table I: Assignment of peaks for CaCO3			

-containing PVC (Figure 2c). The main peaks in the IR spectrum of CaCO

 were detected at 2513, 1796, 1418, 876, 848, and 713 cm

. This clearly indicates that peaks in the bands of 2400–2550 cm

 were the characteristic absorptions of CaCO

 was Raman active, but not IR active and thus was not included in Figure 2. The peak assignments for CaCO

-free PVC are listed in Table II. Figures 2b and 2c show that the main peaks of CaCO

-containing PVC were both detected in the region of 2800–2900 cm

 and at 1430, 1330, 1250, 1200, 1096, 970, 850, 700, and 615 cm

 indicative peaks of strong intensity at 1430, 850, and 710 cm

 were all overlapped by the wide and intensive absorption peaks of PVC at 1430, 848, and 689 cm

. Only the weak indicative peaks at 2513 cm

. As shown in Figure 2c, peaks at 2513 cm

 were very weak and hard to integrate. If the CaCO

 content is low, then it is difficult to discriminate CaCO

DOP is often added into PVC in many products, so we offer an example using IR to discriminate PVC containing DOP and PVC containing both DOP and CaCO

. Figure 3 shows the IR spectra of PVC containing CaCO

 and DOP (Figure 3a) and PVC containing DOP (Figure 3b). In Figure 3a, absorption peaks at 2517, 1794, 1400, 873, and 711 cm

; peaks at 2960, 2927, 2873, 1400, 1287, 967, 711, and 615 cm

 should be assigned to PVC; peaks at 2960, 2873, 1727, 1600, 1461, 1125, 1075, 1040, and 743 cm

 represented DOP. In Figure 3b, peaks at 2960, 2930, 2874, 1400–1460, 1275, 962, 691, and 617 cm

 should be assigned to PVC and peaks at 2960, 2874, 1726, 1462, 1074, 1041, and 743 cm

 should be assigned to DOP. The peak assignment of DOP is listed as follows: 2960 cm

, benzene ring skeleton vibration; 1461 cm

, = C-H in ortho substituents of benzene in-plane bending mode; 1040 cm

, = C-H in ortho substituents of benzene out-plane bending mode. Comparing Figures 3a and 3b, the peaks at 2517, 1794, and 873 cm

Figure 3: IR spectra of (a) PVC with CaCO3and DOP and (b) PVC with DOP.			

were also investigated using IR to find the carbonates' characteristics and observe the tiny differences caused by metal ions in IR spectra. The results are shown in Table III. These results indicate that the peaks in the region of 2400–2550 cm

 were the characteristic absorptions of carbonates. The weak absorption in the 690–740 cm

 region should be assigned to C-O out-plane bending; the intense absorption in the 850–880 cm

 region should be assigned to the C-O in-plane bending; and the wide and intensive absorption in the 1400–1500 cm

 region should be assigned to the C-O stretching vibration. The 1700–1800 cm

 region might be the typical band of the combination of 1080 cm

 region was not reported in the literature and was assumed to be caused by the coupled frequency of 1080 cm

 as verified in many carbonates in Table III. MgCO

appeared different from the others because it existed in the hydroxide form and the peaks in the 2400–2550 cm

 region were too weak to identify. The effects of different metal ions on the IR peak positions are also illustrated in Table III. The tiny, but characteristic differences in IR peak positions would be of great help to identify the carbonates in PVC especially with the lack of elemental analysis instruments such as SEM-EDS. 

Figures 2 and 3 illustrate that the intense indicative peaks of CaCO

 were all overlapped by the absorption peaks of PVC. Only the weak absorbance at 2517 cm

 that were frequencies of combination can be distinguished. Since PVC contains a low amount of CaCO

 were very weak and sometimes they did not even appear, which made it difficult to detect CaCO

Figure 4 shows the Raman spectrum of PVC in the bands of 100–1200 cm

. According to Dollish (15), the main Raman shifts of relative chlorides in liquid solutions and their corresponding vibration modes are as follows: 349 cm

, C-Cl in trans HClC = CHCl in-plane bending mode; 628 cm

C = CHCl symmetric stretching mode; 711 cm

, C-Cl in HClC = CHCl symmetric stretching mode. The vibration modes mentioned above may also exist in the Raman spectra of the PVC products, but the corresponding peak positions shift because of the phase transformation. As shown in Figure 4, the main peaks of PVC in Raman spectrum were 360, 634, 694, 960, 1109, and 1176 cm

C = CHCl asymmetric stretching mode based on the results from IR (16). Peaks in the 1000–1200 cm

 region might be assigned to C-C stretching mode and CH

Figure 5: Raman spectrum of PVC with CaCO3.			

Figure 5 shows the Raman spectrum of PVC containing CaCO

 were extracted as the indicative peaks of CaCO

stretching vibration) was extremely intense and can be used as the best indicator of carbonates (Figure 6). 

Figure 6: Comparison of Raman spectra of PVC with CaCO3and PVC.			

In conclusion, we developed a novel method using IR and Raman microscopy to discriminate carbonate-containing and carbonate-free PVC. The results indicate that IR spectroscopy can discriminate not only carbonate-containing and carbonate-free PVC, but different carbonates clearly even without SEM-EDS. In some cases that involve PVC containing a low amount of carbonates, Raman spectroscopy can be effective (when IR spectroscopy is no longer available) with the sensitive indicative peak of carbonates at 1080 cm

This study was financially supported by the State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (KF2011-09). 

Jungang Lv, Jimin Feng, Yong Liu, Zhaohong Wang,

 are with the Procuratoral Technology and Information Research Center, Supreme People's Procuratorate, in Beijing, China. Please direct correspondence to: 

is with the Biology Institute of Shandong Academy of Sciences in Jinan, China. 

 is with the Agro-Environmental Protection Institute, Ministry of Agriculture in China. 

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Characteristic Raman Frequencies of Organic Compounds

A method is presented using IR and Raman microscopy to discriminate carbonate-containing and carbonate-free PVC that helps forensic scientists trace suspects, avoid wrong certificates of authenticity, and characterize and analyze carbonate components.

Yong Liu

Articles

Differentiation of Inks Used for Seals by Confocal Raman Microscopy and Fourier Transform Infrared Microscopy

Discrimination of Carbonate-Containing and Carbonate-Free Polyvinyl Chloride with Fourier Transform Infrared Microscopy and Raman Spectroscopy