Photofading of ballpoint dyes studied on paper by LDI and MALDI MS

The determination of the age of an ink entry from a questioned document is often a major problem and a controversial issue in forensic sciences. Therefore, it is important to understand the aging process of the different components found in ink. The aim of this work is to characterize the degradation processes of methyl violet and ethyl violet, two typical ballpoint dyes by using laser desorption/ionization (LDI) and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS), and to evaluate the possible application of the method to forensic examination of documents. The mass spectrometric methods were first tested and were found to be adequate for the purpose of this work. Moreover, it is possible to analyze the dye from a stroke directly from the paper (LDI-MS), so the sample preparation is minimized. The degradation of the dyes methyl violet and ethyl violet in strokes from a ballpoint pen was studied under laboratory conditions influenced by different factors such as light, wavelength of light, heat, and humidity. Then, strokes from the same ballpoint were aged naturally in the dark or under the influence of light over one year and then analyzed. The results show that the degradation of these dyes strongly depends on light fluence. Humidity also increases degradation, which can be explained by the basicity of the paper. The influence of heat on the degradation process was found to be rather weak. It was also observed that the dyes from the ink strokes did not show significant degradation after one year of storage in the dark. In conclusion, the storage conditions of a questioned document and the initial composition of the dyes in the ink have to be known for correct interpretation of the age of an ink entry. Measurements over longer periods of time are necessary to follow the degradation of dyes exempt from light exposure. LDI was found adequate and very useful for the analysis of ballpoint dyes directly from paper without further pretreatment.     

Classification and dating of black gel pen ink by ion-pairing high-performance liquid chromatography

A novel approach for classification and dating of the black gel pen ink entries on document was developed based on ion-pairing high-performance liquid chromatography (IP-HPLC). Ninety-three black gel pens were collected and divided into two groups, dye-based and pigment-based, by preliminary solubility test. The chromatographic conditions for separation of the dye-based black gel pen inks were optimized and the dye components in inks were satisfactorily separated by using 40 mmol/L tetrabutylammonium bromide as ion-pairing reagent. According to the number and the chromatographic retention times of the main dye components, the 50 dye-based inks were categorized into four classes. The inks within a class can be further identified by the percentage of each dye component. The compositional changes of the dye components in the black gel pen ink entries on paper were investigated in light and natural aging conditions and it has been found that the dye components in the ink entries underwent obvious decomposition, and the decomposing extent of the dye components was related to the aging time. The results can provide scientific evidences for dating of the suspicious black gel pen ink entries on documents.     

Characterizing different types of black fountain pen ink using the chemical approach

Fountain pen ink contains dyes, pigments, or nanoparticles as colorants; water and ethylene glycol or an organic solvent as the vehicle. The dyes in fountain pen ink are usually negatively charged acid dyes. In this study, various types of black fountain pen ink (5 dye-based and 5 nanoparticle-based) were investigated by ultraviolet–visible (UV–Vis) absorption spectroscopy, Ion Pair High Performance Liquid Chromatography Diode-Array Detector (IP-HPLC-DAD), Matrix Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF-MS), and Liquid Chromatography Quadrupole Time-of-Flight Mass Spectrometry (LC-Q/TOF-MS) analysis. UV–Vis spectroscopy of fountain pen ink samples showed different profiles despite their similar color. MALDI-TOF-MS analyses successfully differentiated 10 of the inks. Nanoparticle-based ink, which is usually darker than dye-based ink, showed the same UV absorption at 240 nm. The use of LC-Q/TOF-MS, however, allowed the 5 dye-based fountain pen ink samples to be differentiated from one another. On the basis of the combined results of all analytical methods mentioned above, the discriminating power values were 0.71–1.00. Among them, MALDI-TOF-MS could successfully distinguish fountain pen ink samples with similar black colors and provide an easy analytical approach for the differentiation and identification of various types of fountain pens.     

Studies on the degradation of blue gel pen dyes by ion-pairing high performance liquid chromatography and electrospray tandem mass spectrometry

Ion-pairing high performance liquid chromatography (IP-HPLC) was utilized to monitor the composition changes of blue gel pen ink entries on paper stored in different light conditions and natural environment. The chromatographic conditions were optimized by comparing the separation efficiencies of the blue gel pen inks using a series of ion-pairing reagents, including ammonium carbonate, ammonium acetate, triethylamine acetate, tributylamine acetate, tetrabutylammonium bromide and dihexylammonium acetate. It has been found that tributylamine acetate was a suitable ion-pairing reagent for separation of the inks on the common C18 column. The analysis results of the ink entries on paper in different aging conditions showed that the tendency of composition change in natural aging condition was similar with those in fluorescent light and UV light conditions, respectively. One main component dye of the blue gel pen ink, Acid Blue 9, and its degradation products were identified by ion-pairing high performance liquid chromatography coupled with electrospray tandem mass spectrometry. The results showed that the main degradation products originated from the Acid Blue 9. It gave a reasonable explanation for the changing rules of the relative content of the dyes in the blue gel pen ink. The results obtained can provide scientific evidences for dating of the blue gel pen ink entries on documents.     

Invisible Security Ink Based on Water‐Soluble Graphitic Carbon Nitride Quantum Dots

Stimuli‐responsive photoluminescent (PL) materials have been widely used as fluorescent ink for data security applications. However, traditional fluorescent inks are limited in maintaining the secrecy of information because the inks are usually visible by naked eyes either under ambient light or UV‐light illumination. Here, we introduced metal‐free water‐soluble graphitic carbon nitride quantum dots (g‐CNQDs) as invisible security ink for information coding, encryption, and decryption. The information written by the g‐CNQDs is invisible in ambient light and UV light, but it can be readable by a fluorescence microplate reader. Moreover, the information can be encrypted and decrypted by using oxalic acid and sodium bicarbonate as encryption reagent and decryption reagent, respectively. Our findings provide new opportunities for high‐level information coding and protection by using water‐soluble g‐CNQDs as invisible security ink.     

Preparation of photoluminescent nanocomposite ink toward dual-mode secure anti-counterfeiting stamps

Fluorescent photochromism has been applied as an attractive approach for the production of effective authentication substrates to show dual-mode secure encoding. In the current study, novel photochromic and fluorescent nanocomposite ink was developed to introduce a stamped film with strong dual-mode emission for anti-counterfeiting purposes. Inorganic/organic nanocomposite ink was developed from lanthanide-doped aluminate (LDA) dispersed in poly(acrylic acid)-based binder. To produce a transparent film, LDA must be dispersed well in the poly(acrylic acid)-based ink solution. The fluorescent and photochromic nanocomposite ink was stamped effectively onto cellulose documents followed with thermal fixation. Homogeneous fluorescent and photochromic layer was stamped onto paper surface providing a transparent look with the ability to switch to green beneath ultraviolet as illustrated by CIE Lab. The stamped documents were studied by photoluminescence spectra to show an absorption peak at 364 nm, and fluorescence band at 438 nm. The induced security encoding was transparent beneath visible light turning into visible greenish-yellow beneath ultraviolet light indicating a bathochromic shift. LDA was synthesized in the nanoparticle form and characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS). The morphological properties of the stamped documents were examined by infrared spectroscopy (FTIR), scan electron microscope (SEM), EDS, and X-ray fluorescence (XRF). The stamped paper sheets displayed an efficiently reversible photochromism without fatigue under visible and ultraviolet lights. The rheologies of the prepared photoluminescent nanocomposite inks as well as the mechanical performance of the stamped sheets were investigated.     

Application of LC‐MS and LC‐MS‐MS to the analysis of photo‐decomposed crystal violet in the investigation of cultural heritage materials aging

In this work, the accurate liquid chromatography‐ultraviolet‐visible (LC‐UV‐Vis), LC‐mass spectrometry (MS) and LC‐MS‐MS analysis of the photo‐degradation products of crystal violet (CV) is reported. CV is a light fugitive early synthetic dye which had a widespread diffusion into the market starting from the end of the XIX century and was used among others by V. Van Gogh and P. Gauguin in their writings, drawings or paintings. On‐line photodiode array detector enabled simultaneous UV‐Vis spectra acquisition. Many degradation compounds were identified through their exact mass (2 ppm accuracy) and MS‐MS technique. In particular, all CV demethylated products, demethylated Michler's ketone and particularly some compounds that most likely contain oxygen, such as N‐oxides, were found. Fragmentation products are all justified by the proposed fragmentation scheme, in term of precursor exact mass and isotopic profile, characteristic losses in fragmentation and rebuilt structure formula. In particular, we hypothesized the presence of N‐imido oxides and hydroxylamine derivates, never reported before, together with the demethylated derivatives of the studied dyes. All these compounds, although at trace level in our samples, contribute to the discoloration and fading of works of arts made with CV. In particular, demethylation of CV by UV light leads to formation of compounds absorbing at shorter wavelengths than CV (blue shift) or no‐absorbing in visible range (yellow‐colourless) with an overall effect that may appear reddish‐brown. This phenomenon justifies drawings appearing grey or brown on aged yellowed paper, when CV‐based inks or paints were used. The final aim was to better characterize the photo‐degradation of early synthetic dyes (in particular of CV) and to gain a better insight into the discoloration and fading of purple ink strokes made of CV. Copyright © 2012 John Wiley & Sons, Ltd.     

Secret Paper with Vinegar as an Invisible Security Ink and Fire as a Decryption Key for Information Protection

Security inks based on photoluminescent materials are mostly investigated for security applications, such as information encryption and decryption, anti-counterfeiting, and data storage. Although they are invisible to the naked eye under ambient light, they can be detected under ultraviolet or near-infrared light. Herein, a new kind of secret paper made from network-structured ultralong hydroxyapatite nanowires and cellulose fibers has been developed. White vinegar, a common cooking ingredient, is used as an invisible security ink. Covert information on the secret paper written with white vinegar is totally invisible under natural light, but it can be decrypted and clearly read after exposure to fire; the response time to fire is short (<10 s). The ways of writing on the secret paper are diverse by using various pens loaded with white vinegar.     

蓝色圆珠笔油墨紫外光照射变化的研究

在紫外光照射下，通过对蓝色圆珠笔字迹色痕ＦＴ－ＩＲ显微光谱的研究。发现溶剂挥发迅速。其次是环氧树脂的聚合、交联；三苯甲烷染料的分解比较缓慢；而醇酸树脂相对稳定。该研究为字迹色痕的进一步分辨提供了新方法，同时为字迹色痕的“年龄”推断奠定了基础。选用相关峰的相对峰高比来描述这一变化，并进行曲线拟合，消除了因笔力不同而引起的油墨流出量不同造成的误差，这在实际工作中是很有意义的。     

Micro-spectrochemical analysis of document paper and gel inks by laser ablation inductively coupled plasma mass spectrometry and laser induced breakdown spectroscopy

Current methods used in document examinations are not suitable to associate or discriminate between sources of paper and gel inks with a high degree of certainty. Nearly non-destructive, laser-based methods using laser induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to improve the forensic comparisons of gel inks, ballpoint inks and document papers based on similarities in elemental composition. Some of the advantages of these laser-based methods include minimum sample consumption/destruction, high sensitivity, high selectivity and excellent discrimination between samples from different origins. Figures of merit are reported including limits of detection, precision, homogeneity at a micro-scale and linear dynamic range. The variation of the elemental composition in paper was studied within a single sheet, between pages from the same ream, between papers produced by the same plant at different time intervals and between seventeen paper sources produced by ten different plants. The results show that elemental analysis of paper by LIBS and LA-ICP-MS provides excellent discrimination (> 98%) between different sources. Batches manufactured at weekly and monthly intervals in the same mill were also differentiated. The ink of more than 200 black pens was analyzed to determine the variation of the chemical composition of the ink within a single pen, between pens from the same package and between brands of gel inks and ballpoint inks. Homogeneity studies show smaller variation of elemental compositions within a single source than between different sources (i.e. brands and types). It was possible to discriminate between pen markings from different brands and between pen markings from the same brand but different model. Discrimination of ~ 96–99% was achieved for sets that otherwise would remain inseparable by conventional methods. The results show that elemental analysis, using either LA-ICP-MS or LIBS, provides an effective, practical and robust technique for the discrimination of document paper and gel inks with minimum mass removal (9–15 μg) and minimum damage to the document's substrate.