The use of laser desorption/ionization mass spectrometry in the analysis of inks in questioned documents

Determination of the age of a handwritten or ink printed questioned document can be an important consideration in forensic cases. Most often the age of a document is determined by the chemical behavior of the dyes that make up the ink. Exposure of the dyes to environmental factors such as oxygen and ultraviolet or visible light cause them to degrade. Often this degradation can be correlated to the time since the exposure of the ink to the elements began. A number of methods have been used to track the aging of inks on paper. This paper reports the use of laser desorption mass spectrometry as a valuable tool in not only elucidating the structures of dyes used in inks but tracking the change in their chemistry as they age. This study also explores methods for artificially aging documents using ultraviolet and visible light.     

DATINK pilot study: An effective methodology for ballpoint pen ink dating in questioned documents

Establishing the approximate age of an ink entry from a questioned document is often a complicated task and a controversial issue in forensic sciences. Among the existing approaches, the analysis of solvents in ballpoint inks may be a useful parameter for determining the age of ink on paper. In recent years, several ink dating methods have been proposed. These methods have been based on the analysis of common ink solvents using gas chromatography/mass spectrometry (GC/MS) as the analytical platform. Despite these recent methods, several questions remain. The aim of this work was to develop an ink dating methodology (DATINK) for documents written by ballpoint pens based on the disappearance of volatile solvents from the ink entry. Multiple solid-phase microextraction (MHS-SPME) coupled to GC/MS was used to measure the solvents from ink entries made with four BIC^® ballpoint pens. The β parameter, the remaining fraction of the analyte in the system after one equilibration, corresponding to the successive extractions was considered for modelling a mathematical equation for later ink age dating. Preliminary tests of DATINK method showed that it was possible to detect the presence of ink solvents on documents up to the studied five years. The analyses of different real samples of known age were analyzed in terms of β values, which provided a mean relative error of 21%. The proposed use of β parameter for estimating the absolute age of ballpoint ink entries has shown promising results with a standard deviation of β ranging from 0.002 to 0.004.     

Forensic Identification of Dyes in Ballpoint Pen Inks Using LC–ESI–MS

The aim of this work was to develop a new technique using flow injection analysis combined with LC–ESI–MS which allows identification of dyes in ballpoint pen inks. A sample preparation procedure for the extraction of dyes from ballpoint pen strokes has been developed. The characteristic group of ions for each sample of 21 studied ballpoint pen inks corresponding to the present dyes has been determined using flow injection method. LC separation conditions for identified dyes have been optimized on reversed-phase sorbent based on silica gel. The best composition of the mobile phase for the dyes mixture LC separation was 0.1% aqueous formic acid and acetonitrile. Detection of dyes was carried out using mass spectrometry with electrospray ionization in positive and negative modes after reversed-phase liquid chromatography separation. Dye composition of ink was additionally confirmed comparing the data obtained from the literature. Flow injection analysis allows obtaining intensive ions of unknown strokes. It is difficult to get this information using only chromatographic methods, because dyes peak intensity can be low and noise of basic line high. Flow injection method allows distinguishing the analyzed 21 ballpoint pens by determining a characteristic set of dyes. The developed flow injection technique is very simple and quick. As a result, a novel approach for the identification of dyes in the ballpoint pen inks by flow injection analysis with LC–ESI–MS and UV detection without using standard dye samples has been established. It can be an effective alternative to the existing LC–DAD–MS and IR spectroscopy methods.     

Nondestructive classification and identification of ballpoint pen inks by Raman spectroscopy for forensic document examinations

As a non-destructive analytical method, Raman spectroscopy often provides insufficient information to identify or differentiate the ink used for the preparation of a questioned document. In this study, blue and black ballpoint pen inks deposited on paper substrate were examined in situ by conventional Raman spectroscopy. Inks were successfully classified based on the total number of prominent bands in Raman spectra. It was found that more than 90% of the samples of the same type and color could be differentiated visually using only Raman spectra, i.e. 94 and 95% for blue and black inks, respectively. As a result of this study, a flow chart has been constructed for blue and black ballpoint pen inks allowing their systematic identification. Raman spectroscopy proved to be a fast and precise technique for forensic ink analysis.     

薄层色谱法分析圆珠笔色痕形成时间

 利用薄层色谱法对 70种市售的蓝色圆珠笔油墨的染料成分进行了研究。通过与染料标样的薄层色谱数据比较 ,发现国内圆珠笔厂生产的圆珠笔所用的油墨中的染料主要包括碱性紫、碱性蓝及铜酞菁 ,只是不同厂家的油墨中几种染料的配比不同。根据薄层扫描仪对油墨各斑点的扫描结果 ,确定了圆珠笔油墨成分中碱性紫和铜酞菁染料的峰高比值随着书写时间变化的关系 ,从而为法庭科学工作中鉴定圆珠笔字迹的色痕形成时间提供了可靠的依据。     

Sensitivity of redox reactions of dyes to variations of conditions created in mass spectrometric experiments

Redox behaviour of four imidazophenazine dye derivatives under mass spectrometric conditions of matrix-assisted laser desorption/ionization (MALDI), laser desorption/ionization (LDI) from metal and graphite surface, electrospray, low temperature secondary ion mass spectrometry (LT SIMS) and fast atom bombardment (FAB) was studied and distinctions in the reduction-dependent spectral patterns were analyzed from the point of view of different quantities of protons and electrons available for reduction in different techniques. The reduction products [M + 2H](+*), [M + 3H](+) and M(-*), [M + H](-) were observed in the positive and negative ion modes, respectively, which permitted to suggest independent occurrence of reduction and protonation/deprotonation processes. LDI from graphite substrate was the only technique that allowed us to obtain abundant negative ions of all dye derivatives. The yield of field ionization (FI) or field desorption (FD) mechanism to ion formation under LDI from rough graphite surface has been addressed. The sensitivity of reduction of the dyes to variation of reduction-initiating agents confirms high redox activity of the dyes essential for their functioning in natural and artificial systems.     

About the plausible contribution of field ionization in the mechanism of the formation of dyes of ions under conditions of laser desorption/ionization from a nanostructurized graphite surface

An approach is proposed for the estimation of the contribution of field ionization (FI) to the mechanism of dye ion formation under the conditions of laser desorption/ionization (LDI) from a nanostructurized graphite surface. As test systems, rough graphite layers with dyes, e.g., imidazophenazine derivatives applied to them were chosen; these ensure FI in a strong electric field. The dyes form three neutral precursors upon reduction and various types of ions in different ionization methods. It was found that the mass distribution within the group of peaks formed by the initial dye molecule and the products of its reduction in the positive ion mode upon LDI from a rough graphite surface is shifted to lower masses by one atomic mass unit in comparison to the distribution recorded for LDI from a smooth metal support. The analysis of plausible pathways of ion formation has shown that such a shift may be due to the superposition of ions formed by the FI mechanism on a graphite substrate with a number of ions formed by protonation in LDI with no dependence on the support type. In the negative ion mode, the registration of LDI dye spectra succeeded only if the graphite substrates used favored negative FI and electron emission enhanced by the field.     

Determining the relative age of blue ballpoint ink by gas chromatography

A method for identifying the writing age of blue ballpoint pen ink has been established due to the imperative demand in forensic laboratories. The content of the volatile components in blue ballpoint pen ink were determined by gas chromatography (GC). The absorbance of dye in blue ballpoint pen ink was measured by UV-Vis spectrometry A writing age curve of the ink was established using the ratio of the content of the volatile components to the dye, based on the identification of 74 kinds of blue ballpoint pen ink of domestic and international origins. The change of benzene alcohol or phenoxetol with the writing age was also tested. Different kinds of blue ballpoint pen ink were detected and the repeatability of the experiment was investigated. The results indicate that the method is reliable, sensitive, systematic and is especially suitable for practical use. __________ Translated from Chinese Journal of Chromatography, 2005 (3)(in Chinese)     

气相色谱法确定蓝色圆珠笔油墨字迹的形成时间

选择适当的内标物,利用气相色谱法测定蓝色圆珠笔油墨中展色剂苯甲醇和苯氧基乙醇这两种组分的含量(以峰面积计),以展色剂的峰面积、内标物的峰面积与紫外-可见光谱测得的参比物墨水中的染料的吸光度值这三者之比为纵坐标,以字迹形成时间为横坐标,确定了字迹形成曲线,从而建立了确定字迹形成时间的方法。选择了不同种类的蓝色圆珠笔油墨进行检测,获得了良好的实验效果,为圆珠笔油墨字迹形成时间的推断建立了一种可靠而又系统的方法。该方法适用于实际办案。     

Forensic analysis of inks by imaging desorption electrospray ionization (DESI) mass spectrometry

Desorption electrospray ionization mass spectrometry (DESI-MS) is employed in the forensic analysis of documents. Blue ballpoint pen inks applied to ordinary writing paper are examined under ambient conditions without any prior sample preparation. When coupled to an automated moving stage, two-dimensional molecular images are generated. Proof-of-principle experiments include characterization of a simulated forged number and examination of older written records. This application of DESI has advantages over extractive techniques in terms of speed and sample preservation. The effects of the desorbing solvent composition, in this case a mixture of methanol and water, and of flow rate, are evaluated. Results suggest that the solubility of the analyte (dyes Basic Blue 7, Basic Violet 3 and Solvent Blue 26) plays an important role in desorption from the paper surface.     

羧基化石墨烯对4种离子型染料的吸附脱色

合成的羧基化石墨烯（G-COOH）用FT-IR进行表征,并对G-COOH用于水溶液中甲基紫、中性红、灿烂黄和茜素红4种离子型染料的吸附性能进行了研究。 考察了吸附剂用量、吸附时间、初始浓度以及溶液pH值等条件对吸附效果的影响。 同时,研究了甲基紫染料的脱附性能,结果表明,用NaOH/EtOH混合溶液洗脱甲基紫,洗脱率可达88.2%,洗脱后的G-COOH可再利用。 从热力学角度探讨得出,G-COOH对阳离子染料甲基紫和中性红的吸附行为能够较好的符合Langmuir等温吸附模型,而对阴离子染料灿烂黄和茜素红的吸附行为则能够较好的符合Freundlich等温吸附模型,计算的吸附参数表明,G-COOH对4种染料的吸附过程容易进行。 动力学研究表明,G-COOH对4种离子型染料的吸附行为均能较好的符合准二级吸附模型。 该实验研究表明,在处理染料废水时,G-COOH为相当优异的吸附剂。     

Positive and negative‐mode laser desorption/ionization‐mass spectrometry (LDI‐MS) for the detection of indigoids in archaeological purple

Laser‐based ionization techniques have demonstrated to be a valuable analytical tool to study organic pigments by mass spectrometric analyses. Though laser‐based ionization techniques have identified several natural and synthetic organic dyes and pigments, they have never been used in the characterization of purple. In this work, positive and negative‐mode laser desorption/ionization mass spectrometry (LDI‐MS) was used for the first time to detect indigoids in shellfish purple. The method was used to study organic residues collected from archaeological ceramic fragments that were known to contain purple, as determined by a classical high‐performance liquid chromatography‐based procedure. LDI‐MS provides a mass spectral fingerprint of shellfish purple, and it was found to be a rapid and successful tool for the identification of purple. In addition, a comparison between positive and negative mode ionization highlighted the complementarity of the two ionization modes. On the one hand, the negative‐ion mode LDI‐MS showed a better selectivity and sensitivity to brominated molecules, such as 6,6'‐dibromoindigo, 6‐monobromoindigo, 6,6'‐dibromoindirubin, 6‐ and 6’‐monobromoindirubin, thanks to their electronegativity, and produced simpler mass spectra. On the other hand, negative‐ion mode LDI‐MS was found to have a lower sensitivity to non‐brominated compounds, such as indigo and indirubin, whose presence can be established in any case by collecting the complementary positive‐ion LDI mass spectrum. Copyright © 2013 John Wiley & Sons, Ltd.     

Laser desorption/ionization mass spectrometry of dye‐sensitized solar cells: identification of the dye‐electrolyte interaction

Dye‐sensitized solar cells (DSCs) have great potential to provide sustainable electricity from sunlight. The photoanode in DSCs consists of a dye‐sensitized metal oxide film deposited on a conductive substrate. This configuration makes the photoanode a perfect sample for laser desorption/ionization mass spectrometry (LDI‐MS). We applied LDI‐MS for the study of molecular interactions between a dye and electrolyte on the surface of a TiO₂ photoanode. We found that a dye containing polyoxyethylene groups forms complexes with alkali metal cations from the electrolyte, while a dye substituted with alkoxy groups does not. Guanidinium ion forms adducts with neither of the two dyes. Copyright © 2015 John Wiley & Sons, Ltd.     

Laser-Induced Thermal Desorption Facilitates Postsource Decay of Peptide Ions

We investigated the thermal mechanism involved in laser desorption/ionization (LDI) of thermally labile molecules from the flat surfaces of amorphous Si (a-Si) and crystalline Si (c-Si). a-Si was selected for this study because of its thermal property, such as low thermal conductivity; thus, it was predicted to be highly susceptible to laser-induced surface heating. By virtue of lack of surface nanostructures, the flat surfaces offer a simple model system to focus on the thermal mechanism, avoiding other effects, including possible non-thermal contributions that can arise from the physical existence of surface nanostructures. For the energetics study, the internal energies of substituted benzylpyridinium ions produced by LDI on the bare and coated surfaces of a-Si and c-Si were obtained using the survival yield method. The results, including LDI thresholds, ion yields, and internal energies all suggested that the LDI mechanism would be indeed thermal, which is most likely promoted by thermal desorption caused by laser-induced surface heating. In addition, the LDI process driven by laser-induced thermal desorption (LITD) was also found to be capable of depositing an excessive internal energy in resulting LDI ions, which underwent a dissociation. It exhibited the essentially same features as in postsource decay (PSD) in MALDI-TOF/TOF mass spectrometry. We report that the LDI process by LITD offers not only a way of intact ionization but also a facile means for PSD of peptide ions, which this work demonstrates is well suited to peptide sequencing using TOF/TOF mass spectrometry.     

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.     

Laser desorption/ionization mass spectrometry on porous silica and alumina for peptide mass fingerprinting

We investigated a variant of desorption/ionization on porous silicon (DIOS) mass spectrometry utilizing an aqueous suspension of either porous silica gel or porous alumina (pore size of 60 and 90 A, respectively). Laser desorption/ionization (LDI) from samples directly deposited on a stainless steel surface without any inorganic substrates was also achieved. Synthetic peptides designed to cover large sequence diversity constituted our model compounds. Sample preparation, including material conditioning, peptide solubilization, and deposition protocol onto standard matrix-assisted laser desorption/ionization (MALDI) probe, as well as ionization source tuning were optimized to perform sensitive reproducible LDI analyses. The addition of either a cationizing agent or an alkali metal scavenger to the sample suspension allowed modification of the ionization output. Comparing hydrophilic silica gel to hydrophobic reversed-phase silica gel as well as increasing material pore size provided further insights into desorption/ionization processes. Furthermore, mixtures of peptides were analyzed to probe the spectral suppression phenomenon when no interfering organic matrix was present. The results gathered from synthetic peptide cocktails indicated that LDI mass spectrometry on silica gel or alumina constitutes a promising complementary method to MALDI in proteomics for peptide mass fingerprinting.     

纳米材料辅助负离子激光解吸电离-飞行时间质谱分析小分子研究进展

基质辅助激光解吸电离-飞行时间质谱（MALDI-TOF MS）作为一种软电离质谱技术,目前已被广泛用于蛋白质、多肽、核酸、聚合物等大分子分析。由于传统有机化合物基质在低相对分子质量（小于700 Da）区域的干扰,该技术在小分子物质分析方面受到很大限制。为克服传统有机化合物基质在低相对分子质量区域的干扰,近年来以纳米材料为代表的无机基质材料备受关注。相对传统有机化合物基质或纳米材料正离子模式,基于纳米材料的负离子激光解吸电离（LDI）有效避免了正离子模式下一种化合物会产生多种加合物的问题,具有图谱简单易于解析、灵敏度高、重现性好等优点。该文综述了近5年来纳米材料负离子LDI-TOF MS技术在小分子分析方面的研究进展,以期拓展该技术在小分子分析方面的应用。     

Simplification of Ink Dating for Forensic Analysis by Thermal Microdesorption Gas Chromatography–Mass Spectrometry

A simplified procedure for obtaining measurements useful for ink dating is presented for forensic analysis. A thermal microdesorption device was employed with gas chromatography–mass spectrometry (GC-MS). The release of phenoxyethanol from the ink entries on paper was studied at several temperatures during a single analysis. A separate thermal desorption unit was not required. Two methods measuring the amount of phenoxyethanol liberated at four temperatures and two temperatures were proposed. The effect of heating of ink samples to 70?°C for 30?minutes applied to fresh and old samples was studied. An illustration how to distinguish between fresh and old samples by the analysis of the same ink entry after the time period of two months is provided.     

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。     

Surface chemical modification to control molecular interactions with porous silicon

Interactions between porous silicon (pSi) particles and probe molecules were evaluated to determine the effect of pSi and probe molecule chemistry on adsorption. Methylene blue, ethyl violet and orange G dyes were chosen for investigation as they possess distinct functionalities and charges. Several distinct pSi surface species were produced via thermal oxidation at 200-800 °C and their effect on adsorption investigated. The adsorption mechanisms were elucidated from equilibrium adsorption and desorption isotherms. Methylene blue adsorption was attributed to electrostatic attraction where a gradual increase in adsorption with oxidation temperature was observed. Significant methylene blue desorption was observed at pH 3, confirming adsorption occurs via electrostatic attraction. Ethyl violet demonstrated an increase in plateau adsorption capacity and affinity with increased oxidation temperatures and adsorption was initially attributed to electrostatic attraction, however desorption of ethyl violet was not observed, thus indicating potential chemisorption. Orange G exhibited high affinity adsorption for Si(y)SiH(x) terminated surfaces but no orange G desorption was detected, indicating a chemisorption adsorption mechanism. It has been successfully demonstrated that the surface modification of pSi enabled the manipulation of molecular interactions. By interacting probe molecules with similar functionalities to drug molecule with pSi, greater understanding of drug-pSi interactions can be ascertained which are of great importance. pSi surface chemistry can be tailored to enable control over molecular interactions and ultimately dictate loading, encapsulation and release behavior.