Introduction
Tasks often requested for the analysis of documents involve distinguishing inks on the same or different documents and also identifying the source or date of a particular ink or paper. Usually these types of determinations are conducted with the help of analytical methods. There are many such methods available and the document examiner should know their capabilities and limitations.
Optical Examinations
Practically each examination of a questioned (Q) document starts with simple optical methods which allow observations in ultraviolet (UV), visible (natural daylight, filtered or unfiltered artificial light) or near infrared (IR) regions of the electromagnetic spectrum. These methods analyze color and luminescence of ink on paper, security fibers in paper, etc. They may also help in viewing obliterated writings, exposing alterations, erasures and substitutions, and discriminating between writing inks that appear to be of the same color.
The morphology of ink lines on paper is examined with the help of optical microscopy that uses visible light for illumination, and glass lenses for magnifying and focusing. This may allow classification of inks being examined, discrimination between inks being compared, or may, in rare occasions, individualize the writing instrument through its performance characteristics. In combination with spot color tests, this technique is also used for paper fiber analysis that allows the type of fibrous raw materials used for making the paper of Q documents to be determined.
Scanning electron microscopy is used when a highly magnified image (down to the nanometer range) of a micro-fragment of ink on paper or of paper itself is desired, for example, in cases when the sequence of crossing strokes is under examination.
Chemical Reactions
Spot chemical (color or crystal) tests are known to have been used for more than a hundred years for detecting both inorganic and organic ingredients of inks and paper. The spot or solubility tests are carried out both in situ (on the document itself) or on a removed sample. These tests are used to differentiate ink formulas, to presumptively identify the constituents of an ink formula, or to select a solvent suitable for the following extraction of the ink.
Spot color and solubility tests have been used for determining the sequence of crossing strokes of different inks and for evaluating the relative age of inks of the same formula and on the same paper.
Spectroscopic Techniques
Spectroscopic methods measure the absorption, emission, or scattering of electromagnetic radiation by atoms or molecules of compounds. The resulting spectra of the absorption, emission, or scattering of light are functions of wavelength and depend on the energy level structure of atoms or molecules. These spectra are useful for characterizing and identifying (e.g. with infrared spectra) compounds.
X-ray fluorescence is commonly used for solids in which secondary X-ray emission is generated by excitation of a sample with X-rays. The technique has found extensive applications in determining the elemental profile of the ink and paper of suspect currency. This profile is then compared with the profile of genuine currency to uncover inconsistencies.
Energy dispersive X-ray microanalysis combined with scanning electron microscopy (SEM-EDX) is an important analytical method due to its ability to examine surface morphology with high resolution and depth of field, and to produce qualitative and quantitative elemental analyses of selected microareas by detection of characteristic X-rays. Many writing and printing inks contain detectable elements that can be used for characterization and comparison by SEM-EDX. In addition some writing inks have detectable trace rare organometallic compounds added which indicate the year of its production. Finally, the technique is a useful tool for the classification and differentiation of photocopier toners. In particular, it is useful in recognizing monocomponent process toners as they contain magnetic carrier materials (magnetite, ferrite) which are easily detected by SEM-EDX.
SEM-EDX has also been applied to the characterization of trace elemental profiles of pencils. The SEM-EDX analysis of both plain and coated photocopy paper has been used to provide a comparison, detect batch differences or to ensure that the contents of the minor inorganic components detected in the ink or toner samples cut out of the document, are not the result of their contact with the paper.
Other analytical methods that also have been used for determining the elemental composition of ink and paper are inductively coupled plasma mass spectro-metry (ICP-MS) and neutron activation analysis (NAA).
Ultraviolet and visible absorption spectroscopy is used mostly for the analysis of organic materials. It measures the wavelength and intensity of absorption of near-ultraviolet and visible light by a sample.
UV-visible reflectance microspectrophotometry has been applied to measuring reflectance (absor-bance) electronic spectra of ink on paper. The method allows discrimination between similarly colored inks at a considerably higher degree of certainty than it could be done using optical microscopy or evaluation by the unaided eye. Microspectrofluorimetry has been used for measuring the emission spectra of ink on paper and of some additives to paper (fluorescent fibers, optical brighteners).
Infrared spectroscopy measures the wavelength and intensity of the absorption of mid-infrared light by a sample. As the wavelengths of IR absorption bands are characteristic of specific types of chemical bonds, IR spectroscopy can be used to identify compounds. It should be stressed, however, that, if the components of interest are analyzed without isolating from the matrices, their chemical identification is practically impossible; as a rule, only characterization of the major functional groups of the compound can be accomplished. In order to produce conclusive identification, either peak-to-peak correlation using the spectrum of a known sample or a comprehensive software library of IR spectra would be required.
Fourier transform infrared (FT-IR) spectroscopy has been used for the characterization of organic components in many materials commonly examined during document analysis (ink, paper, photocopier toners, correcting fluids, etc.).
Diffuse reflectance infrared Fourier transform spec-troscopy (DRIFTS) has been found to be a reliable, reproducible and selective technique for the classification and identification of photocopier toners. Compared with conventional dispersive IR spectroscopy, the DRIFTS technique provides spectra with a significantly improved signal-to-noise ratio, and therefore, it more effectively extracts data from toners that are normally highly absorbing in the infrared due to the large proportion of carbon black content.
Recently, FT-IR microspectrophotometry (a microscope attachment allows the infrared beam to be focused on an extremely small area) has been extensively used for the characterization and differentiation of writing inks and photocopier toners.
IR spectroscopy can also be used for document analysis in combination with other techniques. Thus, ink resin can undergo pyrolysis (see below), followed by IR analysis of the volatile gases generated. In most cases, spectra of the pyrolysis products resemble those of the parent substances. Even when they do not, the spectra are fairly reproducible; thus the reference spectrum of a known substance prepared in the same manner can be used for comparison with the material (ink, toner, paper) analyzed.
Raman spectroscopy (an emission technique in which a laser is directed onto the sample and a very small fraction of the scattered radiation displaced from the laser wavenumber by the vibrational wave-numbers of the sample, is measured) is used for the analysis of inks and photocopying toners in a manner similar to IR spectroscopy.
Chromatographic Techniques
Chromatography is a method used to separate, characterize and identify (e.g. with mass spectrometry) the components of a mixture. Since its introduction in 1903 chromatography has become a separation method that is now a widely accepted and recognized technique.
In document analysis, chromatographic techniques are extensively used for the characterization, comparison, source determination and dating of ink.
Paper chromatography
In paper chromatography the mixture to be separated is allowed to soak along the paper by capillary action; the cellulose in the paper acts as the adsorbent. The technique, as well as paper electrophoresis, has been used for differentiating ink samples.
Thin-layer chromatography (TLC)
This is a form of liquid chromatography that is used for separating nonvolatile organic and inorganic compounds. Among other analytical techniques applied to document analysis, TLC has been most extensively used both for discriminating inks and for identifying ink formulas (by comparison with a ‘complete’ set of standards, see below).
A typical procedure for the TLC analysis of ink is as follows. A sample of ink dissolved in an appropriate solvent, is deposited as a spot (or a band) on the starting line of a TLC plate that consists of a stationary phase immobilized on a glass, aluminum or plastic plate. The constituents of the sample can be identified by simultaneously running standards with the unknown. The bottom edge of the plate is placed in a reservoir with a solvent (mobile liquid phase); the solvent moves up the plate by capillary action. When the solvent front reaches a certain height (e.g. the other edge of the stationary phase), the plate is removed from the solvent reservoir. Inks are mixtures of many components, which move up the plate at different rates due to differences in their partitioning behavior between the mobile liquid phase and the stationary phase. Most separated components of inks are easily detected on the resulting chromatogram due to their own color. The other separated spots (colorless vehicle components) can be visualized with UV light or by treating the plate with an appropriate chromogenic or fluorogenic reagent.
Besides the characterization and differentiation of writing ink and the chemical identification of dyes removed from currency involved in a robbery and exposed to an exploding dye-pack, both conventional TLC and high performance thin-layer chromatography (HPTLC) have been applied to discriminating between stamp pad and typewriter ribbon inks, printing inks (including those used in counterfeit currency), photocopier toners containing dyes mixed with the carbon black pigment, jet printer inks and papers (tinting materials, optical brighteners, sizers and other components of paper can be separated and used to discriminate between paper samples). The method has been used both in its normal phase (hydrophilic stationary phase, e.g. silica gel) and reversed phase (hydrophobic stationary phase, e.g. RP-18 modified silica gel) versions, including gradient elution of ink samples by automated multiple development. Postchromatographic derivatization has been used for the visualization of separated chromatographic zones of colorless organic components of inks and paper. Scanning TLC densitometry has shown a high discriminating power with regard to inks that are indistinguishable to the eye having subtle differences in relative proportions of their dye components.
For over the past twenty years, different approaches using TLC have been used for determining the age of ink on documents. According to a so-called ‘static’ approach that deals with the analytical profiles of inks that do not change with age, the examiner determines the age or source of inks by using a collection of reference standards or by detecting tags, e.g. optical brighteners or other unique components specially added by the manufacturer. If the manufacturer of the ink analyzed is identified and its formula is shown to be unique (through a tag or unique formula known only by the manufacturer), the manufacturer’s files are consulted to determine the initial production date of the ink. This allows one to establish whether a Q ink was available or not at the time the document was allegedly prepared. One obvious limitation here is that only a few inks actually contain unique dating tags.
Another ink dating approach measures the ‘dynamic’ characteristics of an aging ink, i.e. those that change with age. Several ink-dating techniques based on TLC, evaluate the age (date) of a Q entry relative to reference samples which are known dated entries written by ink of the same formula as the Q entry. These techniques primarily use TLC to identify a Q ink formula. However, it should be emphasized that, in fact, unless one is certain that the formula is proven to be unique (see above), the identification of the Q ink formulation with 100% certainty is hardly possible. The reason for this is that, on the one hand, inks of the same type and of similar color are very similar in their dye components (separated and detected by TLC) and, on the other hand, no matter how comprehensive the collection of reference samples is, it will never be complete. Hence, it follows that unless the formula is unique, there is always a possibility that a true match is not in the standard ink library.
This circumstance is of extreme importance and it should always be kept in mind when the examiner uses any ink dating technique that is based on the ink formula identification approach.
High performance liquid chromatography (HPLC)
HPLC is a form of liquid chromatography in which the stationary phase is packed in a separation column.
Components of a sample to be analyzed are separated by injecting a plug of the sample onto the column. These components pass through the column at different rates due to differences in their partitioning behavior between the mobile liquid phase and the stationary phase. The presence of analytes in the column effluent is detected by measuring a change in refractive index, UV-visible absorption at a set wavelength, fluorescence after excitation with a suitable wavelength, or electrochemical response. The separated analytes can also be identified with the help of a mass spectrometric detector.
HPLC has successfully been applied to the characterization and differentiation of ballpoint inks. It enables a high discrimination between inks having similar dye composition by separating and comparing their colorless components such as the ink vehicle components which are reliably detected in the UV region of the electromagnetic spectrum. If inks that are to be compared are on different papers, samples taken from the papers should be analyzed by the same procedure used for the ink-on-paper to ensure that sizers, optical brighteners, tinting materials and other chemicals that may present in the paper would not interfere with the analysis. The modern HPLC provides the examiner with a highly sensitive multi-wavelength detection system (diode array detector) which will provide not only chromatographic profiles of the inks being compared but also the in-situ recorded UVand visible spectra of each eluting peak in the chromatogram. Obviously, such a combination of chromatographic and spectral data improves the ability of HPLC to discriminate between closely related inks. The ability of HPLC to discriminate between similar ink samples is also enhanced by increasing the resolving power with gradient elution.
HPLC has also been used for the analysis of non-ballpoint pen inks, as well as printing inks (including those used in counterfeit currency), photocopier toners and paper.
Capillary electrophoresis (CE)
Performing electrophoresis in small-diameter capillaries allows the use of high electric fields resulting in very efficient separations. Due to electroosmotic flow, all sample components migrate in pH buffer towards the negative electrode. A small volume of sample (a few nanoliters) is injected at the positive end of the capillary and the separated components are detected near the negative end of the capillary. CE detection is similar to detection in HPLC, and includes absorbance, fluorescence, electrochemical and mass spectrometry. Two versions of CE known to have been used for ink and paper analysis are capillary zone electrophor-esis (CZE) and micellar electrokinetic capillary elec-trochromatography (MECC). In CZE separation is solely based on charge, but MECC enables separation of both charged and neutral or even hydrophobic molecules; it becomes possible by adding organic solvents and surfactants to the pH buffers.
CE has recently been applied to the analysis of ballpoint, roller ball, fineliner and marker pen inks, and has shown a very high resolving power that allows the efficient separation of both major and minor components of ink dyes (including their substitution derivatives and isomers) and, therefore, the discrimination between inks with similar dyes from different sources or different batches. The amount of ink-on-paper needed for the analysis is comparable to HPLC and TLC. To detect peaks on the ink electro-phoregram caused by the paper’s constituents (optical brighteners, etc.), blank paper samples of similar size as those taken from the inked paper should also be analyzed.
Gas chromatography/mass spectrometry (GC/MS)
Gas chromatography (GC) is the most widely used analytical technique in forensic laboratories. The technique primarily involves the use of three components: an injector, a separation column (in a thermo-stated oven) and a detector. After vaporization in the heated injector, the sample is then transferred to the column through the use of a carrier gas. The individual sample components mix with the gas, travel through the column and are selectively retained by the stationary liquid phase contained within the column. Finally, a detector is utilized to produce a signal to a recording device. The resulting gas chromatogram is a series of peaks, each of which is characteristic of a particular substance.
It has been shown that the most selective GC determination of components of the complex mixtures can be achieved by the coupling of a micro-mass-spectrometer (mass selective detector) and capillary GC. Mass selective detector uses the difference in mass-to-charge ratio of ionized molecules to separate them from each other. Molecules have distinctive fragmentation patterns that provide structural information usually sufficient for identifying substances separated by GC.
Thus, gas chromatography/mass spectrometry (GC/MS) produces a mass spectral fingerprint for each sample component eluting from the column and, therefore, can allow discrimination between compounds having a very similar chromatographic behavior (close retention indices).
GC/MS has been used for the ink characterization,batch origin determination and ink comparison. In the scan acquisition mode, the method allows identification of an ink’s volatile solid ingredients among which can be nonreacted low molecular mono- or oligomers, reagents and also proprietary additives that are often contained in the resins, polymers or other components of ink vehicles (carriers). It has been shown that, even in old ink-on-paper, high boiling vehicle solvents can be detected and identified using the selected ion monitoring (SIM) acquisition mode; the detector is set to monitor ions specific to the solvents commonly used in the manufacture of inks.
Recently, the unique ability of GC/MS to efficiently separate ink volatile components and to quantify them at down to picogram level has been successfully used for developing ink dating techniques applicable to ballpoint, porous tip and roller pen inks, stamp pad inks, inks for jet printers, and other inks containing high-boiling vehicles.
Pyrolysis gas chromatography
GC is capable of separating volatile organic substances. Therefore, it is not directly applicable to the analysis of such nonvolatile substances as resins in inks or sizing materials in paper. However, pyro-lysis of similar nonvolatile substances leads to their breakdown (thermal decomposition) into smaller compounds which are volatile enough to be analyzed by GC. A pyrolysis device is directly connected to the inlet of the gas chromatograph and the compounds produced by pyrolysis are separated and detected by the chromatographic system. The resulting pyrogram is a highly specific pattern of peaks which is a ‘fingerprint’ of the substance analyzed.
Pyrolysis GC with mass spectrometric detection (PyGC/MS) has been used for the characterization of nonvolatile organic components in inks and photocopier toners. The technique has provided high discrimination between closely related inks and toners.
