Chapter 7
Analytical Techniques Used in Document Examination

In this chapter, the principles behind the main categories of analytical techniques will be described in a relatively straightforward manner with the intention that those less familiar with the chemistry involved will at least be able to understand the main ideas behind the methods. For those with more chemical knowledge, more in depth information and examples of published papers that use the techniques can be found in the Further Reading section of the Preface.

Ink, being a coloured material, is obviously amenable to a simple visual examination. For most people, colour discrimination is generally very good and this provides a useful starting point for comparing inks. However, other more objective methods are available that use the optical and chemical properties of ink. The scientific value of any technique can be compared to that of other techniques available so that the potential (evidential) gain of using a more sophisticated method can be weighed up against the probably less costly and less time-consuming simpler technique. Thus, while it may be possible to use ever more sophisticated pieces of equipment, their use needs to be justified in terms of both the likely evidential gain and the practical costs.

Chemical comparison and analysis of the materials on documents essentially involves the examination of the materials from which documents are composed, typically inks (which come to be on a document via handwritten entries, typescript and computer printers and conventional printing), toners and paper (and occasionally other materials such as adhesives may be pertinent to a case). The composition of inks and toners in particular has developed over the years and the technological instrumentation available has become increasingly sophisticated. As new analytical methods are developed, they are often applied to the examination of inks and toners, and for this reason the literature contains a wide variety of approaches to their analysis. No single method has become the standard. However, there are overall processes that can be followed, and these are described in ASTM 1422-05 and 1789-04.

An operational laboratory will not contain the whole range of equipment needed for all the various methods available. The methods used by a forensic document examiner will be constrained by the equipment either in their laboratory or to which they may be able to gain access, for example in a university department. Other constraints are: (i) the sample preparation needed, particularly if it involves removal of ink from the document since such a ‘destructive’ technique may not be sanctioned by the investigator as it materially changes (albeit minimally) the document, (ii) the cost of the technique and (iii) the expertise involved in using the technique and interpreting the results obtained by the experts.

Different techniques are likely to be better for particular types of ink so there is no one standard technique that is suitable for all case examinations. For this reason, it is a necessary preliminary step to identify the means by which the ink came to be on the paper (is it original pen-on-paper ink or from a scanned copy printed with a computer printer?) and this is generally determined by microscopic examination as described in the relevant previous chapters. Once this has been determined, then the techniques available to a particular laboratory can be surveyed to decide which method (or methods) is best to take the examination further.

In most situations there is no shortage of ink to examine, so if one method does not assist then it may be possible to try other methods (for example infrared spectroscopy and Raman spectroscopy could be tried and if they do not help then x-ray fluorescence could be tried (Li et al., 2014; Zięba-Palus & Kunicki, 2006). Such approaches reflect the uncertain nature of the ink and paper combination being examined and the storage conditions and the effects this will have.

In many cases, the forensic need is to compare samples of ink (are these two inks similar or different?) rather than to analyse them (what is this ink made up of?).

In forensic document examination, materials (such as ink) that might be examined using one or other of the techniques described below are often already associated with a substrate (such as a sheet of paper). Thus, ink is probably on a document (rather than still in the pen) and this raises the possibility that the ink and the paper may interact in some way that could affect the results of a chemical examination. For this reason, it is good practice to examine not only the material of interest (the ink) but also to run a ‘blank’ from the substrate so that any components that are from the substrate can be ‘subtracted’ from the result for the test substance. For example, if a blue ink was removed from a sheet of paper and when examined components A, B, C and D were present, but when the paper on its own (with no ink) was examined it contained component D, then the presence of component D in the ink sample may be due its presence in the paper rather than the ink. (Being very cautious, the coincidental possibility that component D was present in both the paper and the ink may need to be investigated if relevant.) The important point, however, is that any potential ‘contamination’ of the test substance by its association with another material must be taken into account when interpreting the results of the various techniques.

While there are many chemical analytical techniques available to the forensic document examiner, there a smaller number of underlying principles on which they are based. These include the separation of components of a mix (typical of that found in inks), the identification of the molecules present and the identification of the elements present. (See Box 7.1 for a brief description of some of the key concepts to assist readers who are less familiar with chemistry.)

The net result is an array of techniques that use different properties of the chemicals present in, say, a sample of ink to either compare or analyse different samples of forensic interest and a number of the methods that exploit these properties will now be described.

7.1 Chromatography

The purpose of chromatography is to separate mixtures of components in a test material (such as ink). In some cases this separation may be sufficient, for example when comparing two or more materials to see if they do or do not contain the same mix of components. However, as each separate component is isolated by the chromatographic process, there is the possibility to then analyse each separated component to determine what it is.

The test material is separated by moving it in a mobile phase (liquid or gas) through a stationary phase. Components with a high affinity for the stationary phase move more slowly through the column, while those with a lower affinity emerge from the column earlier. A number of stationary phases can be used, but examples are alumina or silica materials, which have the appearance of a white powder-like substance. In practice, the components of a test mixture are dissolved into a solvent (the mobile phase) and the different components are separated according to how they interact with the material in the structure which forms the stationary phase that is present in the experimental set up. (The nature of the interaction is complex and can vary according to many different factors. In general, experience has shown that certain combinations of stationary phase, mobile phase and experimental conditions – such as temperature – are optimal for different types of chemical being tested.)

There are variations of this general separation process that broadly fall into three categories:

  • thin layer chromatography, in which the stationary phase is a thin layer of material supported, for example, on a glass or aluminium plate;
  • liquid chromatography, in which the solution containing the mixture is forced through a thin column packed with a stationary phase material; and
  • gas chromatography where the test mixture is in gaseous form and passes through a narrow column the surface of which is coated with a stationary phase material.

Once the chemicals have been separated, there has to be a way of detecting them. With thin layer chromatography of inks, for example, the coloured components are visible to the naked eye. But if a chemical is not coloured, then other means of detecting it must be used. There are a number of such detectors that depend on the properties of the chemicals being analysed. The mechanisms by which detectors work are complex and not relevant here, but the output from the detector is related to the amount of material being detected (analogous to how intense a coloured spot is on a TLC plate to the naked eye). This means that the output from a detector is quantitative. The quantitative data then provide a means to form a database of known chemicals and their chromatographic properties against which the test material can be compared, hence suggesting the identity of the test chemicals present.

Confirmation of the identity of chemicals present can be achieved in a number of ways. For example, the output from the gas chromatography column can be analysed using other methods, typically mass spectrometry (see Section 7.2 below). This double approach is one that is widely practised in forensic chemistry as it is unlikely that two chemicals will behave similarly when analysed using two different analytical methods. In other words, inadvertently mis-identifying a chemical becomes less likely the more techniques that are used to analyse it, and in practice two techniques (such as chromatography and mass spectrometry) are normally sufficient to provide reliable identification.

7.1.1 Thin layer chromatography (TLC)

When used to examine ink present on a document, TLC is a destructive method as it requires a sample of ink to be removed. The sample size needed is small, however, and providing appropriate permission is obtained to take a sample, the slight damage to the document is not usually likely to cause any subsequent (legal) difficulty regarding the ‘completeness’ of the document. TLC is also relatively cheap to perform and, although it does require careful practice to perfect, it is a fairly straightforward technique to use.

In thin layer chromatography, a sample of ink is removed from the document or taken directly from a pen using a solvent. A small spot of the ink sample is placed close to one edge of a chromatography ‘plate’ (typically a layer of powder-like silica gel on a glass support). The plate is then placed into a shallow bath containing another solvent (the mobile phase), which moves up the plate (the stationary phase) by capillary action and as it does so separates the components in the ink samples which can then be compared one with another.

However, interpreting the results using TLC requires some caution. As noted above, it is always necessary to run a paper ‘blank’ and care needs to be taken as it is possible for documents to be accidentally contaminated with inks from, for example, other pages which come into contact with it, not least because ink is a liquid material when placed on the paper and it may take days or weeks to fully dry. Further, while ink formulas do vary from one manufacturer to another, there are likely to be similarities in ink composition between manufacturers since certain dyes and pigments are widely used (added to which there is the possibility that an ink manufacturer could supply their product to more than one pen manufacturer). Because of such factors, it is generally possible to show differences between inks by the different separated components present. However, if two samples show similar patterns of separated components this may simply be a reflection of the wide availability of some ranges of pens in the marketplace rather than providing significant evidence that the same pen/ink was used.

Assessing the separated components on a plate can be done visually, perhaps using other light sources such as ultraviolet light to illuminate the plate. This obviously is a non-quantitative approach, although some approximation of the amount of a separated component may be possible depending on the intensity of the spot on the TLC plate (a faint spot indicating that not much of the component is present, a darker spot indicating more is present). However, there are methods to measure the amounts of material present for each separated component, such as densitometry (which measures optical density of material present) or by using image analysis software. The position of the separated component is usually measured in relation to the position that the mobile phase reached – referred to as the Rf value. Hence two components of an ink that have similar colour but which have different Rf values are in fact different chemicals. A typical plate is shown in Figure 7.1.

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Figure 7.1 A typical thin line chromatography plate showing the separation of ink components. Each component has its own separation factor, known as Rf, which is calculated by dividing the distance the component has migrated (X) by the distance travelled by the solvent (Y).

Published articles that have used TLC to examine inks include:

  • Roux et al. (1999), in which the usefulness of TLC is compared to optical methods of examining ink;
  • Djozan et al., (2008), which uses image analysis software to interpret the separated spots on the plate;
  • Neumann and Margot (2009a,b,c), which are a series of papers that focus primarily on TLC of ink and how the evidence should be handled to achieve reliable results.

7.1.2 High performance liquid chromatography (HPLC)

In HPLC, the two-dimensional plate that is used in TLC is replaced by a tubular column (typically about 3 mm in diameter and up to about 25 cm in length) that is packed with a stationary phase material, such as silica or various polymers. The ink to be examined is forced through the column in a solvent under pressure. Separation of components of the ink again depends on their interaction with the stationary phase, with different components coming out of the column at different (retention) times. There are various parameters that can be changed with this technique, such as the solvent used, the pressure applied and the particle size of the stationary phase, all of which can affect the results obtained.

While TLC produced a physical record of the separation of the components in ink on the plate, HPLC produces a stream of solvent from the far end of the column and the presence of material in the solvent has to be detected by some means. One such is a UV detector, which responds to changes in the ultraviolet response of the material as it emerges from the column. The extent of the detector response is related to the amount of material being detected and so provides a quantitative result.

Papers that have used HPLC to examine inks include:

  • Kher et al. (2006) in which ballpen inks are analysed and the results statistically processed using principal component analysis and linear discriminant analysis;
  • Wang et al. (2008) used a variant known as ion pairing HPLC to compare fountain pen inks;
  • Liu et al. (2006) classified gel pen inks also using ion pair HPLC.

7.1.3 Gas chromatography (GC)

In GC, the material being analysed is heated to make it a gas and this is mixed with a carrier gas (the mobile phase) which passes the sample through a thin tube, or column, which is coated with a particular material (the stationary phase) that can interact with the gas material being analysed thereby affecting its passage along the tube. Different gases interact with different stationary phases in complex ways. The emergence of the test gas from the column (having passed through the column assisted by a carrier gas, such as hydrogen, in a manner similar to the pressure forcing solvent through a HPLC column) is detected (there are a number of detection methods) and the time taken reflects the interaction between the material being tested and the stationary phase coated on the column. Some chemicals do not readily form gases at the required temperatures used in GC, and such materials may need to be chemically pre-treated so as to make them more volatile (a process known as derivatisation).

Papers that have used GC to examine inks include:

  • Bugler et al. (2005) used GC-MS to analyse the non-coloured components of ballpen ink;
  • Li et al. (2014) examined gel pen inks.

GC is widely used with mass spectrometry (see Section 7.2).

7.2 Mass spectrometry (MS)

Mass spectrometry works by ionising compounds to generate charged molecules and molecule fragments and measuring the mass to charge ratio. The sample to be analysed is typically bombarded with electrons, which may cause molecules to ionise and to break up into fragments. The charged fragments and molecules are then subjected to a magnetic field that separates them according to how much charge and mass they have. The output requires the detection of the charged particles and the results are compared against a database of known substances to identify the components in the test material. Mass spectrometry generally requires the substance being analysed to be volatile (so that it can enter the gas phase relatively easily). Recent developments such as electrospray mass spectrometry mean that even molecules of low volatility can be analysed by mass spectrometry.

Papers that have used MS to examine inks include:

  • Williams et al. (2009), in which electrospray ionisation was used to examine ballpen, gel pen and rollerball inks;
  • Coumbaros et al. (2009) used a variant of MS known as time of flight secondary ion mass spectrometry (TOF-SIMS) to examine ballpen inks;
  • Gallidabino et al. (2011) and Weyermann et al. (2012) used the same variant of MS to examine ballpen inks and gel pen inks respectively.
  • Houlgrave et al. (2013) used another variant of MS known as AccuTOF DART (Direct Analysis in Real Time) Mass Spectrometry to analyse components of inkjet inks.

7.3 Spectroscopy

Spectroscopy uses the fact that chemicals (such as those present in inks, for example) can interact with visible light or light of other wavelengths (see also Box 8.1). The wavelength absorbed depends on the atoms and the bonds present between them in the molecules being analysed (see Box 7.1).

In practice, the result of a spectroscopic analysis is a spectrum showing peaks and troughs at different wavelengths, and this can be compared to the spectrum from other samples.

Comparing inks by their response to visible, infrared and ultraviolet light is a standard technique in document examination and the results can be recorded using suitable cameras. Various pieces of equipment have been devised, aimed at document examination, that exploit these properties and are also very valuable when looking at some of the optical devices found in security documents (see also Chapter 5). The use of such equipment would be routine following a visual and/or microscopic examination of the document when assessing it for evidence of alteration (see Chapter 8).

7.3.1 Infrared spectroscopy

The bonds that link the atoms together in the chemicals being analysed can vibrate in a variety of ways, such as symmetrical and asymmetrical vibrations, rocking and twisting. The nature of the vibrations that occur will determine the infrared absorption spectrum for that substance.

In practice, a beam of light is shone onto the sample, and when the frequency is the same as that of a vibrational energy in the sample, the frequency is absorbed. This can be measured and is indicative of the material present.

Papers that have used infrared spectroscopy to examine inks include:

  • Dirwono et al. (2010) used a variant known as Fourier transform infrared spectroscopy (FTIR) to examine the red inks of seals that are commonly used in some east Asian countries instead of a signature.
  • Almeida Assis et al. (2012) analysed black toner with diamond cell FTIR,
  • Sonnex et al. (2014) used infrared spectroscopy to help identify forged currency.

7.3.2 Raman spectroscopy

If the light shone onto the sample is of a single wavelength (monochromatic light typically from a laser beam), then the scattering of that light is dependent on the vibration of the molecules present. This is the principle behind Raman spectroscopy. The information obtained is similar to infrared spectroscopy but, because of the difference in the methods, it yields additional information about the test chemicals. The technique is non-destructive (assuming a low power laser is used) and non-invasive and is thus an ideal method of forensic analysis.

One problem that can occur in Raman spectroscopy is fluorescence caused by the laser. Even though the fluorescence is relatively weak it is much stronger than the Raman scattering and saturates the detector, which is designed for very low intensity light levels. One way to overcome this problem is to treat a small portion of the ink with a gold or silver colloid. This quenches the fluorescence and provides a greatly enhanced Raman spectrum (over a million times increases in sensitivity have been claimed) of the fluorescent material. This variant is known as Surface Enhanced Resonance Raman Spectroscopy (SERRS).

Papers that have used Raman spectroscopy to examine inks include:

  • Seifar et al. (2001) and White (2003) in which SERRS is used to examine ballpen inks;
  • Raza & Saha (2013) used Raman spectroscopy to analyse stamp pad inks;
  • Braz et al. (2013) analysed inks using normal Raman and also surface enhanced Raman spectroscopy;
  • Bell et al. (2013) compared liquid and gel inks using standard Raman, SERRS and other established methods to see which were most suitable.

7.3.3 UV-visible (UV-vis) spectroscopy

When light of the correct wavelength is shone at molecules, the electrons in the chemical bonds can absorb energy. Dyes and pigments are commonly found in inks and these molecules absorb light in the visible region of the electromagnetic spectrum. Thus a red ink will absorb light from all but the red part of the spectrum. The red light is reflected and perceived by the eye as red. The absorption of light plotted against wavelength is the so-called electronic spectrum (so called because it is the electrons that are responsible for the absorption). The absorption of the light often extends into the ultraviolet region of the spectrum, and hence the technique is often referred to as UV-vis spectroscopy. Generally, recording the UV-vis spectrum requires the ink to be in solution and thus it has to be removed from the page. More recently, instruments have been developed to measure the spectrum in situ, thus requiring no destructive pre-treatment.

Papers that have used UV-vis spectroscopy to examine documents include:

  • Adam et al. (2008) in which the results obtained from UV-visible absorption spectroscopy of ballpen inks were analysed using a statistical method known as principal components analysis;
  • Causin et al. (2012) used a variant called diffuse-reflectance ultraviolet-visible-near infrared spectrophotometry to distinguish between sheets of paper that were visually similar.

7.4 X-ray fluorescence (XRF)

XRF is based on the fact that when substances are exposed to x-rays (which have a short wavelength and a high energy) they cause electrons to be lost from the atoms present and this creates an unstable ion (see Box 7.1). The lost electron(s) create spaces in the atomic structure of the atom into which other electrons fall to fill in the gap and this leads to the emission of further energetic x-rays, the wavelengths of which are characteristic for different atoms. In this way the identity of the elements present in a test material can be determined by comparing the results against a database of results from known elements – so-called elemental analysis.

Papers that have used XRF to examine inks include:

  • Zięba-Palus & Kunicki (2006) used XRF, infrared absorption and Raman spectroscopy in the analysis of ballpen and gel pen inks;
  • Chu et al. (2013) used XRF to compare laser toners;
  • Trzcinska (2006) used FTIR to classify toners and then XRF to gain further discrimination between toners.

7.5 Electrophoresis

Different components of a mixture are separated in an electric field that is applied through a gel-like medium. Tiny samples are placed onto the gel and an electric field is applied, which has a different effect on different compounds present. The movement of the components is, therefore, determined by the electrical and physical properties of the chemicals being analysed.

Papers that have used electrophoresis to examine inks include:

  • Szafarska et al. (2011) used electrophoresis to compare inkjet inks;
  • Krol et al. (2013) used two variants of electrophoresis to study stamp inks, the variations being in both the electrophoretic process itself and the type of detector used.

7.6 Case notes when scientific equipment is used

The use of equipment must of course be recorded in the case notes. If more than one similar piece of equipment is available, then it is important that the particular machine used is noted (for example, gas chromatography machine number 1 – rather than number 2 – was used). The reason that it is important to record this information is that if at some point the particular piece of equipment is checked and found not to have been working properly, then it may be necessary to repeat the relevant analyses. Of course, it is a requirement that all pieces of equipment are regularly checked and any relevant information should be available to show that the equipment is working properly.

The details that need recording will depend to some extent on the equipment used and the particular circumstances of the case. However, as also noted at the end of Chapter 6, the case notes should contain all relevant information regarding methods used, equipment used, the conditions (such as the type of column used in gas chromatography) and any other relevant details that would allow another person to recreate what was done and come up with the same results.

Some equipment produces large amounts of data and associated paper plots. These of course also form part of the case notes. Much data is also capable of being stored in a computer, be it digital photographs showing inks under specialist lighting conditions or peaks of different elements present in a sample of paper. Electronic records must also be linked in some way to the physical case file containing all other paperwork relating to the case. Indeed, there is a gradual trend away from paper-based records to electronic records but this is far from complete and much information is still stored in traditional filing cabinets.

7.7 Reports in cases where scientific equipment is used

The use of specialist equipment is almost always in the context of some aspect of a case, such as alterations to a document. Where the findings of an expert required the use of equipment, that use needs to be included in the report. The reason for needing to use the equipment usually suffices (as opposed to an explanation of how the equipment works). For example, if thin layer chromatography of ink is carried out, the reason for so doing could be that the inks could not be differentiated visually using specialist lighting so chemical analysis was done. When it is necessary to refer to the technical details of the equipment or its use, non-technical wordings are ideal, but if they cannot be avoided a short explanation as to their meaning will assist the non-expert reader.

References

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