Nicola
Attard-Montalto
*a,
Jesús J.
Ojeda
a,
Alan
Reynolds
a,
Mahado
Ismail
b,
Melanie
Bailey
b,
Lisette
Doodkorte
c,
Marcel
de Puit
c and
Benjamin J.
Jones
d
aExperimental Techniques Centre (ETCbrunel), Brunel University, Uxbridge, UB8 3PH, UK. E-mail: nicola.attard-montalto@brunel.ac.uk
bDepartment of Chemistry, University of Surrey, Guildford, Surrey GU2 7XH, UK
cNetherlands Forensic Institute (NFI), Laan van Ypenburg 6, 2697 GB, The Hague, The Netherlands
dSchool of Applied Sciences, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK
First published on 22nd July 2014
This study thoroughly explores the use of time-of-flight secondary ion mass spectrometry (ToF-SIMS) for determining the deposition sequence of fingermarks and ink on a porous paper surface. Our experimental work has demonstrated that mapping selected endogenous components present in natural fingermarks enables the observation of friction ridges on a laser-printed surface, only when a fingerprint is deposited over this layer of ink. Further investigations have shown limited success on ink-jet printing and ballpoint pen inks. 51 blind tests carried out on natural, latent fingermarks on laser-printed surfaces; up to 14th depletion with samples aged for up to 421 days have resulted in a 100% success rate. Development with ninhydrin was found to affect the fingermark residue through mobilisation of ions, therefore sequencing determination was compromised; whilst iodine fuming and 1,2-indanedione developers did not. This implied that selected development methods affected success in fingermark-ink deposition order determination. These results were further corroborated through inter-laboratory validation studies. The adopted protocol and extensive series of tests have therefore demonstrated the effectiveness and limitations of ToF-SIMS in providing chronological sequencing information of fingermarks on questioned documents; successfully resolving this order of deposition query.
When investigating cases of fraud or counterfeiting, besides recovering the fingermark ridge pattern on a handled document, it is necessary to establish whether the fingerprint has been deposited before or after the surface was written or printed over with compromising ink material. This would allow the forensic document investigator to establish the chronology of a fingerprint on a surface and therefore identify whether an individual is actually associated with the ink-related evidence or simply handled a blank sheet of paper. If it was possible to tell whether a document was handled after inked evidence was deposited onto the surface, a forensic investigator would be able to avoid claims of a suspect handling a pre-printed/signed document: if touched after ink deposition, then the suspect/donor would have handled a pre-printed document. This problem is one of the major challenges in fingermarks associated with document examination, as existing development techniques do not provide any information on chronology or depth of penetration of fingerprints into porous surfaces, making it impossible to determine the order of deposition of fingerprints and inks.
An increasing number of established and emerging analytical characterisation technologies are being implemented to study and retrieve information from fingerprint evidence. These include infrared and chromatographic methods, as well as desorption/ionisation-mass spectrometry techniques, which all have particular features that at present allow or are still being explored to investigate fingermarks. Composition, alteration, presence of contaminants, imaging of ridge detail and increasing the level of understanding of interactions experienced between fingermarks and surfaces, environments, development agents and other processes are all aspects being examined for fingermark analysis, which have been described elsewhere.4,9–15
In cases of fraud or counterfeiting, the examination of questioned documents and fingermark components on these porous surfaces necessitates an investigative method with good spatial resolution, highly surface-sensitive capabilities and minimal sample preparation owing to the complexity of the chronological information required. A recent study into sequencing of fingermarks and inks by Fieldhouse et al.3 reports a good degree of efficiency by electrostatic detection (EDSA) on laser printed inks and fingermarks on paper from surface indentations. In our research we explore the application of time-of-flight secondary ion mass spectrometry (ToF-SIMS) as a suitable technique to establish the chronological sequence of printed inks and fingermarks on paper. This instrumental method targets the chemical composition of a sample, providing a combination of elemental and molecular fragment signatures which can be directly linked to individual constituents within a sample complex. It is therefore much more sample-specific and does not purely rely on physical surface indentations. Although other methods such as matrix-assisted laser desorption ionization (MALDI) and desorption electrospray ionization (DESI) are imaging mass spectrometry techniques with proven success in fingermark analysis,10,13 neither has achieved the spatial resolution or surface sensitivity of ToF-SIMS.16 SIMS uses a finely focussed primary ion beam for the ‘soft’-ionization of a surface through desorption of secondary ions. When coupled with a time-of-flight mass spectrometer, this instrumental method allows for rapid mass-spectral analysis over a wide mass range with good mass resolution. It is also capable of producing high resolution chemical images by rastering the ion beam across the sample surface.17 ToF-SIMS has excellent lateral resolution capabilities, is particularly surface sensitive, and has recently demonstrated applications in the forensic analysis of fingerprints,18–21 papers and paper coatings,22 and in determining the deposition order of fingerprints and ball point inks, however with limited success.18,23
The present work is a development and expansion of our proof of concept study,24 and investigates the use of this technique for chronological determination of inks and natural fingerprints deposited on paper. Rigorous testing has been carried out on a variety of printing papers, overlaid with latent fingerprints acquired from various donors on solid, text and draft laser-printed black and coloured inks. Investigations have explored both latent and developed fingermarks, which were examined in various deposition sequences and at staggered times, with extended ageing periods of over 1 year. Our experimental work has demonstrated that mapping various endogenous components within the fingermark enables the observation of friction ridges from fingermarks on the ink surface when a fingerprint is deposited above a layer of ink. Developers were found to affect the chemical components and resulting ion signal from a fingermark residue to various extents, with certain development methods compromising the ability to determine the order of deposition of these fingerprints on inks. This work therefore also advises the order of which latent fingermarks should be developed on this type of evidence to effectively target the deposition order query.
Variable | Aspect/number tested |
---|---|
Paper | 4 batches: Lyreco (1) and office depot (3 types). ALL 80 g m−2 |
Ink type | 3 laser printers, 2 photocopiers, 1 inkjet printer, 2 black BIC pens (fine and cristal) |
Ink style | Printed laser: solid, draft, text |
Printed inkjet: solid only | |
Ballpoint pen: solid, text | |
Printed ink colours | Black, cyan, magenta, yellow, red |
Donors | 15 for laser printed inks, 2 (one good, one poor) for inkjet printing, 1 donor for ballpoint pens, 3 for blind tests |
Depletions | 1, 2 and 5. Blind tests: up to 14th depletion analysed |
Development | Ninhydrin swabbing (9) and spraying (7); iodine fuming (21) |
Samples analysed | Latent: 119 |
Developed: 37 | |
Blind tests: 51 |
ToF-SIMS was used to individually analyse paper, printed ink and fingermarks deposited on silicon wafer as well as the fingermark-ink FOI/FUI sequencing samples prepared (Table 1). Secondary ion spectra and images were obtained using a Kore Technology Ltd. SurfaceSeer ToF-SIMS spectrometer. Two types of primary ion sources were used: an 114In+ and a 69Ga+ FEI liquid metal ion source. The spectrometer was operated in a pulsed, positive mode with 25 kV applied voltage and 1 μA current. Secondary ions were analysed in a reflectron time of flight mass spectrometer and detected with a dual microchannel plate assembly. A flood gun with low energy electrons was operated simultaneously to compensate for surface charging in insulating samples. Spatial resolution was ∼1 μm diameter, and flight times were recorded with a 0.5 ns time-to-digital converter. Mass spectra acquisition time was set at 60 seconds at a magnification of ×100, and acquired spectra m/z range was between 0–1300 Daltons (Da). Well-defined cation peaks on these spectra between 10–60 Da were generally used to select mapping regions. 512 × 512 pixel chemical maps were acquired over 40–50 minutes at 20 cycles per pixel for 4/5 frames, with a minimum of three areas scanned per sample. Acquired maps measured ∼750 × 750 μm, and were processed where necessary to improve contrast using imaging software. The images themselves were visually assessed to establish whether fingermark traces could be observed over an inked area. All ToF-SIMS spectra were calibrated before analysis.
Development was carried out using standard NFI stock solutions. NIN was prepared by mixing 25 g of ninhydrin (BVDA, Haarlem) and 225 mL of ethanol; with 10 mL ethyl acetate and 25 mL acetic acid (>99% purity, Sigma-Aldrich [Zwijndrecht, the Netherlands]) subsequently added to the stock solution. 1 litre of HFE 7100 (3M St. Paul, USA) was then mixed with 52 mL of NIN until a homogeneous solution was formed. A similar stock solution of IND was prepared by combining 10 g of indanedione (BVDA, Haarlem) and 60 mL of ethyl acetate, 10 mL of acetic acid and 900 mL HFE 7100. A working solution was prepared by mixing 8 mL ZnCl2 (>99% purity, Sigma-Aldrich, prepared with ethanol, ethyl acetate and HFE 7100) with 100 mL indanedione. Both NIN and IND development was executed via immersion, with samples dipped in solutions until wet. These were then air dried for a few minutes and heated for 10 minutes at 80 °C and 20 minutes at 100 °C for NIN- and IND-treated samples respectively.
ToF-SIMS investigations for the inter-laboratory experiments were carried out at the University of Surrey 68–72 days after the samples were developed, and no particular care was taken in the packaging or transportation of the samples. Analysis was executed on an IONTOF GmbH (Münster, Germany) ToF-SIMS 5 spectrometer, employing a 25 keV Bi3+ primary ion beam delivering 0.35 pA of current. Images were acquired at 128 × 128 resolution in the MacroRaster mode over a 5 × 5 mm area. Image data were acquired using 256 cycles per pixel point with 1 scan per pixel and a cycle time of 100 μs.
![]() | ||
Fig. 1 Secondary electron images of photocopied ink (a) at the paper-ink interface and (b) an image of draft printed ink. |
ToF-SIMS data of samples of fingermarks over and under laser-printed ink (FOI/FUI) acquired in this study corroborated the preliminary results published in our proof of concept study.24 Ions in the lower mass range were consistently more prominent in all mass spectra owing to analyte composition and ionising properties of the primary ion beam.22 Mapping of these ions presented clear secondary ion maps with improved ion contrast and material visibility. These lower m/z cations were therefore identified as the most useful species to differentiate between fingermark, paper and ink constituents. Mapped ions included the paper derivatives calcium (Ca2+) and to a lesser extent quaternary nitrogen (C3H8N+) at 40.078 and 58.102 Daltons respectively.22 Silicon (Si+) and SiOH+ at m/z 28.086 and 45.093 Da were identified as the embedding silicon components in laser-printed inks. The organic nature of the samples implied that various hydrocarbon fragments at, m/z 15.034, 27.045, 29.061, 41.071 and 55.098 Da, for example, were identified in all materials being examined (paper, ink, fingerprint). The most prominent hydrocarbon peak generally appeared at C3H5+ (41.071 Da). Relative heights of each component to the surface also affected constituent ion counts. This was as a result of the surface sensitivity of the instrument and subsequent limitations in height resolution over a specific area, with larger distances increasing the time taken for secondary ions to reach the time of flight mass analyser, resulting in contrast differences and identification of laser-printed ink and paper boundaries (Fig. 2).
Sodium (Na+) and potassium (K+) secondary ions of 22.990 Da and 39.098 Da, consistently corresponded to the fingermark deposit. Na+ ions typically dominated as a base peak in fingermark spectra, though these salts sometimes appeared as accretions in maps around pore areas from certain donors. Replicated samples analysed from donors indicated that certain individuals consistently deposited poorer fingermarks, though certain other deposits appeared to be affected by cold weather. This was demonstrated by analysing average ion counts and image contrast in SIMS maps. When a mass spectrum was acquired at a non-pore/Na+ accretion area in poorer donors, this was reflected through comparatively lower peak heights in the mass spectra at these particular areas. Sodium ions also predominated in all examined paper substrates, especially in the Office Depot paper batches. The similarity between Na+ and CxHy+ mass fragments in paper and fingerprint component fractions implied that locating a fingermark deposited on paper from these ions was difficult; whether or not the paper was subsequently printed over with ink (FUI). Occasionally, the sodium ion counts in the fingermark were notably higher than those in the paper, implying that ridges could be identified, but these concentrations varied widely between paper and fingerprint donors, as well as in depletion, day and time of deposition.
Sodium ions were additionally observed as base peaks in mass spectra acquired from cyan and yellow toners, as well as being fairly prominent in magenta mass spectra (Fig. 3). K+ was also abundant in all coloured laser inks, with ion counts in secondary ion maps approximating those in FOI fingermarks, therefore making it difficult to discern ridges in potassium maps. The Na+ peak and respective ion counts on mapping were consistently higher in samples where the fingerprint was deposited over laser printed inks, even in poor donor samples. This implied that determining the fingermark chronology over these inks was still possible (Fig. 3).
Increasing the donor pool, experimenting with different printing styles (photocopies, text and draft printing), and testing aged inks and fingerprints were used to establish relative sensitivity and potential limitations of this ToF-SIMS technique. Following a broad, varied sample study of fingermarks on/under laser-printed ink, factors appearing to affect interpretation were generally a combination of fingerprint donor and printed ink characteristics. Compromising factors included lower ion counts resulting from decreased quantities of natural secretions in collected fingermarks, either as a result of them being significant depletions in a depletion series, and/or because the deposit was acquired from a poor donor, and/or on a cold day. Furthermore, as the paper provided similar ion fractions to the distinguishing secondary ions in the fingermarks, identifying ridges in FOI samples was found to depend on the amount of ink deposited on the paper surface, with text and draft-printed ink making data analysis more difficult. Fingerprint age, ink age, as well as ink colour did not appear to particularly affect interpretation, as was demonstrated through the successful identification of FOI/FUI sequences where fingermarks and inks had been aged for over a year. This indicated that ion mobility through a stratum of ink with time did not compromise FOI/FUI determination in this type of printed ink. Nevertheless, aged fingermarks on draft-printed 300 dpi inks were more difficult to identify, especially after the samples had been aged for a substantial time period (S60 Fig. 8). Furthermore, although fingermarks over coloured laser-printed inks could not be distinguished in respective K+ maps; it was always possible to ascertain an FOI fingermark from the elevated ion counts in Na+ maps.
This wetting-penetration effect was reflected in results obtained, where SEM-EDX analysis generally did not allow for the discernment of printed areas on paper as there was no particular difference in height, composition, or in the appearance of paper fibres, presence of voids and crystallinity of filler aggregates within the paper (Fig. 4). Interface edges of coloured ink and paper in effect required light marking with a ball point pen to identify printed and non-printed paper. A variation in colour was only observed in black ink-jet printed area; though EDX results did not identify any particular difference between this black ink and paper components. Compositional analysis in all samples merely indicated that calcium was dominant in the paper, whether or not it was inked over, and that the inks were organic. This organic nature was confirmed by ToF-SIMS analysis, with most ink fragments observed characteristically denoting long-chain hydrocarbon fractions, with similar major mass spectral peaks obtained from cyan, magenta and yellow inks.
![]() | ||
Fig. 4 SEM images of (a) yellow ink-jet ink on paper (BSE, interface) and (b) filler aggregates in an inked area of the paper. |
Sequencing trials of fingermarks over and under printed inkjet inks were executed on fresh (‘wet’) and older (‘dry’) samples. A slight difference in the strata height and composition of the paper and inked areas was observed in the maps, allowing for identification of paper-ink interfaces, and of printed areas. Initial ion maps analysed from fresh FOI/FUI samples with freshly printed inks gave poor results, mainly owing to outgassing effects experienced on UHV conditions, which resulted in poor ion counts. Chemical images designated swollen printed paper fibres, indicative of the absorption of these ‘wet’ inks into the paper cellular structure, which expanded as a result of this ink application. Fingermark ridges were near-invisible in this fresh-sequencing setup, although an increased amount of salts were observed in a few FOI samples; typically those obtained from the thumb and index fingers from a better donor. This was not a consistent result and therefore implied the methodology was unsuitable for ridge identification on freshly printed ink-jet paper surfaces.
Observation of aged samples showed that printed paper fibres had dried and returned to their original shape and structure. Fibre edges in the black printed areas were somewhat less defined than those in the coloured samples, suggesting that this black ink was deposited as a thicker layer. This could also account for the backscatter greyscale difference between the paper and black ink observed in the SEM images (Fig. 4). Although ion counts in aged samples were improved, sequencing determination of FOI and FUI fingermarks was still inconclusive. In black inkjet-fingermark examples, no ridges were observed in the FOI/FUI samples examined, making it impossible to ascertain whether a fingermark was at all present in a mapped location. In the coloured ink samples, ridges were observed in the majority of the sodium ion maps in the inked-over areas of the paper. These ridges, however, were not limited to FOI sequences but were also observed in the FUI samples as in Fig. 5c.
Fig. 6 schematically demonstrates the differences between the two printing techniques. Toners in laser-jet printing effectively fuse onto a surface in a ‘dry’ process, forming the discrete layer of ink observed in SEM images and SIMS maps. Inkjet ink is conversely deposited in solution, therefore penetrates into the paper, colouring the paper fibres and drying level to the surface (Fig. 6a). When a fingerprint is deposited onto a printed document, a mixture of fingermark components are brought into direct contact with the document surface. The isolating stratum of fused Si-based toner in laser-jet ink is assumed to decrease the porosity of the paper surface, implying that the fingermark components rest on top of this relatively ‘impermeable barrier’ formed by the ink layer. The altered porosity also explains why migration of components is not observed after samples are aged for significant periods of time, implying that fingerprint components are not allowed to interact with the porous paper surface and therefore remain on the ink surface and are identifiable by ToF-SIMS. Conversely, inkjet ink does not form this isolating layer but is deposited by the printer as a ‘wet’ solution. This is demonstrated in the visible paper fibres of the inked-over areas, even after the ink has dried (Fig. 4 and 5).
![]() | ||
Fig. 6 Diagram showing difference (a) between ‘wet’ ink-jet printing and (b) and the ‘dry’ process of laser printing. |
Paper fibres were still visible through the ink layer, especially when a single line of ink was drawn on the surface. Comparatively low sodium ion levels and the presence of basic violet peaks in the ink nonetheless sufficiently allowed for discrimination between paper and ink, and for the visualisation of a paper-ink interface. This also facilitated friction ridge discernment, generally from elevated Na+ levels in FOI samples. Ridge detail was easier to ascertain on ‘solid’ inks, rather than those over inked lines, owing to the elevated ink coverage. Paper fibres were, however, still mostly visible with this ink type.
Our FOI/FUI order of deposition experiments also supported previous deposition order experiments with ball point pen inks.23 The investigations carried out in this study however looked at different mass fragments: Na+ in preference to the ink peak intensities at m/z 372 and 358 Da. This was because these ions were endogenous to the fingerprints and were also the focus of our study, as well as being the base peaks in the majority of the mass spectra. The smaller field of view provided by our instrument also meant that smaller, ink-dominant areas could be mapped. Results consistently presented elevated Na+ levels in FOI samples, but were found to be similarly inconclusive in FUI samples, with some Na+ accretions observed in FUI examples. Variation in ion levels was attributed to the variability observed between samples, with influencing factors including quantity of material in the fingermark residue itself as well as ink thickness in the manually drawn ink lines on a paper surface. This in turn affected the overall thickness of the isolating layer of ink and its ability to mask sodium ions in the paper. Improved results were acquired with thumb and index prints, where ridge detail was consistently observed in FOI examples (Fig. 7a and b). Line-drawn examples were more difficult to interpret. The protocol, therefore, although providing an improved result to inkjet FOI/FUI sequencing, still gave an inconclusive result.
Iodine (IOD) was explored as a non-contact alternative, solvent-free developer. Its vapours are assumed to be physically absorbed into the fatty acids in the fingermarks in a transient, reversible process.32 This made it particularly desirable for developing the fingermarks in this study owing to a potentially less impinging and detrimental effect on fingermark constituents when compared to NIN development.
Initial maps generated from samples developed with IOD presented strong Na+ and K+ secondary ion signals over the ink in FOI samples. This observation was consistent, even with 5th depletions. Results were corroborated through a six-donor sample study that looked at split prints, where one half of print was left undeveloped and the other half fumed with iodine. The simultaneous ToF-SIMS analysis and mapping of these samples allowed for effective comparison of data, where mass spectra were both stacked and overlaid for comparative purposes, and maps compared as shown in Fig. 10. There were no prominent differences in relative peak heights between non-fumed and fumed fingerprints. Minor peak-height variations observed were assumed to be a result of inter-sample differences, owing to the presence of more ridges/material/salts in one half of the print than in the other, within a particular area, despite comparisons being made from splits of the same fingermark.
Fig. 10 shows the total, Na+, K+ and Ca2+ secondary ion maps, and compares undeveloped vs. fumed, FOI and FUI maps; from a known, relatively poor donor. The total and Ca2+ maps indicate the location of the printed text, whereas Na+ and K+ maps show the difference in abundance of these ions over the ink in the FOI vs. FUI samples. FOI results clearly show that the fingermark overlies the ink, whether the sample was IOD fumed or left undeveloped. The latent FUI sample also presents an elevated Na+ signature along fingermark ridges on the paper beneath the ink, indicating that in this particular example, it was also possible to identify the FUI on the paper. These observations were corroborated in maps acquired from the other 5 donors.
The final blind sample test was executed to test the viability of the experimental protocol through analysis of a laser-printed document with fingermarks possibly deposited, before or after the ink present. This therefore made the test quasi-operational, it being the scenario expected with samples acquired from forensic investigation cases. Five out of the six fingermarks deposited on the laser-printed text from this quasi-operational study were located through iodine development, and all samples were correctly identified as FOIs following ToF-SIMS analysis. Fingerprint samples were further developed with NIN, and developed fingermarks showed excellent contrast and ridge detail, therefore indicating that the methodology for FOI/FUI determination (IOD fuming and ToF-SIMS analysis) did not affect further (amino acid) development for ridge-pattern identification.
![]() | ||
Fig. 11 Positive ion SIMS maps of (A) a fingermark deposited over the ink and (B) ink lines deposited over a fingermark, post-development with NIN. |
Sample development by NIN immersion again compromised the ability to identify whether the sample was an FOI/FUI, with K+ and other ion fragments appearing over the ink, even in FUI samples. The ‘NIN effect’ described previously was corroborated by the results obtained in this inter-laboratory validation study, from both known and blind-test samples analysed (Fig. 11).
1,2-Indanedione (IND) was preferentially selected owing to its frequency of use in the NFI as a developer for fingermarks on paper. Development with this reagent gave excellent results, although one sample demonstrated that the quality of donor residue affected interpretation confidence. K+ ion maps (m/z 39.098 Da) generally demonstrated clear ridge detail over the ink only when the fingermark had been deposited over this inked layer. Potassium ions could not be identified on the paper substrate using this ToF-SIMS, or over the inked line when an FUI sample was examined. These observations were corroborated in the blind tests studies as demonstrated in the example provided in Fig. 12 below.
This journal is © The Royal Society of Chemistry 2014 |