G. Reeda; Dr N. Nic Daeida; Dr K. Savagea & Dr K. Fauldsb. aCentre for Forensic Science & bCentre for Molecular Nanometrology, WestCHEM, Department of ...
A Comparison of Two Raman Spectrometer Systems for the Analysis of Gel Pen Ink Writings on White Office Paper Using 785nm Excitation Wavelength G. Reeda; Dr N. Nic Daeida; Dr K. Savagea & Dr K. Fauldsb aCentre
for Forensic Science & bCentre for Molecular Nanometrology, WestCHEM, Department of Pure & Applied Chemistry, University of Strathclyde, Glasgow, UK in partnership with Foster & Freeman Ltd, Vale Business Park, Evesham, Worcestershire, UK
BACKGROUND Ink analysis can provide significant forensic evidence in an investigation where the authenticity of a document is in question. The standard approach to examination involves observation of colour and appearance of ink on a document by microscopy, their behaviour under different infra-red and ultraviolet lighting conditions, followed by separation and comparison of their soluble dye components by Thin Layer Chromatography. These examinations may identify the type of pen and ink used to create the document, i.e. Ballpoint, roller-ball or felt tip, as well as whether two or more writings on the same document have been written with inks of different chemical composition thereby suggesting an alteration to the original content of the document. With the development and increase in popularity of the gel ink pen over the past decade a problem now exists for the forensic scientist. Due to their predominant use of insoluble pigments to impart colour, TLC has limited effectiveness for analysing gel inks, thereby reducing the ability to discriminate between gel inks of different chemical composition, i.e. different brands. Research underway at the Centre for Forensic Science is investigating alternative ways to discriminate between gel inks, one of which is an instrumental analytical technique called Raman Spectroscopy. RAMAN SPECTROSCOPY A laser of single excitation wavelength (i.e. 785nm) can be focussed through a microscope onto ink on paper. The incident radiation interacts with chemical bonds in the ink molecule resulting in radiation of longer wavelength scattering from the surface. This radiation is detected and converted into a Raman spectrum displayed on a computer screen. The spectrum is a series of peaks at different wavenumber positions, which correspond to different chemical bonds present in the ink molecule. Since different brands may have different chemical composition, the Raman spectrum of two or more ink entries can be visually compared to determine the likelihood of them having been made by the same ink or brand of ink. Figure 2: Classification groups for 29 models of blue gel ink based on Raman spectra produced using Foster & Freeman FORAM 785nm
EXPERIMENTAL Ink lines (~2cm length) were drawn on standard white office paper, using a total of 167 blue gel ink pens representing multiple samples of 29 different models from 19 different brands available on the worldwide market. Each ink line was analysed twice at different points along the length using a 785nm excitation laser from two different Raman Spectrometer systems. The Renishaw InVia was equipped with a x50 microscope objective and operated at 10% of its laser power intensity to generate the best quality and reproducible spectra, whilst the Foster & Freeman FORAM 785 was operated using a x20 objective and 100% of its laser power intensity.
Figure 1: Raman spectra of Pentel Hybrid DX blue gel ink on office paper taken using two different Raman spectrometer systems at 785nm excitation wavelength; (left) Renishaw InVia [x50 objective, 10% laser power] and (right) Foster & Freeman FORAM 785 [x20 objective, 100% laser power]
RESULTS & CONCLUSIONS For a given brand of gel ink, repeat spectral measurements of the same ink line exhibited good reproducibility, as did ink lines made by different pens of the same brand and model. A comparison of the spectra from a given brand generated by both spectrometer systems compared well with the exception of a lower overall intensity observed in the spectra produced from the FORAM instrument as demonstrated in Figure 1. Given this instrument is equipped with a lower magnification of objective (x20) than the Renishaw InVia (x50) this could account for this difference. Further work is currently underway to investigate the influence of using a higher magnification on the intensity of the Raman spectrum. For all brand/model combinations, spectra generated by both instruments was considered to be visually indistinguishable. Based on the spectral data generated by the FORAM instrument, Raman spectra for a given brand/model combination was grouped visually into six different classes based on the presence or absence of peaks as shown in Figure 2. To confirm these visual classifications, a multivariate statistical approach was applied using SPSS 17.0 software. Based on the presence or absence of wavenumber positions, Hierarchical Cluster Analysis of the FORAM spectral data was undertaken as shown in Figure 3. The resulting dendrogram confirmed that the brand/model combinations for a given class could be discriminated from other classes even when spectra were considered highly similar using visual pattern recognition.
FIGURE 3: Hierarchical Cluster Analysis dendrogram produced using SPSS 17.0 based on the presence or absence of wavenumber positions in Raman spectra of 29 brand/model combinations of blue gel ink produced using a FORAM 785 (coloured circles indicate a different class, with the same colour indicating spectral groups that are highly similar but distinguishable)
FUTURE WORK Investigations of black and red coloured gel ink on office paper, as well as blue gel ink, using 785nm excitation wavelength is continuing. Other excitation wavelengths, i.e. 514nm, 633nm, 685nm and 830nm, and the application of multivariate statistical analysis for discrimination of brands is also continuing with a view to recommending a standard analytical approach to analysing gel inks using Raman Spectroscopy.
