An evaluation of analytical techniques, including Raman Spectroscopy, for use in forensic document examination

Abstract

Forged documents are comnonly encountered in the investigation of crime. Forensic document examiners need to be able to employ techniques that are non-destructive, reliable and admissible in court. In this work, techniques currently in use for ink
analysis, including FLE, TLC, UV - Vis, IR-ATR and Raman spectroscopy, have been reviewed and, where possible optimised in order to evaluate the level of discrimination possible between different ballpoint and gel pen inks. To date, information comparing such a wide range of techniques to relatively large sample sets has not been published.
Particularly, lacking is analysis of ballpoint pen inks from the UK, ballpoint pen ink colours other than black and blue and any analysis of gel pen inks. The results vary widely depending on the combination of ink colour and type, paper and the technique being used. FLE achieved the best discrimination for blue gel pen inks giving a DP of 0.92. TLC is most effective for black and red ballpoint pen inks, giving a DP of 0.89 for both. The highest DP for UV - Vis, 0.86, is achieved for black gel pen inks and for IR-ATR it is red gel pen inks that are most effectively discriminated between, with DPs of 0.88 and 0.87. The Raman spectroscopy analysis used a greater number of combinations of papers, and experimental parameters, i.e. different excitation wavelengths and 'standard' Raman spectroscopy or SERRS with silver or gold colloid, therefore, it is not possible to select a single type and colour of ink to which the technique is best suited.
The pairs of inks discriminated by the five techniques are not mutually exclusive. In real casework it is unlikely that all the instrumentation would be available and there will be financial constraints on how many techniques can be carried out. Therefore, using the data obtained here, protocols have been suggested in order to predict the technique or combination of techniques that is most likely to achieve discrimination of two different inks for each pen ink type and colour. Raman spectroscopy or SERRS are required for 23 out of the 28 recommended protocols suggesting that they would be valuable tools for document examination laboratories.
However, for Raman spectroscopy, although it is a non-destructive method of analysis, when the SER.RS enhancement technique is used, which is effectively non-destructive, some problems with reproducibility were found. Causes of the irreproducibility have been put forward, such as inhomogeneity of the ink and the paper substrate and ink and paper interactions, but no single cause has been identified. Despite this, if care is taken with analysis and interpretation, SERRS results can be useful.