Abstract
Using the technology available to forensic investigators today, it is not currently possible to distinguish (with certainty) whether a fingerprint is above or below a layer of ink. This has implications where a suspect’s fingerprint may be found on a document, but they claim to have handled the paper before any ink was put down on the paper, e. g. “I only put the paper in the drawer”. Furthermore, most of the techniques destroy the sample, which in a forensic context is not recommended. Fundamental experiments analysing fingerprints and inks on basic substrates such as silicon and paper have been carried out. A protocol for determining the deposition order of overlapping undeveloped fingerprints and inks using time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been developed. The developed protocol is based on using the normalised standard deviation of the intensity of an ink line, where that ink line overlaps with a fingerprint. This method produces a numerical value for the sample, thereby removing any potential human error in judgement when imaging the sample alone. After further testing it was shown that this first protocol was not robust. A new method and protocol was developed based upon using the entirety of the data produced in the secondary ion image. This new protocol is based upon using the fingerprint ridge pattern as a mask on the ink line signal, rather than using line scans, which are less representative of the data collected. Further research revealed that when samples involving a fingerprint overlapping with ink are chemically developed the ability of the protocol to identify the deposition order was affected. In some samples, the chemical development process appears to have had a positive effect on the protocol. The results presented indicate that diazafluoren-9-one immersion could be used preferentially over ninhydrin when investigating questioned documents. These results could affect the order in which forensic investigators use different chemical development techniques to analyse evidence. Currently it is common practice to use more than one technique to develop fingerprints. The results presented could lead to recommendations about the order in which chemical development techniques are used. Techniques, such as ToF-SIMS, require the use of a vacuum chamber into which the sample is placed for analysis. Currently, there is no published literature on the effect that low pressures systems (such as vacuum chambers) have on the chemistry of fingerprints. This research shows that pressures as low as 2x10-3 Torr have the ability to remove 20-26% (depending on the exact pressure) of a fingerprint’s mass. This suggests it would be beneficial to develop high pressure systems in order to carry out analyses in order to detect all chemical species present in a sample. It was also found that some vacuum chambers can in fact add mass to a sample (believed to be from chemical contamination).