Limitations of Inkjet and Spot Amino Acid Targets for Fingermark Reagent Research and Monitoring – and some Observations

The performance of amino acid mixture impregnated paper test targets for monitoring, and potentially accrediting or validating, the effectiveness of ninhydrin and similar fingermark reagent formulations is reported. Rather than simple visual indicators, optical density measurements have been made of target areas. A ‘spot’ target method has been introduced as a way of adjusting the amino acid areal concentration; these are shown to behave similarly to inkjet-printed targets used elsewhere. After treatment with ninhydrin, these were scanned for optical density measurement. Amino acid levels in the targets were adjusted for best discrimination between reagent formulations. However, the intensity of reagent reaction and hence colour density, with pure amino acid targets, is shown to be substantially greater than with many actual fingermarks. Known poor formulations have produced target block processed intensities close to those of the extensively researched and considered best formulations. Substantial numbers of two types of commercial test targets, promoted for validation of amino acid fingermark reagents, were incorporated into the later stages of the investigation and have apparently validated every solution tested, even those radically different, or known to be very poor operationally, so they also seem to have no significant discriminatory or ‘validation’ capability. A small-scale split fingermark trial with ninhydrin and 1,2-indandione/zinc was carried out with full strength and dilute UK formulations, and a BKA formulation. Significant differences in fingermark performance between reagents was observed in these trials, but none of the spot test targets showed easy visual discrimination. The commercial targets again appeared to validate all formulations. The ‘commercial test targets’ and ‘experimental spot targets’ evaluated here are hence not capable of clearly validating treatment formulations and protocols for quality control purposes.