Evaluation of fire extinguisher dry powder as an "in situ" development powder for latent fingermarks

Latent fingermarks are present on surfaces of evidence items in many cases. However, current fingermark development methods fail to detect all the invisible fingermarks, and it can be very time consuming and labour-intensive when processing large crime scenes and bulky evidence items with large surface areas. In this study, an "in situ" latent fingermark detection method that makes use of dry powder from a fire extinguisher was evaluated. Its selectivity and sensitivity were compared with those of the current fingermark development methods using traditional fingerprint powders and cyanoacrylate fuming. Powders in three commercial brands of fire extinguishers were first characterized using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX). The analysis revealed the diverse morphological characteristics of the powders, ranging from curved, irregular to sharp-edged forms. This study involved the development and evaluation of over 1700 fingermarks deposited by three donors. Similar to traditional fingerprint powders, Eversafe fire extinguisher dry powder displayed notable superiority in developing sebaceous and natural fingermarks by giving sufficient ridge details and clear secondary details on various glass substrates, whereas cyanoacrylate fuming proved to be less effective in comparison. Even for old fingermarks aged up to 12 weeks old, over 88% had a grade of 3 or 4 and were categorized as "useful" for potential identification, which has provided preliminary evidence for the feasibility of using fire extinguisher dry powders for the development of latent fingermarks. The results indicated that the fingermark detection method was quick, simple and cost-effective, highlighting the need for further investigation into the capabilities and limitations of this approach using a larger cohort of donors to simulate operational conditions.