Wet powder suspensions as an additional technique for the enhancement of bloodied marks

The enhancement of marks in blood on dark surfaces poses significant challenges to the forensic scientist. Current methods of enhancement include the sequential use of acid dyes (acid yellow, acid violet and acid black). Acid yellow is used to greatest effect on lighter deposits of blood on a non-porous background, and is visualised using a light source which causes it to fluoresce [1]. However, further enhancement with acid violet and acid black produces a dark product which may fail to improve the contrast of the mark against a dark background. The use of wet powder suspensions (WPSs) has been proposed as a complementary procedure for use in fingermark enhancement, beyond its typical use in the enhancement of marks on adhesive surfaces. In this investigation, the use of WPS was tested in conjunction with conventional acid dye treatments on marks in blood deposited on a selection of substrates. The results demonstrated that white WPS alone or together with acid dyes results in an overall enhancement of mark quality (p<0.005) on marks deposited on smooth non-porous surfaces. The technique was shown to not interfere with subsequent presumptive tests on blood. However WPS treatments were shown to reduce the amount of DNA recoverable from the marks, resulting on an average decrease of 91% compared to untreated controls. The decline in DNA yields was shown to result in a decrease in the quality of the DNA profiles obtained. The enhancement properties of WPS were evaluated by electron microscopy. It was shown that the titanium dioxide particles in the WPS primarily interact with the non-bloodied part of the mark, thus producing a contrasting effect with the background and acid dyes.