The applicability of fluorescence lifetime to determine the time since the deposition of biological stains
Age estimation of bloodstains is of importance to a forensic investigation in that it gives an indication as to when the crime was committed and can discern whether the bloodstains resulted from a single event or were deposited on different occasions. To date, there is no established method for determining the age of bloodstains and therefore this work investigates the potential of using fluorescence lifetime measurements to determine the time since the deposition of bloodstains. Experiments were carried out using human blood obtained from 6 donors. The results showed a decrease in the fluorescence lifetime within the first 91 hours of deposition, after which the fluorescence lifetime plateaued, illustrating a clear distinction between fresh and old bloodstains. The results also show the applicability of this method to stains originated from a blood/saliva mix (5:1 ratio). Saliva stains aged with the same parameters and conditions as blood showed no change in the fluorescence lifetime, suggesting that this phenomenon only takes place in blood and not saliva. Further analysis on test stains revealed that measuring the fluorescence lifetime over a period of time provides information on their freshness and can therefore form the basis of a valuable bloodstain dating technique.