Raman spectroscopy of blood samples for forensic applications.

We investigated Raman scattering from human blood as a function of parameters that are relevant for forensic field analysis, such as substrate, sample dilution, individual from which the sample originates, and age of the sample. Peaks characteristic of blood components and in particular the hemoglobin peaks were routinely detected when blood was deposited on substrates that are not strongly luminescent, such as plastic, metal utensils and dry wall. Raman scattering from blood proved quite sensitive and blood samples with a dilution up to 1:250 could be measured for an excitation power of ∼2 mW measured at the sample plane. The sensitivity of Raman scattering to diluted blood allowed measurement using blood reconstituted from fabric substrates, thereby alleviating issues related to luminescence and scattering from the substrate. The dependence of Raman scattering on sample age and individual was also investigated. We found that the relative intensities of scattering peaks depended on sample age and history. For example, the relative intensity of oxyhemoglobin peaks decreases after blood has dried. Fresh blood drawn directly from a donor without intermediate storage exhibits also scattering peaks at 1155 and 1511 cm(-1) which disappear after drying. The origin of these peaks is under investigation. We noticed, however, that they were not found in blood that had been stored for longer than one week in EDTA containers before analysis, thus requiring the use of fresh blood for future studies and validation purposes. The relative intensity of scattering peaks was also found to be somewhat dependent on the donor and, for a same donor, on the day on which blood was drawn.