Ethnic background and gender identification using electrothermal vaporization coupled to inductively coupled plasma optical emission spectrometry for forensic analysis of human hair
Multi-elemental analysis of human hair was performed by solid sampling (SS) electrothermal vaporisation (ETV) inductively coupled plasma optical emission spectrometry (ICP-OES) and multivariate analysis then carried out to discriminate general ethnicity and gender. This approach is advantageous over digestion and nebulisation, as it eliminates possible contamination or analyte loss that can occur during digestion, requires a smaller sample mass for analysis, and eliminates the dilution that follows digestion. Hair samples were first washed with water and hexane, and then dried before being ground up. Thirteen samples from different people with various ethnic backgrounds were analysed. Aliquots (of 4 mg) were placed in graphite boats that were then introduced in the ETV furnace using an autosampler. No quantification of the elements in hair was necessary. Point-by-point internal standardization with an Ar emission line was systematically carried out to compensate for sample loading effects on the plasma. Then, peak area was integrated and divided by the actual sample mass before being used in combination with multivariate analysis. Elements showing the highest loadings with principal component analysis (PCA) were considered for linear discriminant analysis (LDA). One group of elements, Li, Mo, S, Sr, Cr, K, Ni, Zn and Pb correctly identified all 13 samples for general ethnicity with LDA, whereas this was not possible with PCA. On the other hand, both PCA and LDA correctly assigned all samples for gender using Mg, S, Sr and Zn. The larger number of elements (9 for general ethnicity versus 4 for gender) is commensurate with the greater variety of ethnicities. The technique was also successfully applied to two blind samples from people living in cities as far as 430 km away from where the samples used for the training set were collected. Clearly, SS-ETV-ICP-OES in combination with LDA shows great potential as a tool for forensic analysis of hair.