In situ trace element analysis of human hard tissues by Laser Ablation ICP–MS

After the advent of the laser ablation system (LA), the analysis of micro-samples of hard materials has spread to many scientific fields (i.e. geology, engineering, archaeometry, anthropology). The Laser Ablation Coupled with a Mass Spectrometer (as in the LA–ICP–MS) allows to investigate any hard material at the scale of a few µms, for example, defining the elemental compositional profile of bone, teeth and nephroliths, measuring any element with an atomic mass between 2 and 255 amu (i.e. heavy metals as Pb, Cr, Cd, As, etc.). Moreover, one of the main advantages offered by LA, compared to more traditional techniques, is the drastic reduction of analytical time, because the sample is analysed as it is without chemical dissolution or separation of elements. In this work, we have tested and developed a routine to measure all trace elements in human bones and teeth with the LA–ICPMS housed at the University of Modena and Reggio Emilia. We analyse all trace element but we have focused here our attention on Zn and Pb, given their importance and significance in dentistry, human health and forensic pathology studies. We have then analyzed two human teeth, measuring their trace elements compositional profile and drawing conclusions on the health history of these two individuals. Materials and Methods. We analysed two human teeth (molar 1HT–R and premolar 2HT–R) sampled from two post-medieval mummies (Roccapelago, Modena, 16th – 18th cent.). The concentrations of Pb and Zn were measured in situ through a 213 nm Nd:YAG Laser Ablation system coupled to a quadrupole ICP–MS system (Thermo Fisher Scientific® X SeriesII). Before analysis, each sample was cut in half with a diamond low-speed wheel saw, to expose the inner parts of the tooth. Eighteen LA–spots (diameter 100 µm) were analysed along two profiles (n=10 sample 1HT–R; n=8 sample 2HT–R), exploring enamel and dentine. The elemental concentrations of our samples have been corrected through a calibration with NIST 1486 Bone Meal as a multi-element standard. To correct any matrix-related effect, the calcium concentration, measured with an ESEM Quanta 200, has been used as the internal standard. To ensure the accuracy of the method, a two-tailed t-test was performed on two certified elements of the standard. Statistical analysis shows no significant difference between expected and measured values (p<0.05; mean of 3 analyses). Results. The first tooth (1HT–R) shows a Zn concentration ranging from 230 in the outer enamel to 110 ppm in the inner primary dentine (x = 150 ± 38 ppm, 1σ) and, respectively, a Pb concentration ranging from 3 to 1 ppm (x = 2 ± 1 ppm, 1σ). From the outer enamel to the inner dentine, the second tooth (2HT–R) shows a Zn concentration ranging from 301 to 123 ppm (x = 177 ± 66 ppm, 1σ) and a Pb concentration ranging from 6 to 2 ppm (x = 3 ± 1 ppm, 1σ). Conclusions. Our results are in agreement with Pb and Zn profiles described in literature for human teeth (Humphrey et al. 2008). The concentrations of both elements tend to increase from the enamel-dentine junction (EDJ) to the enamel external surface and tend to decrease from the EDJ to the inner dentine. The Pb concentration of both samples is on average lower than the threshold of poisoning reported for human bones (5 ppm; Hess et al. 2013). The Zn concentrations are low, in particular in the outer enamel. As reported by Lynch (2011), this could be related to tooth wear or to the old age of the two individuals (~ 40 yr). The LA–ICP–MS is a non-destructive in–situ analytical technique, fundamental in forensic pathology and human health studies to measure the elemental composition of human hard tissue and in particular, in outlining the history of the tooth by trace element compositional profile from the inner to the outer portions of the tooth.