Effects of particle size, storage conditions, and chemical pretreatments on carbon and oxygen isotopic measurements of modern tooth enamel

Isotopic analysis of human tooth enamel can provide life history information useful in forensic identification. These applications depend on the availability of reference data documenting isotopic values for individuals with known life history and on the comparability of data from reference and case work samples. Here we build on previous methodological research, which has largely focused on paleontological and archaeological samples, and conduct experiments using enamel from modern human teeth targeting three sample preparation variables (sample particle size, storage conditions, and chemical pretreatments). Our results suggest that differences in particle size affect the efficiency of sample reactions during pretreatment and analysis, with coarse particles giving reduced loss of enamel carbonate during acid pretreatments but producing slightly higher oxygen isotope values than fine particles during analysis. Data for samples stored in dry and ambient environments following pretreatment were indistinguishable, suggesting no exchange of oxygen between samples and ambient water vapor. Finally, chemical pretreatments with a range of commonly used reactants and conditions showed a pervasive, moderate oxygen isotope shift associated with acetic acid treatment, which may be caused by exchange of enamel hydroxyl groups with reagents or rinse waters. Collectively, the results emphasize the importance of methodological standardization to improve comparability and reduce potential for bias in the forensic application of tooth enamel isotope data.