Research On The Characteristics Of Stable Isotope Ratios In The Fingernails Of Residents In Different Areas In Shaanxi Province

Stable isotopes have significant advantages in tracing studies,and analyzing stable isotopes in human tissue can infer information such as an individual’s place of residence and dietary habits,which have practical implications for criminal investigations.Human nails grow steadily and contain stable isotope information that reflects an individual’s living area and food changes,making them a natural"recorder"of a person’s life trajectory,with broad prospects for law enforcement applications.Currently,there is poorly researched in the field of forensic science related to stable isotopes in China.Therefore,this study optimized the inspection method for analyzing human nail samples using isotope ratio mass spectrometry（IRMS）,collected 241 human nail samples from 10 major cities in Shaanxi Province,and investigated and analyzed the ratio characteristics and influencing factors of hydrogen（H）,oxygen（O）,carbon（C）,and nitrogen（N）stable isotopes in nail samples from different regions.We established a discriminant indicator and analysis model for stable isotope source tracking in human nails.The main conclusions are as following:This study optimized and selected multiple parameters for the sample pre-treatment method,injection volume,type of reference materials,temperature of the cleavage furnace,combustion furnace,and chromatographic column,as well as the data result correction method for stable isotope analysis of human nails using IRMS.Using this method,after grinding,cleaning,and drying the nail samples,the optimal injection volume was 250μg～300μg,and the optimal temperatures for the cleavage furnace and chromatographic column for H and O stable isotope analyses were 1300℃and 80℃,respectively,while those for C and N stable isotope analyses were 900℃and 80℃,respectively.The detection accuracy of the four stable isotopes of H,O,C,and N in human nail samples was verified by three-point calibration of the reference materials,and the results showed thatδ²H<1.08‰、δ¹⁸O<0.24‰、δ¹³C<0.06‰、δ¹⁵N<0.06‰.Using the established method,we detected H,O,C,and N stable isotopes in 241 human nail samples from 10 major cities in Shaanxi Province,and statistically analyzed their distribution characteristics and influencing factors.The results showed that there was a positive correlation between H and O stable isotopes and between C and N stable isotopes in human nails.There were different in stable isotope distribution among residents in different cities,but there were also some overlap.However,after dividing them into three larger regions,namely,northern Shaanxi,central Shaanxi,and southern Shaanxi according to geographical space,the differences in stable isotopes between the three regions were significant（P<0.05）.Furthermore,geocentric factors such as altitude,latitude and longitude,and drinking water had a significant impact on the H and O stable isotopes in human nails,while daily dietary habits such as vegetables,meat,eggs,dairy products,and seafood had a significant impact on the C and N stable isotopes in human nails.Gender and age did not significantly affect stable isotope ratios in human nails.This study investigated the indicators and statistical analysis models for stable isotope source tracking in human nails.The validation results of single-index and multi-index combinations showed that using data on indicators of H,O,C,and N stable isotopes in human nails for comparison and discrimination could effectively improve the accuracy of source tracing.The selection results of the statistical analysis model showed that the multilayer perceptron neural network（MPL）model had the highest accuracy in classifying and processing the four stable isotope data indicators in human nails,and the accuracy of source tracing and judgment was higher than that of the nearest neighbor element（NNA）and radial basis function neural network（RBF）models.Using these four indicators and the MPL model to classify 10city samples,the results were consistent with the distribution characteristics of stable isotopes among cities,and the source tracing effect between northern Shaanxi,central Shaanxi,and southern Shaanxi regions were good.The results of this study show that the stable isotope ratios in human nails are highly correlated with an individual’s place of residence and daily dietary habits.Analyzing stable isotope ratios in human nails has practical significance for tracing living areas and life trajectories.