Rachael Smith Thesis: Using XRF to Resolve Commingling of Human Remains

My workspace while conducting XRF analysis

My thesis uses x-ray fluorescence and trace element analysis to determine if it is possible to resolve commingling using the elemental composition of human bones.  X-ray fluorescence is a type of non-destructive element identification method that bombards a sample, in this case bone, with high energy x-rays which excite atoms causing them to release energy which is specific to each element.  The XRF device measures which elements are detected and at what concentrations in parts per million.  Commingling occurs when multiple skeletonized individuals are mixed together in a single assemblage.  There are a variety of events that can cause human remains to become commingled.  These can include single events such as disasters and mass graves, or over multiple events like a reused burial area.  When archaeologists come across these commingled assemblages it can be difficult to get any useful information from it.  It is important to attempt to resolve the commingling and identify individuals because more specific research questions can be answered, and it might be possible to return such individuals to their loved ones.

This project focuses on the possibility of using the non-destructive

The XRF at work analyzing a vertebra

XRF to resolves commingling which can then lead to identification of individuals.  The remains are from the Arch Street Project which houses the burials that were excavated from the First Baptist Church Cemetery in Philadelphia, PA.  To do this, I have three main research questions: is there elemental variation within a bone, is there variation within an individual, and is there variation between individuals.  For the within bone variation, I sampled six bones (cranium, humerus, tibia, femur, sacrum, and os coxa) at different locations.  I then used RStudio analyses to compare the values for each sample locations for each bone.  For the within individual variation, I tested these six bones plus three vertebrae, clavicle, and a rib.  The last analysis I conducted compared all these bones between the individuals.  For all analyses, I used RStudio which has been an interesting adventure into statistics.  The statistics I used included nonparametric statistics, two-way ANOVA, and a multivariate ANOVA known as a MANOVA.  The last and overarching analysis I will conduct is a mock commingling which will be used to either prove or disprove my hypothesis that XRF can be used to resolve commingling.

The results that appear when the XRF finishes it analysis. The peaks indicate elements and concentrations

The theory behind this project is that overtime elements such as zinc, iron, and even lead replace the calcium in the hydroxyapatite that makes up the bone.  The individual’s metabolism, physiological health, and exposures to chemicals during life can determine the concentrations of each element within the bone.  Because each person has different physiologies and different life experiences, I believe the element concentrations within their bones will also be different.  The main question is are they different enough to separate individuals.  Another problem is that bones vary in density and thus element concentrations based on the location on the bone and the type of bone being sampled.  Trabecular bone is porous and less dense than cortical bone which makes up the shaft of long bones.  The trabecular bone might have different elemental concentrations but is also much more susceptible to diagenesis or the changes that occur post-burial.  Diagenesis can change the elemental concentrations within bone.  One particularly common diagenetic contamination is lead which can be introduced into the bone through soil and ground water.  There are a lot of factors that can impact the elements within bone.  My hope is that this research will be able to identify useful methods for distinguishing individuals in a commingled assemblage and allow the reassociation and identification of those individuals.

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