Speciation of Pb in the tidemark of human articular cartilage

The vast majority of the toxic tracelement lead (Pb) is stored in the skeleton. We have recently shown that Pb specifically accumulates in the tidemark (TM) of human articular cartilage at much higher levels compared to trabecular bone. However, the accumulation mechanisms as well as the chemical species of Pb at the TM, which is the border between calcified and non-calcified articular cartilage, are unknown. To check if X-ray absorption spectroscopy is feasible to determine the chemical form of Pb in human bones, we recorded Pb L3 edge EXAFS spectra at the TM of a human femoral head and of a synthetic apatite with 400 ppm Pb as reference substance. Since the estimated thickness of the TM is in the order of 10-20 μm it is necessary to measure the spectra with a small beam focus (here about 150 μm×150 μm) as it is available at the X-ray beamline of the Synchrotron Radiation Laboratory for Environmental Studies (SUL-X) of the synchrotron radiation source ANKA. For a quality check of the small focus setup the reference sample with its size of about 100 mm2 was also measured at the bulk X-ray absorption spectroscopy beamline of ANKA (ANKA-XAS). Because of the low Pb concentration of sample and reference substance the fluorescence mode was chosen to record the X-ray absorption spectra at both beamlines. Fluorescence radiation was collected with a 7 element Si (Li) detector (Gresham, now e2v) at SUL-X and with a 5 element Ge detector (Canberra) at ANKA-XAS. In both cases a silicon (111) crystal pair with a fixed beam exit was used as monochromator.The spectra obtained from the standard material showed no significant differences in noise, indicating that at SUL-X spectra with a quality similar to the bulk measurements can be achieved. Since Pb EXAFS amplitudes are weak compared to other elements we focus in the first data evaluation on the extended XANES (X-ray Absorption Near Edge Structure) region to achieve information about the chemical bond of Pb in hum