Elemental analysis of bone mineral by backscattering of alpha particles

Backscattering of 2 MeV alpha particles has been applied for the first time to studies of the inorganic substances of human bone. Bone samples taken from the femoral shaft (dense bone) and iliac crest (spongy bone) of 18 cadavers were analysed for the calcium, phosphorus and oxygen content, after lyophilisation and heating for 1.5 h at 500 degrees C to remove water and organic material. The backscattering method produces a spectrum in which all the elements present in the sample except hydrogen can be seen at once. The reproducibility of the measurements was less than 2%. No previous studies were found in the literature of the measurement of elemental oxygen in bone samples. Ca/P ratios varied from 1.97 to 2.47, Ca/O ratios from 0.76 to 0.91 and P/O ratios from 0.34 to 0.41, the latter two being about 10--15% lower than expected if bone mineral were hydroxyapatite (Ca/O = 0.96 and P/O = 0.44). The results are consistent with previous work on Ca/P ratios but the low Ca/O and P/O ratios could not be explained without postulation of crystal or hydroxyl water in bone mineral. Support for the explanation was gained by differential gravimetric and thermal analysis.