X-ray emission analysis of human plasma by proton excitation of thick samples in air.

Thick plasma samples have been irradiated in air by 2.3 MeV protons for trace elemental analysis by proton-induced X-ray emission spectroscopy. Accurate calibration curves were obtained for Fe, Zn and Sr and detection limits in dry plasma were estimated to be 0.08 PPM for Fe, 0.065 PPM for Zn and 0.12 PPM for Sr. Low temperature ashing of these samples improved the detection limits by a factor of 3 but reduced the precision and resulted in loss of bromine from the plasma. Mass absorption coefficients for low energy X-rays in dry plasma were measured and X-ray attenuation in the sample was found to prevent the detection of trace elements with Z less than 26. The method was used to determine Sr in plasma from a patient with bone disease, the Sr being a non-radioactive bone-seeking tracer element. The results, which were in the 3--13 PPM range, were in good general agreement with the analysis of the same samples by flame photometry.