IMPROVEMENTS IN THE CALIBRATION OF CD-109-K X-RAY-FLUORESCENCE SYSTEMS FOR MEASURING BONE LEAD IN-VIVO

A Cd-109 K x-ray fluorescence (XRF) system using a point source in a b ackscatter geometry is described. The suitability of plaster-of-Paris phantoms as targets for intercalibration standards was evaluated. When the phantom concentrations were measured by inductively-coupled-plasm a mass spectrometry (ICPMS), the calculated phantom concentrations und erestimated true concentrations by an average of 15%. Since calculated values are used to calibrate the K XRF system, in vivo bone-lead conc entrations may be similarly underestimated. The difference between cal culated and measured concentration is attributable to impurities in th e plaster of Paris (e.g. calcium carbonate). The ICPMS-measured concen trations were used to calibrate the K XRF system. The same phantoms we re also measured as 'unknowns' by a bone-lead measurement system (Abio med, Danvers, MA, USA). The commercial system overestimated the lowest -concentration phantoms and underestimated the phantoms with concentra tions above 15 mu g Pb/g plaster of Paris, The commercial system and o ur system were compared by measurement of the new phantoms in air and in water. The K XRF system exhibited better precision in both situatio ns. On the basis of this work, we recommend that plaster-of-Paris phan toms used to calibrate K XRF measurement systems be analysed first by ICP,MS or another valid analytical technique.