Validation of X-Ray Fluorescence-Measured Swine Femur Lead against Atomic Absorption Spectrometry. (Articles)

Article excerpt

The aim of this study was to apply the technique of [sup.109]Cd-based K-shell X-ray fluorescence (XRF) bone lead measurements to swine femurs and to validate the concentrations obtained therefrom against an independent chemical measurement of bone lead: atomic absorption spectrometry (AAS). The femurs ranged in lead concentration from 1.0 to 24.5 [micro]g of lead per gram of ashed bone, as measured by AAS. On average, XRF overestimated AAS-measured femur lead by 2.6 [micro]g/g [95% confidence interval (CI), 1.1-4.0 [micro]g/g], approximately 2 [micro]g/g poorer than that observed in studies of human tibiae. Measurements of swine femur and, by extension, of nonhuman bones may require adjustment of the XRF spectrum peak extraction method. Key words: atomic absorption, lead poisoning, spectrometry, spectrophotometry, X-ray fluorescence. Environ Health Perspect 109:1115-1119 (2001). [Online 19 October 2001]

Exposure to lead is monitored most commonly by measuring blood lead levels, and in the United States the criteria for lead poisoning and lead toxicity are based on blood lead as a standard. However, the biologic residence time of lead in blood is approximately 36 days (1), and it is therefore an indicator only of recent lead exposure. Moreover, the concentration of lead in blood is a composite index that reflects the equilibrium among current exogenous exposure, excretory loss, and the movement of lead between bone and other deep compartments (endogenous exposure). The relative contribution to the blood lead level of each of these sources varies with the levels of current exposure and body burden.

Lead is stored in the human body predominantly in calcified tissues; 90-95% of the total lead burden is contained within bone in nonoccupationally exposed adults (2,3). The turnover rate of lead in bone is slow; quantitative estimates of the characteristic residence time vary, but there is a consensus that it is on the order of years or even decades (1,4-6). Throughout childhood and most of adult life, lead exposure from both environmental and occupational sources increases lead concentration within calcified tissue. Bone lead content thus reflects integrated or cumulative lead exposure (7).

Bone lead can be measured noninvasively and in vivo by the technique of [sup.109]Cd photon-induced K-shell energy-dispersive X-ray fluorescence (XRF) (6,8,9). The 88.034 keV gamma rays emitted by a [sup.109]Cd source are used to excite the lead atoms contained in bone. The lead atoms in the bone subsequently de-excite and may thereby emit X rays of energy specific to lead. The lead X rays are recorded by a radiation detector and, when compared with calibration data, yield a measure of the lead content of the bone. Although the technique delivers a radiation dose to the subject, the radiation dose and consequent risk arising from an XRF bone lead measurement are very small for all age groups, including children (10).

Three previous reports in the literature compare [sup.109]Cd KXRF to independent chemical measurements of lead in bone in humans. Somervaille et al. (11) analyzed 30 bone samples and found no evidence of a statistically significant difference between XRF and atomic absorption spectrometry (AAS). Hu et al. (12) analyzed eight locations from three legs and reported a correlation coefficient of 0.98 between XRF and AAS. Aro et al. (13), using inductively coupled plasma mass spectrometry, measured eight cadaver legs and also reported the agreement with XRF as correlation coefficients.

Herein we report results of a comparison between XRF and AAS measurements of the lead concentration in 44 swine femurs. Validation of our XRF method using animal bones interests us because animals that have undergone controlled dosing with lead may serve in the future as a source of calibration, validation, or standard reference materials for human XRF measurements. …