An interlaboratory comparison of bone lead measurements via K-shell X-ray fluorescence spectrometry: validation against inductively coupled plasma mass spectrometry

Abstract

¹⁰⁹Cd-based K-shell X-ray fluorescence spectrometry (hereafter, for brevity, XRF) is used, often in epidemiological studies, to perform non-invasive, in vivo measurements of lead in bone. We conducted the first interlaboratory study of XRF via the circulation of nine goat tibiæ in which the mean lead value ranged from 4.0 μg g⁻¹ to 55.3 μg g⁻¹ bone mineral. The test tibiæ were subsequently analyzed via nitric acid digestion followed by lead determination by inductively coupled plasma mass spectrometry (ICP-MS) – along with certified reference materials for bone lead – thus providing measurement traceability to SI units. Analysis of dried bone for lead via nitric acid digestion and ICP-MS yields mass fraction data in units of μg g⁻¹ dry weight. The mean bone lead value based on ICP-MS analysis ranged from 1.8 μg g⁻¹ to 35.8 μg g⁻¹ dry weight. For comparison purposes, XRF-measured Pb values (μg g⁻¹ bone mineral) were converted into the ICP-MS-measured units (μg g⁻¹dry weight bone) by multiplying the former by the average ash fraction from the nine tibiæ. Eight of the XRF systems did not yield a significant bias for any of the nine tibiæ; one system was biased for one of the tibiæ; two systems were biased for two tibiæ; one system was biased for four tibiæ; two systems (813-1 and 804-2) were biased for five tibiæ and one system (801-1) was biased for six of the nine tibiæ. Average bias for the systems (under those particular operating conditions) that were biased for the majority of samples ranged from −2.6 μg g⁻¹ (−15.7%) to 5.1 μg g⁻¹ (30.7%) dry weight bone. All participants now have the ICP-MS data, allowing any corrective actions deemed necessary to be implemented. The ICP-MS data, however, indicated that the lead mass fraction varied considerably with the sampling location within the tibiæ, to the extent of exceeding XRF variability for the higher lead values. Material heterogeneity is an unavoidable reality of measuring lead in bone.