APPENDIX A
Detailed methodology for XANES, m-XRF and m-XRD
Bulk X-Ray Absorption Near-Edge Structure (XANES)
Synthesis of Pb reference compounds. Pyromorphite [Pb5(PO4)3Cl] was synthesized as described in Fomina et al. (2005). Pb adsorbed on cysteine (pH 7.0) was obtained following a procedure similar to that described for Zn by Beauchemin et al. (2004). Goethite was synthesized according to the procedure of Schwertmann and Cornell (1991). To remove non-crystalline Fe, the goethite was washed with ammonium oxalate in the dark for 2 hours using the method described in Ross and Wang (1993). A solution of 1M KCl was used to remove the oxalate, followed by multiple washings with DI water to decrease the ionic strength (Alcacio et al. 2001). The goethite was finally resuspended in 10 mM KCl for the adsorption of Pb. Pb was adsorbed on the goethite at a final rate of 350 mmol Pb per kg goethite at pH 6.5. Humic acids were purchased as Na salts and subjected to dialysis in a cellulose membrane tubing with 3500 Da pore size to remove Na ions (Xia et al., 1997; Moon et al., 1991). The washed humic acids contained in the tubing were then immersed in a Pb nitrate solution at a ratio of 300 mmol Pb/kg (pH 5), agitated for 48 hours following an adaptation of the method used in Sarret et al. (1997), and centrifuged to recover the wet paste used for XANES analysis. The ratio of 300 mmol Pb per kg humic acids (pH 5) should be below the maximum Pb sorption capacity for humic acids (Xia et al., 1997). The three Pb soaps (Pb stearate, Pb palmitate and Pb azelate) were precipitated following the hydrothermal method described in Plater et al. (2003) and freeze-dried for XANES analysis.
Data collection and fitting. The reference compounds were finely ground and diluted in boron nitride to achieve an optimized edge step of one for data collection in transmission mode. Two to four scans were collected and averaged for each Pb reference compound. The household dust samples were directly mounted behind Kapton tape in a 380-µm teflon holder and analyzed in fluorescence mode using a solid-state germanium, multielement detector. Four to seven scans were collected and averaged for each dust sample.
Least-squares linear combination fitting was done using Athena 8.054. No energy shift was allowed for the fitting and no constraint to sum to 1 was imposed. The XANES fitting range varied from -40 to 100 eV (relative energy). The R-factor, reduced chi-squared values and sum of fractions before normalization were adopted as goodness-of-fit criteria.
Micro X-Ray Fluorescence (m-XRF) and X-Ray Diffraction (m-XRD)
For microspectroscopic analysis at the Advanced Photon Source, the dust samples were spread thinly on Kapton® tape and covered with ultralene. They were then placed in a cardboard slide-holder and attached to a vertical plastic sample holder oriented at 45o to the beam in the horizontal plane. The incident X-ray beam was focused using a pair of Kirkpatrick-Baez mirrors, and a monochromatic incident beam was achieved using a Si(111) double crystal monochromator. During m-XRF map collection, the incident X-rays were tuned to 18 KeV and focused to a spot size of 20 µm in the vertical direction by 20 µm horizontally. The motorized sample stage was translated in the X and Y directions through the beam path at 20 mm steps with a count time of 1 s per step.
Micro-XRD patterns were collected in transmission geometry using a MarCCD camera (Mar USA, Evanston, IL) at 2048 x 2048 pixels and with the incident X-ray beam energy tuned to 18 KeV (λ = 0.6881 Å). The distance between the sample and the X-ray detector and geometric corrections were calculated with FIT2DTM software (Hammersley, 1998) using CeO2 as the diffraction standard. Following these corrections the Kapton® tape and ultralene were subtracted from the data and the resulting 2D images were processed to produce 1D diffraction in the computer program, FIT2D (Hammersley, 1998).
REFERENCES
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