Reconsideration of methods and standards: Digestion of diaper wipes and use of matrix-matched calibration standards for dust lead analysis

Diaper wipes are widely used for sampling residential dust for lead analysis. A thicker type of diaper wipe was incompletely digested and had low recoveries (~40%) of lead on stock solution spikes using existing protocols. A modified protocol was applied to various quality control samples prepared with thicker diaper wipes in 134 batches of field samples. Modifications included a larger reagent volume (20 mL), more concentrated acid (50% HNO₃ followed by concentrated HNO₃), 3 h on the hot plate, and squeezing wipe residues during filtration. Seventeen batches were reanalyzed using matrix- matched standards. Acceptable lead recoveries were obtained for stock solution spikes (88%) and spikes prepared with leaded dust-SRM 2582 (88%), SRM 2589 (96%), and CRMO 14-050 (99%). Matrix-matched calibration standards increased mean lead recoveries by an additional 8%. Our protocol may provide a basis for a standard operational procedure for wipe digestion and analysis. Differences in estimates of dust lead loadings attributable to the type of wipe and to sample preparation and calibration procedures have implications for risk assessment, clearance testing, and comparability of laboratory data. Reconsideration of current protocols for wipe materials, wipe digestion, and judging laboratory performance (including the ELPAT external proficiency testing program for wipes) is warranted.