EVALUATION OF EXTRACTABLE ORGANIC ZINC AS A QUANTITATIVE MARKER FOR TIRE TREAD PARTICLES IN ENVIRONMENTAL MATRICES

Dichloromethane extractable organic zinc has been proposed as a robust quantitative marker for tire-tread particles in environmental samples including air, soil or sediment; however, to date a detailed analysis of matrix effects on method performance metrics such as precision, accuracy and detection limit has not been completed.  In this study, a standard homogenized artificial test soil used in ecotoxicity testing and consisting of sand, clay and Sphagnum peat moss was spiked with cryogenically ground tread (<200 micron).   Particles representative of passenger vehicle tread, truck tread or a mixture of both types were used with spike concentrations ranging from 500 to 50,000 ppm. Bulk tread and spiked soil samples were extracted in dichloromethane followed by digestion of the organic extract in nitric acid and quantification of zinc by high resolution-ICP-MS.  Modifications to the previously published reflux condenser extraction protocol, including conventional Soxhlet extraction, ultrasonic extraction and shake-flask extraction, were evaluated to optimize organic zinc recovery from the artificial soil matrix.  Additionally, tests were completed to assess the effect of sample weight, extraction time, potential analyte losses during the extraction protocol, use of an alternate solvent (chloroform) and potential positive bias from inorganic zinc (as ZnO).  The results of this study emphasize the importance of considering whole-particle matrix spike and laboratory control samples when evaluating the performance of quantitative environmental marker methods for complex materials such as tire tread particles.  With suitable modifications, this method is likely to be useful for the routine determination of tread content in environmental matrices.