Use of Mixed Solvent Systems to Eliminate Sorption of Strongly Hydrophobic Organic Chemicals (SHOCs) on Container Walls.

Sorption coefficients of strongly hydrophobic organic chemicals (SHOCs) are normally measured in aqueous systems to determine the sorption coefficient (K_OC). SHOCs are defined as neutral organic chemicals that have K_OC values ≥ 10,000. Since SHOCs can sorb on container walls leading to overestimation of K_OC values, we used mixed solvent systems to characterize this potential error. Sorption of DDT and anthracene on Teflon (PTFE), polycarbonate (PC) and polypropylene copolymer (PPCO) centrifuge tubes were measured in mixtures of methanol-water that allowed sorption on containers until 100% recovery of the sorbate. The sorption coefficents (K^M) obtained using mixed solvents were evaluated using the Solvophobic theory. The values of K^M decreased exponentially with inreasing fraction of methanol (f^c). Plotting ln K^M VS. f^c, yielded a straight line with a slope that was unique to each sorbent-sorbate combination. From the line, the aqueous sorption coefficient (K^W) and percent recovery (%R^W) were extrapolated at f^c = 0.  For sorption on PTFE, 100% recovery of the two chemicals was at f^c ≥ 0.45. The 100% recovery from sorption of both chemicals on PC and PPCO was at f^c ≥ 0.90. In water (f^c = 0) the calculated recovery of DDT and anthracene when placed in PTFE was 32% and 48%,respectively. However, the calculated aqueous percent recovery (%R^W) of DDT and anthracene from sorption on PC and PPCO was 2 ≤ %R^W ≤14. The data obtained indicate that sorption on container walls is a serious source of error that can reduce the integrity of the analyte and might be one of the causes for the large variability in literature K_OC values, when sorption experiments for SHOCs were determined in aqueous systems.