Tandem-cartridge solid-phase extraction followed by GC/MS analysis for measuring partition coefficients of association of polycyclic aromatic hydrocarbons to humic acid

A tandem-cartridge solid-phase extraction system combining reversed-phase separation and dynamic ion exchange followed by GC/MS analysis was studied for measuring partition coefficients (Kdoc) of association of polycyclic aromatic hydrocarbons (PAHs) to humic acid. Time course batch experiments revealed that the association of PAHs to humic acid included a slow stage with equilibrium time of 5-7 days. The disequlibrium and retention of the PAH-humic acid associate during reversed-phase separation lead to systematic negative error for the measured partition coefficients; more accurate results were achieved with a three-cartridge tandem system in which the negative error was assessed and corrected via the PAH analytes measured on the second reversed-phase cartridge. The potential and advantages of use of C18 disk cartridges instead of conventional C18 cartridges to set up the tandem system for measuring the partition coefficients were also studied. With the tandem-cartridge system, the measured partition coefficients did not change with most experimental conditions, but a trend of decreasing partition coefficients with humic acid concentration was observed, implying possible nonlinear association of the PAHs and humic acid.