Detection Limit and Surface Coverage Determination for Tributyl Phosphate on Soils by Static SIMS

The minimum detectable limit (MDL) and semiquantitative determination of surface coverages of tributyl phosphate (TBP) on soils by static SIMS are reported. Two soils with similar bulk chemistries having surface areas of 3 and 20 m²/g were exposed to TBP (solution-phase and gas-phase exposures). These samples were investigated using two SIMS instruments, one utilizing a quadrupole mass analyzer and the other an ion trap mass analyzer. The SIMS response plotted as a function of exposure exhibited a leveling of the signal at a surface concentration estimated at 1 monolayer. The overall results of these studies were similar for both solution- and gas-phase exposures and for both soils. A minimum detection limit for TBP on both soils was determined to be 0.06 ± 0.03 monolayer (∼70 pg/mm²) for the quadrupole SIMS. Preliminary results suggest that the MDL for TBP analysis on soils using ion trap SIMS is lower; however, because the ion trap SIMS is still evolving, the MDL for that instrument was not determined. These results provide a basis for evaluation of static SIMS for use as a screening tool and illustrate the potential for investigating submonolayer surface coverages of organic contaminants on environmental surfaces by static SIMS.