Anal. Chem. 1996, 68, 1309-1316
Detection Limit and Surface Coverage Determination for Tributyl Phosphate on Soils by Static SIMS J. C. Ingram,* G. S. Groenewold, A. D. Appelhans, D. A. Dahl, and J. E. Delmore
Idaho National Engineering Laboratory, Idaho Falls, Idaho 83415-2208
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. Environmental analysis is an ever-increasing challenge to the analytical chemist, due in part to the need to know a range of information about a particular sample, including the composition of the matrix, the identity and concentration of contaminants, where these contaminants reside with respect to the sample, and the modes of interaction of those contaminants with each other and the sample matrix. In addition to the complexity of both the sample and the data interpretation, analysis time is also an important factor in environmental work. Typically, many samples must be interrogated to overcome environmental inhomogeneity, which can result in variations in contaminant concentrations; these variations may span several orders of magnitude within a small area. An approach that our laboratory has taken to address some of the challenges in environmental analysis has been to directly analyze the sample surface for organic contaminants using static secondary ion mass spectrometry (SIMS). Although static SIMS has historically been utilized for characterization of polymers, and adhesion and coating processes, it has also shown good sensitivity for organic compounds adsorbed onto sample surfaces. Benninghoven was among the first to recognize that adsorbed organic compounds were responsible for a large fraction of the secondary ion signal observed in static SIMS.1 Recent work in our laboratory 0003-2700/96/0368-1309$12.00/0
© 1996 American Chemical Society
has demonstrated that a variety of organic compounds adsorbed to samples such as rocks, soils, and plant leaves are readily detected by static SIMS.2-5 Static SIMS enjoys several advantages compared to conventional techniques, which rely on solvent extraction and component separation prior to detection. SIMS does not require extraction and separation steps, which results in decreased analysis times (
