Ion-trap SIMS analysis of pinacolyl methylphosphonic acid on soil

The minimum detection limit and semi-quantitative determination of surface coverages of pinacolyl methylphosphonic acid (PMPA) on soil by static secondary-ion mass spectrometry (SIMS) are reported. The soil was exposed to aqueous solutions of PMPA and then analyzed on an ion-trap SIMS instrument. Mass spectrometry/mass spectrometry was utilized to discriminate against the chemical background inherent in environmental samples (such as soil). The quasimolecular ion [PMPA - H]^- at m/z = 179 was trapped and then subsequently fragmented to form an ion at m/z = 95, which is interpreted as a loss of the pinacolyl olefin (C₆H₁₂) to form the methylphosphonic acid anion. The product ion at m/z = 95 was used to investigate the surface coverage of PMPA on the soil. The m/z = 95 product-ion abundance was observed to be linearly related to the PMPA surface coverage between 2 and 0.002 monolayers. The minimum detection limit is estimated at 0.008 monolayer (approximately 12 pg mm^-2, three standard deviations of the blank). These data were compared with analyses performed by using a quadrupole SIMS instrument, which indicates an improvement in sensitivity by the ion-trap SIMS of a factor of 250. The results of this study demonstrate that ion-trap SIMS is a facile approach for determination of phosphonates on soil.