Static SIMS investigation of tetraethylammonium bromide on soil particles using ReO₄^- and Ga⁺ projectiles

Tetraethylammonium (TEN⁺) adsorbed to soil particles (primarily silicate) was investigated using static secondary ion mass spectrometry (SIMS) in order to assess the behavior of the adsorbate under atomic and polyatomic projectile bombardment. Three different instruments were used for the investigation: a quadrupole-SIMS instrument equipped with a ReO₄^- primary ion gun; an ion trap SIMS instrument equipped with ReO₄^-; and an imaging time-of-flight (ToF) SIMS equipped with Ga⁺. In all experiments, TEN⁺ was observed to decrease in abundance with increasing primary ion dose. The disappearance cross-section (σ₁₃₀) for intact TEN⁺ (m/z 130), induced by ReO₄^-, was measured at 670 Ų using the quadrupole, and 560 Ų using the ion trap. The σ₁₃₀ induced by Ga⁺ was measured at 450 Ų using the ToF-SIMS, indicating that the polyatomic projectile was perturbing an area 20-50% larger than the monoatomic. These values are significantly larger than Ga⁺-induced cross-sections in the literature (100-200 Ų), for similar compounds in a more fluid matrix (gelatin). The comparison was extended by measuring the cross-section using ReO₄^- projectiles and a gelatin matrix: σ₁₃₀ in this case was 480 Ų, which is of the order of 150% greater than the same experiment using Ga⁺. It is concluded that ReO₄^- produces a larger σ than does Ga⁺. In addition, the results suggest that disappearance cross-sections are larger on a refractory solid surface (silicate), than they are on a fluid surface (gelatin). The minimum detection limit was estimated for TEN⁺ on soil using ReO₄^- with the quadrupole SIMS instrument, at approximately 5 × 10^-4 monolayers (ML), which corresponds to about 500 ppb. Consideration of this result suggests that a lower detection limit may be achievable using a brighter primary ion beam together with a trapped ion mass spectrometer.