STATIC SIMS INVESTIGATION OF TETRAETHYLAMMONIUM BROMIDE ON SOIL PARTICLES USING REO4- AND GA+ PROJECTILES

Tetraethylammonium (TEN+) adsorbed to soil particles (primarily silica te) was investigated using static secondary ion mass spectrometry (SIM S) in order to assess the behavior of the adsorbate under atomic and p olyatomic projectile bombardment. Three different instruments were use d 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 e xperiments, TEN+ was observed to decrease in abundance with increasing primary ion dose. The disappearance cross-section (sigma(130)) for in tact TEN+ (m/z 130), induced by ReO4-, was measured at 670 Angstrom(2) using the quadrupole, and 560 Angstrom(2) using the ion trap. The sig ma(130) induced by Ga+ was measured at 450 Angstrom(2) using the ToF-S IMS, indicating that the polyatomic projectile was perturbing an area 20-50% larger than the monoatomic. These values are significantly larg er than Ga+-induced cross-sections in the literature (100-200 Angstrom (2)), for similar compounds in a more fluid matrix (gelatin). The comp arison was extended by measuring the cross-section using ReO4- project iles and a gelatin matrix: sigma(130) in this case was 480 Angstrom(2) , which is of the order of 150% greater than the same experiment using Ga+. It is concluded that Real produces a larger a than does Ga+. In addition, the results suggest that disappearance cross-sections are la rger on a refractory solid surface (silicate), than they are on a flui d surface (gelatin). The minimum detection limit was estimated for TEN + on soil using ReO4- with the quadrupole SIMS instrument, at approxim ately 5 x 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 trapp ed ion mass spectrometer. (C) 1998 Elsevier Science B.V.