SIMS of organic anions adsorbed onto an aminoethanethiol self-assembled monolayer: an approach for enhanced secondary ion emission

Secondary ion mass spectrometry (SIMS) was used to monitor the uptake of organic anions from solution by aminoethanethiol (AET) monolayers on Au substrates, as a test of the applicability of this monolayer as a substrate for organic SIMS analysis. Event-by-event bombardment and detection mode coupled with coincidence counting allowed the atomic and polyatomic projectile impacts on a particular sample surface to be compared simultaneously and under the same experimental conditions. The mass spectra produced from the monolayer surface and those from Au and Si blanks demonstrate that the AET monolayer is important to the uptake of the organic anion. The exchanged monolayer surfaces were used to measure secondary ion yields, defined as the number of secondary ions detected per incident primary ion, produced from ultrathin films by (CsI)nCs+ (n = 0-2) projectiles at the limit of single-ion impacts. The yield of a tetradecyl sulfate (IDS) anion was improved by a factor of 200 using the AET substrate instead of the thick salt target. The intact ion and fragment ion yield trends produced from the AET surface were measured as a function of number of atoms in the primary projectile and energy. We observed a yield increase for both the intact ion and the fragment ion with the projectile complexity and energy. The increase in yield per projectile atom was linear for the emission of intact TDS and intact dodecyl sulfate from the AET surfaces. A supralinear yield enhancement, however, was observed for the fragment ion SO3- when the three-atom (CsI)Cs+ cluster was used. The experiments demonstrate that the various organosulfate and suffonates are weakly bound to the AET surface and their adsorption to the AET monolayer is reversible. The utility of the AET monolayer on Au was also tested as a general substrate for the characterization of derivatized organic molecules with biological and industrial importance by TOF-SIMS.

Speciation of sodium nitrate and sodium nitrite using kiloelectronvolt energy atomic and polyatomic and megaelectronvolt energy atomic projectiles with secondary ion mass spectrometry

The negative-ion mass spectra produced by kiloelectronvolt energy (CsI)nCs+ (n = 0-2) and megaelectronvolt energy 252Cf fission fragment projectile impacts on NaNO3 and NaNO2 were collected and compared. The mass spectra generated by impacts of the kiloelectronvolt polyatomic primary ions on NaNO3 were markedly different from those derived from the fission fragment impacts, featuring higher relative intensities of nitrate (NO3-) specific secondary ions (those that reflect the sample stoichiometry). The most prominent secondary ion (SI) peaks produced from NaNO3 by the kiloelectronvolt energy projectiles were NO3- and Na(NO3)2-, both of which relate directly back to the chemical composition of the staring material. Likewise, the most prominent peaks produced by the kiloelectronvolt energy polyatomic projectile impacts on NaNO2 were NO2- and Na(NO2)2-. The fission fragment projectiles produced SI spectra from NaNO3 that were dominated by signals characteristic more of NaNO2, indicating that the megaelectronvolt energy ions induce considerable degradation of the nitrate solid. In addition, the fission fragment projectile produced relative negative SI intensity distributions that are remarkably similar to those reported in earlier studies of the use of laser desorption to produce SI signals from NaNO3. Of the projectiles examined in this study, the 20 keV (CsI)Cs+ projectile generated negative-ion mass spectra that best differentiated NaNO3 and NaNO2, primarily by producing a base peak in the NaNO3 spectrum that was unambiguously representative of the original sample stoichiometry.

Secondary ion yields produced by keV atomic and polyatomic ion impacts on a self-assembled monolayer surface

A suite of keV polyatomic or 'cluster' projectiles was used to bombard unoxidized and oxidized self-assembled monolayer surfaces. Negative secondary ion yields, collected at the limit of single ion impacts, were measured and compared for both molecular and fragment ions. In contrast to targets that are orders of magnitude thicker than the penetration range of the primary ions, secondary ion yields from polyatomic projectile impacts on self-assembled monolayers show little to no enhancement when compared with monatomic projectiles at the same velocity. This unusual trend is most likely due to the structural arrangement and bonding characteristics of the monolayer molecules with the Au(111). Copyright 1999 John Wiley & Sons, Ltd.