Secondary ion yield improvements for phosphated and sulfated molecules using substrate-enhanced time-of-flight secondary ion mass spectrometry

摘要

Aminoethanethiol (AET), at an appropriate pH, forms a charged self-assembled monolayer (SAM) on Au surfaces. We have shown that an AET-SAM can be used to adsorb organic and inorganic anions from aqueous and ethanol solutions. The adsorbed anions can then be sputtered with high efficiency for characterization by time-of-flight secondary ion mass spectrometry. We present here the application of the AET monolayer for the characterization of more complex and biologically relevant organic sulfates and phosphates up to 1150 Daltons, each featuring a charged anionic site to facilitate the adsorption to the monolayer by ion-pair formation. We monitored the mass spectra and secondary ion emission following 20 keV (CsI)_nCs~+ (n = 0-2) projectile impacts on targets composed of thin layers deposited on Si, Au, and AET/Au substrates; and of thick targets (thick with respect to the 10-100 nm range of projectile). For each molecule used in this study, the secondary ion yield increased when the anion was deposited as a thin layer, relative to a thick layer, and is greatest for AET/Au substrates compared to bare Au and polished Si substrates. Additionally, the intact-to-fragment ion yield ratio is lower when sputtering from the solid salt target than from the AET monolayer.