Laser Desorption Fourier Transform Mass Spectrometry Exchange Studies of Air-Oxidized Alkanethiol Self-Assembled Monolayers on Gold

Exchange of self-assembled monolayers (SAMs) of dodecanethiol (C₁₂H₂₅SH) on gold with decanethiol (C₁₀H₂₁SH) was detected with laser-desorption Fourier transform mass spectrometry (LD-FTMS) in the negative ion mode. The amount of adsorption of C₁₀H₂₁SH is dependent on the extent of air oxidation of the C₁₂H₂₅SH SAM. In this study, a partially air oxidized C₁₂H₂₅SH SAM with a 2.2:1.5:1 ratio of dodecanethiolate (C₁₂H₂₅S^-, m/z 201) to the presumed dodecanesulfinate (C₁₂H₂₅SO₂^-, m/z 233) to dodecanesulfonate (C₁₂H₂₅SO₃^-, m/z 249) on the surface is examined. After the sample is soaked in an ethanol solution of C₁₀H₂₁SH for 30 min, the LD-FTMS spectrum shows that both the C₁₂H₂₅SO₂^- and the C₂₁H₂₅SO₃^- are gone, but a decanethiolate peak appears (C₁₀H₂₁S^-, m/z 173) in the same ratio to C₁₂H₂₅S^- (1.19) as the ratio of the combined oxidation products (1.16) prior to soaking. Subsequent air exposure (9 h) of this SAM composed of mixed thiolates gives a similar ratio (1.06) of the fully oxidized sulfonates of both species, which can be completely removed upon subsequent soaking in a solution of C₁₂H₂₅SH, resulting in a SAM of pure dodecanethiolate by FTMS analysis. Negligible exchange occurs when nonoxidized C₁₂H₂₅SH SAMs are soaked in decanethiol solution for 30 min. From these observations, we draw several conclusions. First, the results refute suspicions that sulfonates form in a laser-induced reaction of thiolates with O₂ absorbed in the SAM during air exposure. Second, ionization efficiency of thiolates and sulfonates during LD-FTMS analysis is approximately equal. Third, previous literature reports of exchange experiments should be carefully scrutinized for possible evidence of the effects of air oxidation.