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Finding optimal experimental conditions for generating stable negative ion electrospray ionization ion trap mass spectra (ESI-IT-MS) of alkanethiol-derived species is critical for quantitatively characterizing multicomponent alkanethiol-based self-assembled monolayers by this technique. Since alkanethiolates slowly oxidize in solution, purposeful oxidation of alkanethiols to their fully oxidized form (alkanesulfonates) is advantageous: sulfonates are chemically stable and have little affinity for covalent binding to metal surfaces. We have used ESI-IT-MS to characterize the products of H(2)O(2) oxidation of simple n-alkanethiols in solution and have observed monomeric alkanesulfonate species as well as alkanesulfonic acid/alkanesulfonate adducts, yielding gas-phase dimers and trimers. MS intensities of both monomers and adducts exhibit a dependence on the ion transfer capillary temperature that is alkyl-chain-length-dependent and that appears to be correlated with C-S bond cleavage. The trend in optimal capillary temperatures indicates that entropic effects lead to lower thermal decomposition temperatures for short-chain species relative to the longer-chain homologues. MS calibration data from alkanesulfonate mixtures are characterized by large linear dynamic ranges (10(-6)-10(-3) M) and detection limits influenced by their thermal decomposition. The high degree of precision in the calibration data should facilitate distinguishing among mixed SAMs having similar compositions.

The design and construction of an ultrahigh vacuum multi-imaging scanning electron microscope is described. The microscope is designed to contain two field electron emission colurmrs and can acquire simultaneous digital images from a ldchannel electron spectrometer, a fourquadrant kck-scattered electron ( B E ) detector, an Si(Li) x-ray detector, a SEM detector and the current flowing to ground through the sample. Because there is exact spatial registration between corresponding pixels in each of the images, it is possible to use the image set to make quantitative interpretations of the surface and subsurface chemktry. This is done using mathematical manipulations of the image set, together with models for the SEM, BSE and Auger signals. Techniques are described for setting up the alignment and characterizing the field of view and transmission function of the microscope and its spectrometer. Examples of multi-imaging from simple samples are given. The close coupling between the microscope and its control and interpretation computers provides considerable power for the analysis of inhomogeneous surfaces.

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