SURFACTANT AND MICELLAR INFLUENCES ON DESORPTION AND FRAGMENTATION INLIQUID SECONDARY-ION MASS-SPECTROMETRY

Two steroid glucuronides have been used as model amphiphiles to study mass spectral characteristics of nonionic surfactants in liquid second ary ion mass spectrometry (L-SIMS). The critical micelle concentration s (CMCs) of androsterone glucuronide and 11-ketoandrosterone glucuroni de in glycerol were determined by difference spectrophotometry. In acc ordance with the theory of surfactant behavior in aqueous systems, the CMC of the former was found to be lower than that of the latter. L-SI MS experiments show that the absolute abundance of desorbed protonated molecules of each compound is highest when the analyte concentration is near the CMC. In both cases, fragment ion abundances vary significa ntly with analyte concentration. Above the CMC, observed higher levels of fragmentation can be rationalized by considering that in this conc entration region, low levels of glycerol desorb with secondary ions, h ence, less energy partitioning between MH+ and glycerol is occurring. Energy partitioning with neighboring molecules in the micellar layer i s also low. This causes protonated molecules to retain relatively high levels of internal energy, resulting in increased decompositions. Fra gmentations at the target surface arising via collision cascade proces ses are also likely to become more prominent once complete analyte cov erage of the glycerol drop has been achieved. L-SIMS of equimolar mixt ures of the two compounds at high analyte concentrations reveals that androsterone glucuronide produces higher MH+ signals, indicating that this compound tends to preferentially populate the surface in accordan ce with its lower CMC value. Adenosine, 1-methyladenosine, N6-methylad cnosine, and N6-(2-isopentenyl)adenosine have also been investigated b y L-SIMS. For these nontypical amphiphiles, absolute abundances of pro tonated molecules increase linearly with analyte concentration up to t he solubility limit. The base portion of the nucleoside apparently ind uces sufficient polar interaction with the solvent to substantially di minish surface activity.