Surfactant and micellar influences on desorption and fragmentation in liquid secondary ion mass spectrometry

Abstract

Two steroid glucuronides have been used as model amphiphiles to study mass spectral characteristics of nonionic surfactants in liquid secondary ion mass spectrometry (L-SIMS). The critical micelle concentrations (CMCs) of androsterone glucuronide and 11-ketoandrosterone glucuronide in glycerol were determined by difference spectrophotometry. In accordance 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-SIMS 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 significantly with analyte concentration. Above the CMC, observed higher levels of fragmentation can be rationalized by considering that in this concentration region, low levels of glycerol desorb with secondary ions, hence, less energy partitioning between MH + and glycerol is occurring. Energy partitioning with neighboring molecules in the micellar layer is also low. This causes protonated molecules to retain relatively high levels of internal energy, resulting in increased decompositions. Fragmentation at the target surface arising via collision cascade processes are also likely to become more promiment once complete analyte coverage of the glycerol drop has been achieved. L-SIMS of equimolar mixtures 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 accordance with its lower CMC value. Adenosine, 1-methyladenosine, N 6-methyladenosine, and N 6-(2-isopentenyl)adenosine have also been investigated by L-SIMS. For these nontypical amphiphiles, absolute abundances of protonated molecules increase linearly with analyte concentration up to the solubility limit. The base portion of the nucleoside apparently induces sufficient polar interaction with the solvent to substantially diminish surface activity.