Imaging of domain structures in a one-component lipid monolayer by time-of-flight secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometry with high lateral resolution was used to image the domain structures of a phase-separated pure dipalmit oylphosphatidylcholine monolayer, which was transferred from the air-water interface of a Langmuir film balance to a gold surface. In the two-phase re gion characteristic domain structures became clearly visible in the image m aps of different fragment ions (secondary ions) as well as the molecule ion . Depending on the fragment, the liquid-condensed (LC) domains exhibit both lower and higher secondary ion yields than the liquid-expanded regions of the monolayer. To understand the mechanism leading to this alternating cont rast, we determined the intensities of some secondary ions at different sur face pressures adjusted during transfer. This analysis showed that the seco ndary ion formation in the lipid headgroup region is much more sensitive to the physical state of the monolayer than in the acyl chains. We conclude t hat an increased electrostatic interaction among the closer packed molecule s in the LC phase accounts for the lower yield of head group fragments. The observed increased yields in the LC phase leading to a negative contrast m ay be due to a more fundamental mechanism of formation of the respective se condary ion like weakening of the chemical bonds or favoring proton transfe r.