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 dipalmitoylphosphatidylcholine monolayer, which was transferred from the air-water interface of a Langmuir film balance to a gold surface. In the two-phase region characteristic domain structures became clearly visible in the image maps 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 contrast, we determined the intensities of some secondary ions at different surface pressures adjusted during transfer. This analysis showed that the secondary 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 that an increased electrostatic interaction among the closer packed molecules 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 may be due to a more fundamental mechanism of formation of the respective secondary ion like weakening of the chemical bonds or favoring proton transfer.