Imaging time-of-flight secondary ion mass spectrometry allows visualization and analysis of coexisting phases in Langmuir-Blodgett films

Solid supported monolayers of dipalmitoylphosphatidylcholine (DPPC) mixed with the lipid fluorescence dye nitrobenzoxadiazolphosphatidylcholine (NBDPC) were investigated by high laterally resolved imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS), The lipid monolayers were prepared at the air/water interface of a Langmuir film balance and transferred to solid substrates covered with a thin gold layer by Langmuir-Blodgett (LB) techniques. Prior to and during transfer, the monolayers were held in the two-phase region consisting of the liquid-expanded and the liquid-condensed phase which was verified by fluorescence microscopy. Mass-resolved ion images of the LB films obtained by a TOF-SIMS instrument equipped with a fine focused gallium Liquid metal ion source clearly show the same domain structures as the fluorescence microscopical images. NBDPC is almost completely excluded from the liquid condensed phase as the lateral distribution of the protonated molecular ions of the dye demonstrates. These findings are in good agreement with fluorescence microscopical data. Our investigations substantiate that imaging TOF-SIMS allows the detailed chemical analysis of coexisting phases in LB monolayers.