A complete cold chain freeze-fracture methodology has been developed to test the feasibility of using time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging for the molecular analysis of frozen hydrated biological samples. Because the technique only samples the first few monolayers of a sample, water on the surface of a sample can be a major source of interference. This problem can be minimized by placing a cold trap (fracture knife and housing at -196 degrees C) near the fractured sample that is held at a warmer temperature (-97 to -113 degrees C). This results in removal of surface water and prevents condensation on the surface. Although this approach is effective, it has been found that sample warming needs to be carefully controlled due to the volatility of other matrix molecules and the morphological effects imparted onto the cell surface during drying. By utilizing the above handling technique, it has been possible to demonstrate for the first time that TOF-SIMS imaging technology can be used to obtain images of molecular species across a cell surface with a submicrometer ion probe beam. Images of small hydrocarbons and the deliberately added dopants DMSO and cocaine have been obtained with TOF-SIMS of the single-cell organism Paramecium.

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Imaging time-of-flight secondary ion mass spectrometry is used to chemically resolve the spatial distribution of lipids in submicrometer sections of phospholipid membranes. The results show that it is possible to unravel dynamical events such as chemical fluctuations associated with domain structure in cellular membranes. In this work, a liposome model system has been used to capture the stages of membrane fusion between two merging bilayer systems. Fracturing criteria for preserving chemical distributions are shown to be much more stringent than morphological electron cryomicroscopy studies. Images of membrane heterogeneity, induced via mixing various liposomes followed by fast freezing, demonstrate the necessary sample preparation groundwork to investigate complex, heterogeneous membrane domains. Clear delineation of membrane structure provides direct evidence that specific domains or “rafts” can exist. Moreover, low concentrations of each phospholipid are distributed throughout newly fused liposomes despite the existence of distinct domains. In the liposome model, membrane structure ranges from specific domains to a fluid mosaic of the phospholipids during the fusion event. The availability of mass spectrometric imaging is proposed to facilitate the discovery of functional rafts or substructure in cell membranes before, during, and after events including cell division, exocytosis, endocytosis, intracellular transport, infection of membrane-bound viruses, and receptor clustering. This technology holds the promise to define the biology of cell membranes at the molecular level.

The study of cell membrane lipid and steroid composition and distribution is important for the understanding of membrane dynamics and function. Here we present efforts to chemically image phospholipid distributions on a submicron scale on freeze-fractured and frozen-hydrated liposomes and red blood cells using time-of-flight secondary ion mass spectrometry. Sample preparation by freeze fracturing of membranes is described. Fragments representative of phospholipid headgroups are found to be localized on both liposomes and red blood cells. In addition, the cholesterol molecular ion [M + H] is localized on liposome surfaces.