The answer is isotopes. The NanoSIMS 50 can detect extremely small differences in mass among molecular fragments labeled with different isotopes. And the machine has five detectors, so scientists can look at five different isotopically labeled molecules at a time. The picture from each isotope is overlaid to produce a composite image that provides rich, mass-specific information about chemically what's in that cell membrane.
Boxer compared the concept to learning about people sitting in chairs in a room to the organization of individual molecules in a membrane. ''If you look at the room, you want to be able to see not just the chair but also what kind of chair it is... and I want to see who is nearby, and that chair over there is empty... I'd like to know that you're sitting in a chair and that that chair is different from that [other] chair. And that's what's been missing. We just don't have analytical tools that give us this combination of sensitivity, analytical information and spatial resolution.''
For the Science study, Boxer and colleagues made a spherical lipid vesicle to model a cell membrane, placed it on a small silicon wafer to make the lipid bilayer flatten into two dimensions and organized the flat membrane with a pattern of chrome grids to provide ''landmarks'' on the surface. That system let the scientists track how lipids moved and measure how many of each type of lipid resided in an area.
''We're looking for lateral organization of some sort believed to be present in membranes,'' Boxer said. ''There is now no really good way to measure it. There's got to be something that makes one part of a cell different from another part of a cell. But the forces that are responsible for segregating components, or co