Mapping chemical microscapes of cells.

Mapping chemical microscapes of cells

Superman may have had X-ray vision, but he didn't have an eye for the chemicals inside cells. Two chemists at Cornell University do. Using a single instrument that combines the magnifying power of a microscope with the chemical-analysis prowess of a mass spectrometer , they can uncover the identities and cellular locations of elements such as calcium, boron and magnesium.

At last week's Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, held in New York City, George H. Morrison and Subhash Chandra reported adapting the instrument, called an ion microscope, to reveal cellular details of physiological processes.

"More and more biologists are going to see this stuff and come knocking on the door," Morrison says. In fact, they already have. The researchers have received more than 400 reprint requests for a paper they published last year demonstrating the potential of ion microscopy for biological studies.

At the conference, Chandra described more recent studies on the role vitamin D plays in the absorption of calcium into the bloodstream. In these experiments, the chemists injected calcium into the intestines of two sets of chicks, which differed only in the presence or absence of vitamin D in their feed. The team used a stable, heavy isotope of calcium that would stand out from the lighter isotope normally dominant inside cells. By extracting gut tissue at different times after the calcium injections and using the ion microscope to get "snapshots" of the calcium distribution within different cells of the tissue, the researchers could track calcium absorption.

Although they were not surprised to find that chicks lacking vitamin D absorbed calcium poorly at best, Chandra says the ion microscopy study provides graphic data about exactly where and when absorption occurs. And that insight enables scientists who seek more details about the absorption process to rule out many experiments that would likely prove a waste of time. "No other existing technique can localize elements as well at a subcellular level as can ion micorscopy," Chandra notes. Other techniques often involve hard-to-handle radioactive chemicals or steps that introduce data-skewing perturbations, he adds.

In their analyses, the researchers first bombard a carefully prepared sample with positively charged oxygen ions. These primary ions sputter secondary ions from the sample surface, and an electric field then accelerates the secondary ions into a mass spectrometer, which sorts them according to the energies they carry and their masses. A special electromagnetic lens system ensures that the secondary ions maintain the spatial relationships they had in the sample surface, and a component known as a charge-coupled device detects and digitizes the spatial pattern of the speeding ions.

Physicists and materials scientists have been using ion microscopy for years to study the elemental distributions of silicon wafers and other materials whose properties depend on trace impurities. The difficulty of adapting the technique for biological samples, which require more complicated preparation, has until now kept ion microscopy from the biologists' tool chest, Morrison says. Adds Chandra: "Now we are applying the technique to real-life situations."