Organic Ion Imaging

Researchers
Peter J. Todd

Introduction

Organic ion imaging refers to employing secondary ion mass spectrometry (SIMS) to map the distribution of organic compounds in a range of samples - from fingerprints on glass or stainless steel to compounds in specially prepared tissue sections from the brains or other organs of laboratory animals. What is unique about our approach is the application of tandem mass spectrometry (MS/MS) to identify the structure of the secondary ions we use to image. For example, we were the first to identify the ubiquitous m/z 184 emitted from a variety of animal samples as being phosphocholine known collectively as phosphatidylcholine.

Principal goals of this research are to extend the range of compounds emitted characteristic ions from biologic tissue from lipids to small peptides. We also want to integrate the images we obtain with other imaging methods, such as MALDI, optical imaging and electron microscopy. To this extent, we need to develop sample preparation methods appropriate for SIMS and to improve spatial resolution to better than 1 micron, at the same time, using MS/MS to verify ion structure.

Biologic tissues are complex chemical systems. Images allow us to transfer enormous amounts of data from instrument to person very efficiently. For example, we can all spontaneously recognize a photograph of someone we know. However, a verbal description, no matter how concise and accurate requires more thinking, and a lot more time. By seeing how chemical distributions change, we can immediately develop complex hypotheses about how the chemicals interact.

Analytical Instrumentation

Ion trap secondary ion microprobe shown schematically on left, and in vacuum housing on right, as it exists in the laboratory. The high sensitivity of the ion trap combined with its unique capability of high efficiency MS/MS, makes this an ideal instrument for mapping compounds in tissue. The entire instrument was designed and constructed in the lab, with the ion trap modified from a Themo-Finnigan IT-MS system. The instrument features a 1 cm field of view, and spatial resolution of 5 microns has been demonstrated. 

Representative Data

Mass Spectrum (left) and total secondary ion image (right) from a finger print. Negative secondary ion images of animal samples invariably show fatty acid anions. Fingerprints are loaded with fatty acids.

Future Directions

The future of this project will depend of development of the polyatomic primary ion gun, and improving spatial resolution. At the end of the day, we intend to map the distributions of compounds in individual cells, from components of animal tissue to microbes.