Organic secondary ion mass spectrometry (SIMS) and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry can be used to produce molecular images of samples. This is achieved through ionization from a clearly identified point on a flat sample, and performing a raster of the sample by moving the point of ionization over the sample surface. The unique analytical capabilities of mass spectrometry for mapping a variety of biological samples at the tissue level are discussed. SIMS provides information on the spatial distribution of the elements and low molecular mass compounds as well as molecular structures on these compounds, while MALDI yields spatial information about higher molecular mass compounds, including their distributions in tissues at very low levels, as well as information on the molecular structures of these compounds. Application of these methods to analytical problems requires appropriate instrumentation, sample preparation methodology, and a data presentation usually in a three-coordinate plot where x and y are physical dimensions of the sample and z is the signal amplitude. The use of imaging mass spectrometry is illustrated with several biological systems.

The ability to simultaneously focus a wide mass range of metastable fragment ions formed after the initial ionization event in a matrix-assisted laser desorption/ionization time-of-flight mass spectrometer has been made possible by the development of a new type of ion reflector. This coaxially designed time-of-flight instrument employs a modified single stage reflector whose axial voltage gradient rises differentially in order to produce an alignment of energy focal points for product ions. In contrast, product-ion focusing in conventional constant field reflectors occurs over a broad range of distances from the reflectron exit. Approximately 90% of the product-ion mass spectrum can be collected without adjustment, thereby eliminating the need to scan the reflector voltage. Design considerations of the curved field reflectron, its calibration properties and representative metastable spectra of several peptides are discussed.

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