Chemical microscopy

(analytical chemistry) Application of the microscope to the solution of chemical problems.

A scientific discipline in which microscopes are used to solve chemical problems. The unique ability to form a visual image of a specimen, to select a small volume of the specimen, and to perform a chemical or structural analysis on the material in the selected volume makes chemical microscopy indispensable to modern chemical analysis. See also Analytical chemistry.

Microscopes can be combined with most analytical instruments. For example, a light microscope can be combined with a spectroscope, making it possible to determine the molecular composition of microscopic objects or structures. Similarly, an x-ray spectrometer can be combined with an electron microscope to determine the elemental composition of small objects. See also Spectroscopy; X-ray spectrometry.

Phase analyses can also be made microscopically. The boundaries of amorphous phases can usually be distinguished in the microscope, and an elemental or physical analysis can be used to identify the phase. An example of a physical analysis is the measurement of refractive index. Crystalline phases are even more amenable to microscopical analysis. For example, a polarizing microscope can be used to measure the optical properties of a crystalline phase and thus identify it. Or a transmission electron microscope can be used to select a tiny area of a crystalline phase and identify the crystal structure by means of electron diffraction.

The minimum volume that can be analyzed varies widely with the instrument used. Light microscopes can be used to identify particles as small as 1 micrometer in diameter and weighing about 1 picogram. A field ion microscope has been combined with a mass spectrometer and used to identify single atoms extracted from the surface of a specimen. See also Mass spectrometry.

After a portion of a specimen has been selected microscopically, it can be analyzed in many ways. An experienced microscopist may learn to recognize various structures by studying known materials, using published atlases, or an atlas that the individual microscopist has constructed. If the object or structure cannot be recognized, many means of analysis are available. For example, a polarizing microscope may be employed to identify the object by using optical crystallographic methods. Other light microscopes useful for chemical analysis include phase-contrast and interference-contrast microscopes, microspectrophotometers, the confocal scanning laser microscope, and the laser Raman microscope. Physicochemical methods may be used to measure melting points, or mixed-melt phenomena and dispersion staining may also be used. See also Laser spectroscopy.

Microscopes using other types of image-forming beams serve for chemical analysis. Scanning or transmission electron microscopes are powerful tools for chemical microscopy. Scanning electron microscopes are often fitted with x-ray spectrometers which are capable of both qualitative and quantitative analysis for most of the elements. Other electron microscopes capable of chemical analysis are the Auger electron microscope, field electron microscope, scanning tunneling microscope, and cathodoluminescence microscope. Microscopes which use ion beams, neutron beams, and x-ray beams also have analytical capabilities. See also Auger effect; Auger electron spectroscopy.