Luminescence Analysis

a method of examining various objects by observing their luminescence.

Luminescence analysis involves the observation either of the luminescence of the objects to be examined (for example, the vapors of a gas) or of the luminescence generated by special luminophors used to treat the objects. The apparatus for this type of analysis contains a luminescence excitation source and a recording device. Excitation of an object’s photoluminescence is the method most commonly used; however, certain cases call for the observation of cathodoluminescence, radioluminescence, and chemiluminescence. Photoexcitation is generally produced by quartz-mercury-vapor lamps with light filters attached to isolate the ultraviolet region of their spectrum. Apart from mercury-vapor lamps, the light sources used include xenon lamps, sparks, and lasers. The luminescence is usually recorded visually or by means of photoelectron devices, which permit greater accuracy.

Quantitative and qualitative chemical luminescence analyses are usually used in recording independent luminescence in substances. The concentration of luminescent matter (at low optical thickness and concentrations of less than 10^-4-10⁵ g/cm³) is determined by the intensity of luminescence, with the aid of quantitative chemical luminescence analysis. This method is highly sensitive (reaching several units per 10¹⁰ g/cm³ in the detection of some organic substances) and can be used for purity control. In isotopic luminescence analysis a gas laser beam is used to excite the luminescence of certain isotopes.

Qualitative chemical luminescence analysis makes possible the detection and identification of certain substances in mixtures. Spectrophotometers are used to study the energy distribution in the luminescence spectrum of substances at low temperatures and in viscous solutions (oils). Certain nonluminescent substances are detected by the luminescence of products formed during their interaction with specially added substances.

Precision luminescence analysis makes it possible to discover the differences between seemingly identical objects by the quality of their luminescence. The method is used to diagnose diseases (for example, tissue infected with Microsporum is detected by its bright green luminescence when exposed to ultraviolet light), to determine the damage to seeds and plants caused by disease, and to determine the organic matter content of soil. It is used in analyzing rocks for the presence of petroleum and gases; in analyzing the composition of petroleum, minerals, and rocks; and in grading raw diamonds. Diamonds can be sorted automatically by observing their characteristic luminescence under soft rays. The extrinsic luminescence of objects is often examined by precision luminescence analysis. In scanning for certain chemical elements (for example, the rare earths), rock samples are treated with special compounds that form luminescent complexes with the substances sought. In biology, living tissues are colored with special dyes that interact with biological substances to produce luminescent complexes. For example, the cell nuclei of connective tissue emit a bright luminescence when dyed with acridine orange; if the cells are cancerous, the color of the emitted radiation is altered.

If the examined object does not exhibit intrinsic luminescence, it is sometimes subjected to preliminary treatment involving the addition of a special luminophor. When a liquid is under study, the luminophor is dissolved in it; when a solid is under study, the luminophor is adsorbed onto its surface. The movement of subterranean waters can be observed by dissolving a luminophor, such as fluorescein, in the water and conducting luminescence analysis. Similar methods are used to investigate the movement of coastal sands (the luminophor is adsorbed onto the surface of the grains).

Luminescence analysis is also used in criminal law (to determine the authenticity of documents and to detect traces of toxic substances), in restoration techniques, in flaw detection, in public health (to determine the quality of certain food products and drinking water), and in industrial-sanitary chemistry (to determine the level of harmful substances in the air). Because certain substances (scintillators) luminesce under the action of elementary particles, the methods of luminescence analysis are widely used in nuclear physics. Luminescence analysis that involves the use of microscopes is known as luminescence microscopy.