Trace Analysis

the chemical analytical determination of very small amounts of elements and compounds (less than 0.01 percent) present in the form of admixtures in the major components of the sample under study. Analyses are carried out on samples with weights from fractions of a microgram to several grams, depending on the type of material studied; the micro-constituents, or trace constituents, range in weight from 10^-9 to 10^-4 g. Determinations are made of trace amounts of elements and compounds in, for example, seawater, soil, city air, the air of industrial plants, metals, plant and animal cells, chemical reagents, medicines, drinking water, and food products. Trace analysis is expressed not as a percentage of weight, as is customary in quantitative analysis, but in parts per million (ppm).

Certain features and difficulties are peculiar to trace analysis. As a consequence of the marked difference in the contents of the major and minor species, a separation and preconcentration of microconstituents is almost always required in order to reach the detection limits of the species of interest. The techniques most often used for separating the trace components include those of liquid extraction, distillation and sublimation, ion exchange, chromatography, and coprecipitation. For the trace analysis proper, favored techniques include spectrophotometry in the ultraviolet and visible regions, gas chromatography, atomic absorption, neutron activation analysis, emission spectroscopy, and flame photometry. For example, in a 1–50 microliter sample, a 1 × 10^-5 microgram admixture of hydrocarbon (approximately 10 ppm) may be determined with a flame-ionization detector, while an electron-capture detector will determine a hydrocarbon admixture with a concentration of approximately 0.1 ppm. Flame photometry can determine approximately 0.05 ppm Cs and K and approximately 0.01 ppm Na; neutron activation analysis can determine approximately 0.0001 ppm Al in a 10-g sample.

Trace analysis requires that reagents, water, and other solvents be carefully purified and that dust and possible chemical contaminants be removed from the laboratory atmosphere. Laboratory ware made of polyethylene is used.

Trace analysis is gaining in importance, especially in connection with the development of ecological and biochemical studies, nuclear technology, and the production of semiconductor materials.