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Quantitative Analysis - Instrumental Methods

chemical component analyte response

The presence of many chemical substances can often be found by their response to some external signal. The magnitude of this response is proportional to the amount of substance present. Because electronic equipment is often necessary to generate the external signal and/or to detect the chemical response, these methods of quantitative analysis are called instrumental methods. Instrumental methods are indirect, so the detecting instrument requires calibration to measure the response initially from a sample with a known concentration of analyte. This is necessary to relate the response, which is often electrical, to the quantity of chemical substance. Standard solutions, containing known amounts of analyte, are first studied to calibrate the measuring instrument.

The type of instrumental method used for quantitative analysis varies with the nature of the substance being analyzed and with the amount of analyte thought to be present. While classical analytical methods are suitable for major amounts of analyte present in a sample, 1% or greater, instrumental methods are generally employed for amounts of analyte which may be less than 1% of the sample's total mass. Modern instrumental techniques are capable of analyzing the presence of a component which can comprise 0.0001% or less of its mass.

Table 1 names the more common instrumental techniques used for quantitative analysis and the type of signal they invoke from a chemical system.

A thorough understanding of chemistry is necessary in selecting the proper method for the quantitative determination of a substance. Lastly, the necessary calculations to convert the data obtained into its desired form must be carried out. Computer programs have helped considerably with this last step.

Resources

Books

Harris, Daniel C. Quantitative Chemical Analysis. 4th ed. New York: W.H. Freeman & Company, 1995.

Skoog, Douglas A., and James J. Leary. Principles of Instrumental Analysis. 4th ed. Philadelphia: Saunders College Publishing, 1992.


Gordon A. Parker

KEY TERMS

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Analyte

—The component within a sample that is to be measured.

Classical analysis

—Those procedures in which the desired component is reacted with a suitable chemical reagent, either by precipitate formation or titration.

Gravimetric analysis

—A classical quantitative technique in which an added chemical forms an insoluble precipitate with the desired component. The precipitate is collected and weighed.

Instrumental analysis

—A modern quantitative technique in which some property of the desired component (electrical, optical, thermal, etc.) is measured and related to the amount present.

Titrimetric analysis

—A classical quantitative technique in which a solution of known concentration is reacted exactly with the desired component and a calculation preformed to find the amount present.

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