Microanalysis

Microanalysis (from the Greek mikros- small), methods of qualitative and quantitative analysis (chemical, physico-chemical and physical), allowing operation with the smallest possible amounts of the substance being tested, within several tenths or whole milligrams in quantitative determinations, down to hundredths and thousandths of a milligram or even less in qualitative detection. In some cases, the same reactions and methods are used for microanalysis as in ordinary macroanalysis, only improving and refining the technique: for weighing, microbalances (see Weighing) are used or the test solutions are measured with micropipettes; in volumetric analytical methods, more dilute solutions (1/100 or even 1/1000) are used, measuring them with microburettes, the most advanced of which allow titration with accuracy to thousandths of cm³; in organic elementary microanalysis, detailed by Pregl, the size of all apparatus is correspondingly reduced; instruments have been constructed for determining pH or electrical conductivity in very small amounts of test material (from one drop to 1 cm³); there are devices for micropolarimetry that allow working with liquid amounts of about 0.1 cm³. In most cases, however, the methods of microanalysis are based on new principles and techniques that allow an even greater reduction in the amount of substance required for research. Thus, in qualitative microanalysis, developed by Emich, Behrens and Kley, the substance is identified on the basis of the crystal form of certain characteristic compounds, with observation being made under a microscope. In quantitative microanalysis, for example, when analyzing a filtrate after some precipitation, instead of the exhaustive washing of the precipitate usually used in macroanalysis, one resorts to determining in an aliquot, i.e., in a specific part of the filtrate; instead of filtration in microanalysis, centrifugation is more often used, which allows quantitative collection of even negligible amounts of precipitates. Volumetric determinations are aimed at converting to the most sensitive methods, for example, acidimetry is replaced by iodometry or electrometric titration. Weighing of precipitates is sometimes replaced by direct determination of their volume (microsedimentation). The striving to replace purely chemical methods with physical and physico-chemical methods had an exceptional significance for the development of microanalysis. Colorimetry and nephelometry (see) allow taking for analysis as upper limit amounts of the test substance that for ordinary macroanalysis represent the lower limit of qualitative detectability; at the same time, amounts in hundredths and thousandths of a milligram are determined with accuracy to 1% or even fractions of a percent. The same accuracy is possessed by the manometric methods developed by Barcroft and Warburg, which have become widespread for analyzing composition (for example, content of CO₂ and oxygen in blood - see Barcroft's apparatus), amount of Hb, etc., as well as for tracing tissue and cellular metabolic processes (see Respiration, Glycolysis). Van Slyke's microapparatus allows determining the reserve alkalinity in 0.5 cm³ of blood. With the help of Salge's thermoelement, one can cryoscope 0.5-1 cm³ of liquid. Spectral microanalysis, especially in the invisible parts of the spectrum, has rendered the most valuable services to science, allowing to establish the presence of one or another substance in amounts not detectable by ordinary macroanalytical reactions. Determination of the refractive index (refractometry and interferometry) allows quantitative determination, for example, of protein content in one drop of serum or the action of protective enzymes in the Abderhalden reaction. Microcalorimetry made it possible to study heat production in muscle and nerve with the utmost precision and thereby judge the chemical processes taking place. The first impetus for the systematic application of microanalysis in clinical and experimental medicine was given by Bang's work (see Bang's micromethods). In these fields, microanalysis acquires special importance, since the possibility of working with minimal amounts of test material (in most cases blood) on the one hand facilitates experimentation on small animals, and on the other hand makes repeated analyses possible, allowing to judge not only static changes in composition but also the dynamics of various processes, especially pathological processes, since they find reflection in the chemistry of blood and other tissues. At present, Bang's methods are to a large extent replaced and supplemented by numerous new methods of microanalysis, of which the system of blood microanalysis proposed by Folin and based mainly on colorimetric methods should be noted.