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Chemical Microanalysis

methods, substances, products, required, quantitative, usually and material

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MICROANALYSIS, CHEMICAL. Chemical analysis (see CHEMISTRY, ANALYTICAL) becomes chemical microanalysis when the quantities of substances examined or measured are very small. The power to deal successfully with minute quantities of material has been acquired as a consequence, in the first place, of the increased precision of such instruments as the microbalance and the spectroscope, and, in the second, of a general advance in analytical knowledge and technique.

Microchemical methods are of the highest importance because of their ever-increasing application, not only in chemical, bio chemical, and pathological research, but also to daily life and industry. Artificial products and the products of animal and vegetable life are usually obtained as mixtures, more or less com plex, which can be separated by physical means (distillation, crys tallisation, dialysis, etc.), into component parts consisting each of a more or less pure chemical substance. The identification of these substances and the control of their purity depend ultimately upon chemical analysis. The recognition and estimation of traces of impurities, frequently a matter of great public importance, must generally be carried out by microchemical methods; and it is becoming increasingly appreciated that quantitative microanal ysis often affords the most rapid and economical means of ex amining even bulk products and chemically pure substances. This is particularly true of the products of research, many of which can be isolated only with difficulty and in minute quantities and yet often exercise the most important functions. It is only neces sary to mention in this connection the vital roles played by catalysts and promoters in chemical processes, and those of the vitamins, hormones, and enzymes, in biochemical phenomena.

Microanalysis may be either qualitative, i.e., concerned only with the detection of the kinds of substances present; or it may be quantitative, being then concerned also with the proportions in which the substances occur. Thus, it might be required to know whether strychnine were present in a given drug, or whether human blood were present as a given stain ; or again, it might be required to decide whether the proportion of arsenic in a given foodstuff (beer or cocoa) exceeded a certain limit. Speak

ing quite generally, it may be said that the quantity of material required for, or measured in, a given microanalysis is from to 2% of that in the large-scale or macroanalysis. The methods employed in microanalysis are usually essentially similar to those in the corresponding macroanalysis. This is particularly the case in: Quantitative Microanalysis, in which the classical methods of analysis for elements and radicals (see CHEMISTRY : Ana lytical) are preserved in refined forms. These developments have taken place since 1910, and are due principally to Fritz Pregl of Graz. They have been rendered possible by the elabora tion, by W. H. Kuhlmann of Hamburg and by others, of a chemical microbalance capable of determining loads (up to 20 grams) to within two or three thousandths of a milligram; of being manipulated with ease and rapidity; and of retaining sensi tiveness during many years.

General methods are now in use for the quantitative estimation of the following elements and radicals : Carbon and hydrogen, nitrogen, chlorine, bromine, iodine, sulphur, selenium, tellurium, phosphorus, arsenic, copper ; carboxyl-, methoxyl-, ethoxyl-, methylimido-, and acetyl-groups. In addition, metals in salts; ash content ; water content ; and molecular weight (ebullioscopic method) may be microanalytically determined.

The weight of material required for each determination varies from 0.5 to 15 milligrams (0.0005 to 0.015 gram) ; it is usually from 3 to 5 milligrams (0.003 to 0.005 gram.). The microanalyt ical methods are at least as accurate as the large-scale methods, in some cases more so; they are much more rapid than the latter; and they involve economy in respect of power, bench space, and materials. The necessary skill and accuracy to operate the meth ods is easily acquired by any chemist. There can be little doubt that these methods are rapidly replacing the older and more cum brous processes, and that their general adoption will greatly accel erate the progress of chemical and biological knowledge.

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