The History of Microanalysis MARY 0. HILLIS1 University of Illinois, Urbana, Illinois ICROANALYSIS, under that name, has existed for less than a century, although some of its methods and tests date back as far as the beginning of the Christian era. Indeed, the first known chemical reaction in the wet way (I) to be used in analysis was a test which would today be classed as a spot reaction. This test was developed by Plinius (23-79 A.D.) and was used to detect the presence of ferrous sulfate in verdigris. He accomplished this by the use of a paper impregnated with nut gall, which was blackened when placed in contact with the substance containing ferrous sulfate. This reaction, in fact, has found a place in modern spot analysis, for Gutzeit (2) listed it, along with other spot tests for iron, in a publication appearing in 1929. Boyle (1627-1691) was the fmt to use probably the most common of all spot reactions-the testing for acidity and alkalinity by paper impregnated with plant extracts (3). He used litmus paper and paper treated with the juices of violets and cornflowers. Two other examples of early micro tests are the reactions used in early times for the detection of gold. Tachenius (4) found in 1666 that a drop of a mixture of gold solution with tincture of nut gall, when placed upon paper, produced a metallic spot. The other reaction, still used today in both macro- and microanalysis, is the "Purple of Cassius" test ( 5 ) , developed by Cassius in 1632. This test depends upon the production of a solution of colloidal gold and tin hydroxide by the treatment of auric chloride with stannous chloride. The history of microanalysis during its main period of development can perhaps best be covered by considering separately its various branches-qualitative microscopic analysis, spot analysis, and quantitative microanalysis. The oldest of these branches is microscopic analysis. According to Chamot and Mason (@, Feigl (7), and Meyer (S), microchemistry had its beginning in the middle of the 18th century when Sigismund Marggraf (1709-1782), during his research on cane sugar in red beet juice, first introduced the use of the microscope into chemistry. There is a difference of opinion as to the first investigator to have used microscopic methods in the field of analytical chemistry. Chamot and Mason ( 6 ) credit this to Raspail (1833), who is believed to have been the first to present a system of microscopic tests for chemists and to have suggested the identification of compounds by observation of crystals under the microscope. Deniges (9) and F e d (7), on the other hand, mention Teichman as the "true founder" or "precursor"
Present address: 1
Vassar College, Poughkeepsie, New York.
of microchemistry, whose work (1853-57) described the microscopic identification of small traces of blood through the formation of characteristic hemin crystals. Emich (10) traces the beginning of microchemical analysis hack to a series of investigators-Helwig, Harting, Wormley, and Boricky. Helwig's book, published in 1865, dealt with the use of the microscope in toxicology. A text by Harting on the theory and general use of the microscope also appeared in 1865. In the second volume (1866) the forms of some precipitates were described. Wormley published a book in 1867 on the microchemistry of poisons (11), in which he applied the microscope to the examination of a drop of suspected solution after treatment with the proper chemical reagent. Boricky's dissertation (1877) discussed the new chemical microscopic analysis of minerals and stones, and presented the first rational system of microchemical reactions. His analysis of minerals was based upon their decomposition by fluosilicic acid and the identification of the fluosilicates by means of their crystalline forms. In 1884 Streng (12) published some microchemical observations, listing several new reactions and describing new techniques to aid in microscopic studies. In an earlier paper he had recommended the use of the microscope as a valuable aid in blowpipe analysis, along which lines some work had already been done by Sorby (13) in 1869. A paper by Reinsch (14) in 1881 dealt with the identification of compounds by means of their crystalline forms. In 1885 a .publication by Haushofer (15) described microscopic tests as a supplement to the usual qualitative analysis. In this paper, reactions for the majority of the elements were given, laying a foundation for general microscopic analysis. It was proposed to bring all elements into the range of microchemical analysis, an idea and purpose also expressed by Klement and Renard (16) in their book about a year later. They applied microscopic methods to the qualitative analysis of certain minerals and gave methods of attack, separation, filtration, and other manipulations. The greatest advance in qualitative microanalysis was made by ~ ~ (17), who h was ~ called~ by Chamot ~ s and Mason (6) "the real founder of our present science of microscopic chemical analysis." In his publications during the last two decades of the 19th century, he proposed a large number of microscopic tests and also for the separation of the compounds from other, thus giving greater accuracy to the confirmatory tests. The first important addition to ~ ~work was h made~by schoorl ~ ~ of ' (18)~ in a series papers appearing in the years 1907 to 1909, describing a
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scheme of microchemical qualitative analysis based upon the classical H a system. Emich (1860-1940) made valuable contributions to both qualitative and quant~tativemicroanalysis and was responsible for the development of many of the important techniques used today (10). Benedetti-Pichler (19) called him the pioneer and founder of the science of microchemistry, for he was the first to attempt general chemistry and physico-chemical experiments on a small scale. His work was not limited to microanalysis, but included small-scale experiments in general, inorganic, and organic chemistry. Although the earliest tests, described in the beginning of this paper, belong to the branch of microanalysis known as spot analysis, the true development of this field has taken place for the most part within the last two decades, under the guidance of Fritz Feigl and coworkers. The technique known as "spot analysis" is so called because the analysis is carried out by reactions which take place within the volume of one or two drops, usually upon a spot plate or filter paper. Feigl and Stem (20), in an early report on spot reactions, gave a brief history of this field, especially in regard to the use of filter paper as a medium for the reactions. Schonbein (1861) was the first one to show that the adsorption of different salt solutions in paper varied, and that, therefore, it should be possible to identify a'single component in a salt mixture by capillarity. Goppelsroeder made thorough investigations (1899-1911) of capillary phenomena, but worked chiefly with organic substances. In the methods used by both of these investigators the adsorption studies were made by means of strips of filter paper dipped into the solutions. Bayley (1878-86) introduced a method in which drops of solution were allowed to fall upon filter paper. He noted the formation of concentric rings with varying concentrations, and pointed out a method, based upon this observation, for the detection of cadmium in the presence of copper. Reed noted the zones corresponding to the separation of the solutes, due to their different capillary migration velocities. He also recognized the dependence of these phenomena upon the concentration of the solution. Trey (1898) showed that i t is possible, in a systematic analysis, to detect two elements in the presence of one another through adsorption. He found that in an ammoniacal solution, containing copper and cadmium, the cadmium forms the outer ring and may be detected by treatment with ammonium sulfide, which leads to the formation of a yellow ring of CdS around a black circle of CuS. Holmgren (1899) developed a simple formula which related the size of the nngs to concentration. In 1900 Skraup and scholars investigated the dependence of capillarity upon various chemical and physical factors. Krulla (1909), using the strip method, noted that in solutions of salt mixtures containing different ions, each single component is adsorbed to the same extent as if it were alone in the solution. The addition of similar ions, however, caused a repression of the capillarity of the ion in question. K ~ l l used a this for the qualitative
detection of ions in the following way To the solution being tested, diierent ions were added as long as there was a distinct decrease in capillarity. Each ion that caused this decrease was, therefore, indicated as present in the tested solution. Reactions carried out on textile fibers and observed under the microscope, as described by Emich (21) in 1907, might be classified as spot tests or, perhaps, as a transition between this form of analysis and microscopic analysis. Since 1918 Feigl (22) and coworkers have been applying these special techniques and the use of specific and selective reactions to the analysis of the majority of cations and anions and to many organic compounds. A specific reaction has been defined by the "International Commission on New Analytical Reactions and Reagents" (23) as a reaction which, under the experimental conditions employed, is indicative of one substance (or ion) only. Most of the specific reagents are organic compounds which form colored inner complex salts and coordination compounds with the metallic ions. Comprehensive texts on organic analytical reagents have been published by Mellan (24) and Yoe and Sarver (25). Hauser (26) was the first to suggest the use of spot reaction methods for systematic analysis. He used the tests described by Feigl and Stern (20) for the analysis of the ions in Group I11 of the H2Ssystem. Since that time, a great many schemes of systematic qualitative microanalysis have appeared in the literature. Some apply spot and microscopic tests chiefly as supplementary tests in the classical H2S system and some dispense with the use of H2S altogether, either using other group reagents or making the analysis by the use of specific and selective spot tests. Some of the investigators who have developed these analyses are Tananad (27), Gutzeit (2), Heller (28, 29), Krumholz (29, 30), Engelder and Schiller (31), Gerstenzang (32). Emschwiller and Charlot (33), von Nieuwenburg (34), Davies (35), Benedetti-Pichler (36), Wilson (37), and Emich (10). Quantitative microanalysis has had its greatest development during the last 25 years, although a few attempts a t the quantitative determination of small amounts bad been made a t earlier dates. In 1876 Goldschmidt (38) described a microscopic method for the quantitative determination of silver and gold in grains which were too small to be weighed on the balances of that day. This method depended upon the measurement of the diameter of the particles in question and could be classified under microscopy. Many of the earlier attempts to make quantitative determinations upon small quantities of material gave results which were merely estimates. Prior to the development of accurate methods of quantititive microanalysis toward the end of the first decade of the 20th century, Richter's "Gabelverfahen" method was used when dealing with small amounts of material (39). This method was based upon the assumption that for a given substance there was available a series of reactions, each of which possessed a known and different sensitivity. Using these reactions upon the substance
in question, it was possible to determine which tests worked and which failed to work and from these results to draw conclusions as to the unknown amount. One of the prerequisites for accurate quantitative microanalysis was a balance of high sensitivity. The first microbalance was constructed in 1886 by Warburg and Ihmori (40) and was used by them to measure the water film on a glass surface. In 1901 Salvioni (41) constructed a microbalance which would weigh accurately to 0.001 mg. and could carry a load of 100 mg. Readings were made by measuring the bending of a fine elastic wire by means of an ocular micrometer. The Nernst balance (42), which used a quartz fiber and was constructed in 1902, was probably the most popular balance in the decade following its construction and was used in many of the early microchemical determinations. The oldest method for the determination of molecular weights of small samples was that of Barger (43); published in 1904. This method depended upon the comparison, by means of a microscope, of the sizes of drops of solutions in a capillary. By measuring the drops of solutions of known concentration and comparing them with drops of a solution of the substance whose molecular weight was being determined, the molecular weight of the latter could be calculated. The samples used by Barger weighed 0.31 mg. The new interest in radioactive metals, available in only small amounts, increased the need for more accurate methods of analysis and atomic weight determinations. In 1903, Nernst (4Z),using the balance constructed by him a year earlier, made molecular weight measurements by determining the vapor density of very small quantities of materials. Later in the same year Nernst and Riesenfeld (42) used this same balance in some quantitative determinations of small samples and in the determination of the atomic weight of one of the rare earths. Emich (lo),who was mentioned earlier for his contributions in qualitative microanalysis, has also made valuable contributions to the field of quantitative microanalysis. He was the first to devise procedures of for the quantitative collection of small precipitates and worked out many other quantitative procedures and techniques. He developed methods of quantitative microanalysis for both inorganic and organic substances, using a modified Nernst balance in his early determinations. Janecke (44) made the first when be electrolytic determination on a micro used this method in 1904 to determine the amount of mercury in pathological urine. Brill and Evans (45) for atomic weight de. in 1908 used this same terminations. The Nernst balance was used in both of these investigations. The best known worker in the field of quantitative organic microanalysis is Pregl (46). In 1910, during an investigation on bile acids, he obtained a fission product obtainable a t the time in only very small quantities. It was necessary for him either to use exceptionally Or to find a way large quantities Of the to determine carbon, hydrogen, and nitrogen in those
small samples. Eniich had already illustrated the possibility of analyzing small inorganic samples and this encouraged Pregl to set to work to develop a micromethod for carbon and hydrogen analyses. By the end of 1911 he had succeeded in solving this problem and was able to determine car6on and hydrogen in 7 to 13 mg., and nitrogen in 4 to 8 mg. of organic material. During the next decade he and his students and coworkers adapted many of the common macro analytical methods to a micro scale. Pregl used a Kuhlman balance in his determinations. This balance, still widely used, has a capacity of 20 g. and weighs accurately to 0.01 or 0.02 mg. The balance used by him in 1911 had a sensitivity of 0.001 mg. In 1923 Pregl was awarded the Nobel prize for his contributions in this field. It has been the aim of this paper to present briefly part of the early development of this increasingly important branch of analytical chemistry. There have been many omissions, for the workers and publications in this field, especially in more recent times, have been too numerous to enumerate. The analysis of small quantities of material by flame analysis, spectroscopy, colorimetry, nephelometry, and other important instrumental methods has also been omitted, although these methods are of great importance, especially in industry, where the speed and simplicity of operation make them of great value. LITERATURE CITED
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