Development, Present State, and Outlook of Spot Test Analysis
Authors: Feigl Fritz 
Published: July 2009

When I received the invitation to speak at the Fifty-second Annual Meeting of the A.S.T.M. on some phase of spot test analysis, 28 years had passed since I had shown, in my doctorate thesis, that surprisingly sensitive tests for metals can be made by means of spot reactions carried out on filter paper. In the meantime, isolated applications of spot reactions have grown into spot tes analysis, which is a method of detecting, or sometimes of quantitatively determining, inorganic and organic materials. This development was made possible only because of the active collaboration of workers in many countries. The common efforts have been aimed not merely toward improving a technique, which is fundamentally so simple as carrying out known tests within the volume of a single drop. Far more important has been the discovery of new tests, and especially the demonstration of the possibilities of making practical applications of spot reactions. The utility of spot reactions in testing materials was soon recognized. Indeed, it may be said that spot testing became so popular precisely because it proved to be so valuable in solving problems in the field of testing materials. This statement is supported by the fact that a series of papers on spot test methods are to be presented at this symposium. With respect to its practical application, spot test analysis is an analytical technique. However, there is much more to be said concerning it if it is viewed from a scientific standpoint. Its development is linked with studies of the analytical usefulness of chemical reactions. The results of such research have been of profit not only to spot test analysis but to analytical chemistry in general. The benefit has been twofold. New tests and methods of determination have resulted, and furthermore the knowledge of the scientific bases of analytical chemistry has been deepened. Therefore, it may be of interest to review the development of spot test analysis, to examine its present state, and to consider its future prospects. It is difficult to establish who was the first person to carry out spot reactions. Analytical chemists have long used single chemical tests conducted in drops of solutions on filter paper or on impermeable surfaces. The earliest published instance appears to have been given by Schiff (1). He used paper impregnated with silver carbonate to detect uric acid through the formation of a spot of metallic silver. This instance of a spot reaction was not known to me when, as a student, I made my first trials with spot reactions. These experiments were inspired by reading the classical studies by Goppelsroeder (2) who had investigated the capillary rise and spreading the liquids and dissolved materials in filter paper. Goppelsroeder was concerned primarily with the capillary separation of organic compounds, and he made this the basis of his so-called “capillary analysis.” His paper, however, also contained references to the capillary spreading of inorganic salts. Therefore it seemed quite natural to me to find out whether an inorganic capillary analysis was possible. If I had known that others before me had had the same idea and that they had found that capillary separation of inorganic materials by means of filter paper is possible in isolated cases only, I assuredly would not have busied myself with spot reactions on paper. However, like many young colleagues I was not yet aware of the importance of literature searches; and, in this particular case, neglect of the printed record proved to be an advantage. I had to carry out color reactions in the form of spot tests on the separate zones of the paper in order to detect the materials which had been separated by capillarity. It was impossible in these experiments to overlook the fact that the picture of a reaction when carried out as a spot reaction on paper is quite different from that seen in a test tube. The appearance of the flecks was quite different according to the concentration of the reaction partners, the variety of the paper, and the experimental conditions—a finding that hinted at a regularity which was still unknown at the time. I was fascinated by the romantic and aesthetic features of the development and outward appearance of the flecks and by the fact that many tests exhibited an unexpectedly great sensitivity when they were conducted as spot reactions on paper. My first collaborator, R. Stern, was also fascinated and conceived the idea of trying certain tests with organic reagents in the form of spot reactions. We found not only that sensitive individual tests are possible through spot reactions but that several materials can be detected in a single drop of a solution provided the reagents are chosen properly.