Technikatörténeti szemle 19. (1992)

KÖNYVISMERTETÉS - Papers of the First „MINERALKONTOR” International Conference on the History of Chemistry and Chemical Industry (Veszprém, 12-16 August, 1991)

Around the turn of the century Arthur Konig and F,F. Martens developed an excellent visual spectrophotometer which was subsequently manufactured in the United States by Gaertner Scientific and Bausch & Lomb. While many modifications were made in the next fifty years, the basic operation remained the same. The split light beam was passed through sample and solvent, and where there was absorption by the sample the light intensity was diminished. The key to the operation of the visual spectrophotometer was the determination of the „match point" obtained by adjusting internal prisms at each manual wavelength setting until the intensity of the reference beam and the sample beam were again equal. The data collected in this manner, taken from a calibrated dial used to adjust the intensities, had to be plotted on graph paper point by point to obtain the spectrum. Instruments of this type were in use well into the 1950s, and one author in 1948 praised it for being „free from the complications of photographic manipulation or the operation an maintenance of electrical measurining devices." (5). As with colorimetry there were many inherent difficulties resulting from the basic characteristics of vision. There were a number of different ways in which the photometric fields wery juxtaposed for comparison purposes (Figure 6). Yet the human eye has very low sensitivity at the extreme ends of the visible spectrum, and the matching of intensities by the eye involves subjective as well as objective factors. Under identical conditions different observers may vary considerably in how they perceive the contrast in intensity they need to adjust to make a measurement. Finally, the visual stimulus is a complex function of the wavelength of the exciting radiation being examined. In spite of these inherent sources of error, proponents of visual methods continued to be reluctant to switch to newer techniques. As late as October 1941 Ralph H. Mueller of New York University stated that „the assumption that any photoelectric instrument must be more accurate and reliable than a visual instrument is wholly unwarranted." Yet he reluctantly admitted „that the future does seem to lie in the direction of the photoelectric types, for there is no inherent limit in the attainable sensitivity and objectivity of the measurement. Fatigue effects again incline toward the photoelectric type, although squinting at a microammeter needle for protracted periods is not a pleasant pastime." (6). Photographic spectrophotometry: Replacing the eye with a camera In 1911 Adam Hilger, Ltd. introduced an instrument that replaced the human eye with a photographic plate to detect absorption phenomena. Basically the operation was similar to the visual instrument, using a rotating sector such as the one shown here to alternate the light beam between sample and solvent. The light beams passing through the sample and solvent were then focused on the entrance slit of a standard spectrograph. In this manner it was possible on a single photographic plate to produce a series of spectra that would show the form of the absorption curve when ..spotted." This process required marking the match points where the blackness of adjoining solvent and solution spectra is equal, not a necessarily easy task when it had to be repeated many times. The result of thus ..spotting" the match points on the photographic plate