Introduction

In the 1930s, preliminary research was indicated that several vitamins, particularly Vitamin A, showed absorbance in the UV region. As the likelihood of American involvement in WWII loomed, scientists raced to find a simple way to measure the vitamin content of food. The government needed to know what vitamins were in rations so soldiers would be well-nourished and able to perform their best. (A WWII poster calling for soldiers to eat more Vitamin A is shown in Figure 1.) This research spawned a revolution in instrumentation that was spearheaded by the Beckman DU UV-vis spectrophotometer.

In the 1930s it was discovered that biochemical materials, particularly vitamin A, absorb in the UV region, giving a molecular "fingerprint" that allows easy identification in complex mixtures. The discovery of the fingerprint region was thought to herald a new age where every chemical could be identified easily and then synthesized by knowing its absorption fingerprint. This age has not yet come, and the development of infrared spectroscopy and proton NMR spectroscopy have proven to be more useful tools for structural determination. UV-vis instruments still have a solid role in any modern chemical laboratory for determining concentrations, studying kinetics, and a few other uses.

The first instrument to address the need for UV measurement was the Coleman DM spectrophotometer. The Coleman DM incorporated a tungsten light source (automotive headlamp), grating monochromator, and a standard phototube in a compact unit designed for routine UV measurement. Readout from the Coleman DM was through a Coleman "pH electrometer," which amplified the signal from the phototube and displayed the output. The Coleman pH electrometer was just a slight variation on the original Beckman Model G pH meter, and Arnold Beckman at Beckman Instruments was quick to see the potential for a UV-vis spectrophotometer of this own design.

None of the employees at National Technical Laboratories had experience with optical design, so Beckman contacted Bausch and Lomb in hopes of making the spectrophotometer a joint project. However, war was on the horizon and Bausch and Lomb anticipated a huge demand for binoculars, bomb sights, and other wartime needs, leaving little time for developing a new research instrument with a small applied electronics company. Beckman returned to Pasadena determined to go ahead with his instrument. "We dusted off our physics textbooks to learn about optical ray-tracing and such and ended up with what has proven to be an enduring optical design for the spectrophotometer" (Sturchio and Thackray, 1985).

The first prototype UV-vis spectrophotometer, dubbed the Model A, was developed in 1940. The Model A used a tungsten source and glass Fery prism monochromator until it was discovered that glass was unsuitable for use in the UV. The Model B prototype, shown in Figure 2, replaced the glass Fery prism with a quartz prism, greatly improving its usefulness in the UV. The Model B also utilized a tangent bar mechanism to adjust the monochromator. This mechanism was almost linear in wavelength, but too compressed and sensitive, particularly in the UV, for general use (Beckman et al., 1977). A sure hand, skill, and considerable concentration were prerequisites for operation of the Model B. The Model B, like the Model A before it, used an external Beckman pH meter for readout.

The sensitive tangent bar mechanism of the Model B was replaced in the Model C, shown in Figure 3, by the scroll drive mechanism that would be used in all later Beckman quartz prism instruments. The model C also abandoned the rotary cell compartment of the Model B in favor of a linear sample chamber, but still used an external Beckman pH meter for readout. The end of the Model C was brought about by the realization that it was impractical to house the electronics separately from the optics. Beckman later wrote, "It quickly became apparent that having the amplifier and readout in separate housings was economically undesirable. When these components were incorporated within the monochromator, the instrument was first designated the Model D" (Beckman et al., 1977).

The Model D incorporated the vacuum tube amplification system of the Beckman G in the empty space in the casing surrounding the quartz prism monochromator of the Model D. Finding the raw material for quartz prisms presented its own challenges. The large, high optical quality quartz required by the Model D could only be obtained from Brazil. Quartz was an essential part of radio oscillators, however, and any quartz shipped from Brazil was rapidly purchased by radio manufacturers for their wartime efforts. Because of the importance of UV spectroscopy to vitamin research, Beckman was able to convince the government of his need for quartz prisms, but "only after establishing a wartime priority listing and paying a premium price was it possible to get access to the stores of large, uncut crystals from which prism blanks Écould be cut" (Beckman et al., 1977).

Two more challenges had to be overcome before the Model D could enter production. The first was the design of a suitable UV light source. Tungsten light sources were widely available because of their use as automotive headlamps, gave excellent results in the visible, but had little power in the UV. In fact, Cary and Beckman's original paper describing the model D listed "a standard 32-candlepower automobile headlamp (Mazda No. 2331)" as the tungsten source (1941). Hydrogen lamps were known to produce UV radiation, but in 1940 there were no manufacturers of hydrogen lamps suitable for spectroscopic work. By 1941, National Technical Laboratories had developed and started production of a new type of hydrogen lamp with an enclosed anode and thin blown-glass window.

The most powerful UV lamp is useless without an appropriate detector, and finding a suitable phototube for the Model D was the last major challenge of its development. In 1941, RCA produced phototubes suitable for use from 350 nm to 600 nm and from 600 nm to 1200 nm, but nothing in regular production for use below 350 nm. RCA was also developing an experimental phototube useful to 220 nm, but the batches of this phototube were small and quality was irregular. Despite these problems, the Model D spectrophotometer was introduced at the Summer Conference on Spectroscopy at MIT in July 1941, with the experimental RCA phototube. (Cary and Beckman, 1941)

The Model D was a success, and demand quickly drained the supply of experimental RCA phototubes. RCA was unwilling to produce regular batches of the UV-sensitive phototubes, so Beckman and Cary frantically set about designing their own UV-sensitive phototube before the success of the Model D became its demise. Within months, production began on a new UV-sensitive phototube designed by Howard Cary and Warren Baxter. Instruments equipped with the new phototube were designated DU to signify their enhanced UV performance.

With the advent of the DU, shown in Figure 4, Beckman introduced an instrument which "had higher resolution and lower stray light in the ultraviolet than any other commercial instrument, and it quickly enjoyed a good market" (Beckman et al., 1977). By the end of 1941, 18 DU spectrophotometers had shipped at a price of $723; in 1942, 54 were sold. The first publication written on work using a Beckman DU was published in Industrial and Engineering Chemistry, Analytical Edition in September 1942. Beckman later wrote about the delay between the introduction of the DU and publication, "Those decrying the present delays in publication will be interested to know that the instrument upon which this paper was written was not shipped from Beckman Instruments until June 5, 1942! In subsequent years thousands of technical papers concerning DU applications have been published." (Beckman et al., 1977)

The Beckman DU has proven to be one of the most enduring instrumental designs in the history of analytical instrumentation. From its introduction in 1941 to the end of production of the cosmetically-altered DU-2 in 1975, over 35,000 instruments using the optical design of the original DU, shown in Figure 5, were sold. Major changes along the lifespan of this instrument have been the shift from DC to AC power, the introduction of the DU-2 with its cosmetic freshening and slightly altered control layout, and the production of numerous Beckman and other aftermarket accessories. The introduction of AC power to the DU line was more complicated might be expected because of the necessity to design a single power supply that would work with all existing DU spectrophotometers as well as their accessories, each of which had different voltage and current requirements. The modular design of the DU permitted accessories to be added easily. Some of the more popular, influential accessories were the flame photometry and automatic point-by-point recording accessories, both of which heralded new classes of instruments.

Automatic recording devices notably advanced the science of spectroscopy. Consider Figure 6, a UV spectrum of benzene taken on an original Beckman D spectrophotometer in 1941 (Cary and Beckman, 1941). Each point on the spectrum had to be collected individually and plotted by hand. Collecting a quality spectrum like the one shown in Figure 6 would take an entire day's concentrated effort.

Modern UV-vis spectrophotometers are significantly different in design and function than the DU, but most of the features advertised by Beckman for the DU in the 1940s, as shown by the advertisement in Figure 7, still exist in some form today. Quartz prisms are very rarely seen today, and have been almost entirely replaced by grating monochromators; high quality replica gratings and multiple-order attenuation filters have negated the stray light that led Cary and Beckman to choose a quartz prism in the Beckman Model B prototype. The cost of gratings are now much less than quartz prisms, and the linear dispersion that characterizes grating monochromators has greatly simplified the design of low-cost spectrophotometers. Phototubes are still widely used in low-cost spectrometers, but cost and longevity hint that phototubes may soon be replaced by photodiodes for routine work. The molecular hydrogen lamp designed by Beckman and Cary was the most popular type of UV lamp until it was superseded by the superior intensity of quartz-window deuterium lamp.

Regardless of design, each of today's UV-vis spectrometers can trace its roots to the Beckman DU. The DU was not only the first instrument to probe the UV region with high precision and accuracy, it also introduced the concept of useful electro-optical instrumentation to countless scientists. It is hard to overstate the importance of the Beckman DU. Bruce Merrifield, Nobel Laureate in chemistry, has called the DU "probably the most important instrument ever developed to the advancement of bioscience" (Sturchio and Thackray, 1985a). The DU and other UV-vis spectrometers that followed have changed lives through the analysis and production of chemicals such as polymers, fuels, and pharmaceuticals, and the sales records of the Beckman DU have yet to be matched by any other instrument.

References

Beckman, A. O.; Gallaway, W. S.; Kaye, W.; Ulrich, W. F. "History of Spectrophotometry at Beckman Instruments, Inc." Anal. Chem. 1977, 49(3), 280 A-300 A

Cary, H. H.; Beckman, A. O., Jr. "A Quartz Photoelectric Spectrophotometer." J. Opt. Soc. Am. 1941, 31, 682-689.

Sturchio, J. L.; Thackray, A. Center for History of Chemistry, Philadelphia, PA. Unpublished Interview of Arnold O. Beckman. (Conducted at the University of Pennsylvania, July 23, 1985.), 1985.

Thackray, A.; Myers, M. Jr. Arnold O. Beckman: One Hundred Years of Excellence. Chemical Heritage Foundation: Philadelphia, PA, 2000.