The DU spectrophotometer or Beckman DU, introduced in 1941, was the first commercially viable scientific instrument for measuring the amount of ultraviolet light absorbed by a substance. This model of spectrophotometer enabled scientists to easily examine and identify a given substance based on its absorption spectrum, the pattern of light absorbed at different wavelengths. Arnold O. Beckman's National Technical Laboratories (later Beckman Instruments) developed three in-house prototype models (A, B, C) and one limited distribution model (D) before moving to full commercial production with the DU. Approximately 30,000 DU spectrophotometers were manufactured and sold between 1941 and 1976.
Sometimes referred to as a UV–Vis spectrophotometer because it measured both the ultraviolet (UV) and visible spectra, the DU spectrophotometer is credited as being a truly revolutionary technology. It yielded more accurate results than previous methods for determining the chemical composition of a complex substance, and substantially reduced the time needed for an accurate analysis from weeks or hours to minutes. The Beckman DU was essential to several critical secret research projects during World War II, including the development of penicillin and synthetic rubber.
Background Before the development of the DU spectrophotometer, analysis of a test sample to determine its components was a long, costly, and often inaccurate process. A classical wet laboratory contained a wide variety of complicated apparatus. Test samples were run through a series of awkward and time-consuming qualitative processes to separate out and identify their components. Determining quantitative concentrations of those components in the sample involved further steps. Processes could involve techniques for chemical reactions, precipitations, filtrations and dissolutions. Determination of the concentrations of known impurities in a known inorganic substance such as molten iron could be done in under thirty minutes. The determination of complex organic structures such as chlorophyll using wet and dry methods could take decades.
Spectroscopic methods for observing the absorption of electromagnetic radiation in the visible spectrum were known as early as the 1860s. Scientists had observed that light traveling through a medium would be absorbed at different wavelengths, depending on the matter-composition of the medium involved. A white light source would emit light at multiple wavelengths over a range of frequencies. A prism could be used to separate a light source into specific wavelengths. Shining the light through a sample of a material would cause some wavelengths of light to be absorbed, while others would be unaffected and continue to be transmitted. Wavelengths in the resulting absorption spectrum would differ depending upon the atomic and molecular composition if the material involved.
Spectroscopic methods were predominantly used by physicists and astrophysicists. Spectroscopic techniques were rarely taught in chemistry classes and were unfamiliar to most practicing chemists. Beginning around 1904, Frank Twyman of the London instrument making firm Adam Hilger, Ltd. tried to develop spectroscopic instruments for chemists, but his customer base was consistently made up of physicists rather than chemists. By the 1930s he had developed a niche market in metallurgy, where his instruments were well adapted to the types of problems that chemists were solving.
By the 1940s, both academic and industrial chemists were becoming increasingly interested in problems involving the composition and detection of biological molecules. Biological molecules, including proteins and nucleic acids, absorb light energy in both the ultraviolet and visible range. The spectrum of visible light was not broad enough to enable scientists to examine substances such as vitamin A. Accurate characterization of complex samples, particularly of biological materials, would require the accurate reading of absorption frequencies in the ultraviolet and infrared (IR) sections of the spectrum in addition to visible light. Existing instruments such as the Cenco "Spectrophotelometer" and the Coleman Model DM Spectrophotometer could not be effectively used to examine wavelengths in the ultraviolet range.
The array of equipment needed to measure light energy reaching beyond the visible spectrum towards the ultraviolet could cost a laboratory as much as $3,000, a huge amount in 1940. Repeated readings of a sample were taken to produce photographic plates showing the absorption spectrum of a material at different wavelengths. An experienced human could compare these to the known images to identify a match. Then information from the plates had to be combined to create a graph showing the spectrum as a whole. Ultimately, the accuracy of such approaches was dependent on accurate, consistent development of the photographic plates, and on human visual acuity and practice in reading the wavelengths.
Development The DU was developed at National Technical Laboratories (later Beckman Instruments) under the direction of Arnold Orville Beckman, an American chemist and inventor. Beginning in 1940, National Technical Laboratories developed three in-house prototype models (A, B, C) and one limited distribution model (D) before moving to full commercial production with the DU in 1941. Beckman's research team was led by Howard Cary, who went on to co-found Applied Physics Corporation (later Cary Instruments) which became one of Beckman Instruments' strongest competitors. Other scientists included Roland Hawes and Kenyon George.
Coleman Instruments had recently coupled a pH meter with an optical phototube unit to examine the visual spectrum (the Coleman Model DM). Beckman had already developed a successful pH meter for measuring acidity of solutions, his company's breakthrough product. Seeing the potential to build upon their existing expertise, Beckman made it a goal to create an easy-to-use integrated instrument which would both register and report specific wavelengths extending into the ultraviolet range. Rather than depending on development of photographic plates, or a human observer's visual ability to detect wavelengths in the absorption spectrum, phototubes would be used to register and report the specific wavelengths that were detected. This had the potential to increase the instrument's accuracy and reliability as well as its speed and ease of use.
Model A (prototype) The first prototype Beckman spectrophotometer, the Model A, was created at National Technologies Laboratories in 1940. It used a tungsten light source with a glass Fery prism as a monochromator. Tungsten was used for incandescent light filaments because it was strong, withstood heat, and emitted a steady light. Types of light sources differed in the range of wavelengths of light that they emitted. Tungsten lamps were useful in the visible light range but gave poor coverage in the ultraviolet range. However, they had the advantage of being readily available because they were used as automobile headlamps. An external amplifier from the Beckman pH meter and a vacuum tube photocell were used to detect wavelengths.
Model B (prototype)
It was quickly realized that a glass dispersive prism was not suitable for use in the ultraviolet spectrum. Glass absorbed electromagnetic radiation below 400 millimicrons rather than dispersing it. In the Model B, a quartz prism was substituted for the earlier glass.
A tangent bar mechanism was used to adjust the monochromator. The mechanism was highly sensitive and required a skilled operator. Only two Model B prototypes were made. One was sold: in February 1941, to the University of California Chemistry department in Los Angeles.
The Model B prototype should be distinguished from a later production model of spectrophotometer that was also referred to as the Model "B". The production Model "B" was introduced in 1949 as a less-expensive, simple-to-use alternative to the Beckman DU. It used a glass Fery prism as a chromator and operated in a narrower range, roughly from 320 millimicrons to 950 millimicrons, and 5 to 20 Å.
Model C (prototype) Three Model C instruments were then built, improving the instrument's wavelength resolution. The Model B's rotary cell compartment was replaced with a linear sample chamber. The tangent bar mechanism was replaced by a scroll drive mechanism, which could be more precisely controlled to reset the quartz prism and select the desired wavelength. With this new mechanism, results could be more easily and reliably obtained, without requiring a highly skilled operator. This set the pattern for all of Beckman's later quartz prism instruments. Although only three Model B prototypes were built, all were sold, one to Caltech and the other two to companies in the food industry.
Model D (limited production)
The A, B, and C prototype models all coupled an external Beckman pH meter to the optical component to obtain readouts. In developing the Model D, Beckman took the direct-coupled amplifier circuit from the pH meter and combined the optical and electronic components in a single housing, making it more economical.
Moving from a prototype to production of the Model D involved challenges. Beckman originally approached Bausch and Lomb about making quartz prisms for the spectrophotometer. When they turned down the opportunity, National Technical Laboratories designed its own optical system, including both a control mechanism and a quartz prism. Large, high optical quality quartz suitable for creating prisms was difficult to obtain. It came from Brazil, and was in demand for wartime radio oscillators. Beckman had to obtain a wartime priority listing for the spectrophotometer to get access to suitable quartz supplies.
Beckman had previously attempted to find a source of reliable hydrogen lamps, seeking better sensitivity to wavelengths in the ultraviolet range than was possible with tungsten. As described in July 1941, the Beckman spectrophotometer could use a "hot cathode hydrogen discharge tube" or a tungsten light source interchangeably. However, Beckman was still unsatisfied with the available hydrogen lamps. National Technical Laboratories designed its own hydrogen lamp, an anode enclosed in a thin blown-glass window. By December 1941, the in-house design was being used in production of the Model D.
The instrument's design also required a more sensitive phototube than was commercially available at that time. Beckman was able to obtain small batches of an experimental phototube from RCA for the first Model D instruments.
The Model D spectrophotometer, using the experimental RCA phototube, was shown at MIT's Summer Conference on Spectroscopy in July 1941. The paper that Cary and Beckman presented there was published in the Journal of the Optical Society of America. In it, Cary and Beckman compared designs for a modified self-collimating quartz Fery prism, a mirror-collimated quartz Littrow prism, and various gratings. The Littrow prism was a half-prism, which had a mirrored back face, so that the light went through the front face twice. Use of a tungsten light source with the quartz Littrow prism as a monochromator was reported to minimize light scattering within the instrument.
The Model D was the first model to enter actual production. A small number of Model D instruments were sold, beginning in July 1941, before it was superseded by the DU.
Model DU