DU spectrophotometer
The DU spectrophotometer or Beckman DU, introduced in 1941, was the first commercially viable
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
Spectroscopic methods for observing the absorption of electromagnetic radiation in the visible spectrum were known as early as the 1860s.[4]: 65 [5]: 5 Scientists had observed that light traveling through a
Spectroscopic methods were predominantly used by
By the 1940s, both academic and industrial chemists were becoming increasingly interested in problems involving the composition and detection of
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.
Development
The DU was developed at National Technical Laboratories (later
Coleman Instruments had recently coupled a pH meter with an optical phototube unit to examine the visual spectrum (the Coleman Model DM).[10] 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.[2]: 149–151
Model A (prototype)
The first prototype Beckman spectrophotometer, the Model A, was created at National Technologies Laboratories in 1940. It used a
Model B (prototype)
It was quickly realized that a glass dispersive prism was not suitable for use in the ultraviolet spectrum.[2]: 153 [17]: 16 Glass absorbed electromagnetic radiation below 400 millimicrons rather than dispersing it.[20] In the Model B, a quartz prism was substituted for the earlier glass.[2]: 153 [17]: 16
A tangent bar mechanism was used to adjust the monochromator. The mechanism was highly sensitive and required a skilled operator.[17]: 16 Only two Model B prototypes were made. One was sold: in February 1941, to the University of California Chemistry department in Los Angeles.[2]: 153
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.[21] 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.[22]: 183–184 [23][24][25]
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,
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.[10]
Moving from a prototype to production of the Model D involved challenges. Beckman originally approached
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.[26]: 684–685 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.[17]: 17 By December 1941, the in-house design was being used in production of the Model D.[2]: 154–155
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.[17]: 17
The Model D spectrophotometer, using the experimental RCA phototube, was shown at
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.[2]: 153–155 [17]: 17–18
Model DU
When RCA could not meet Beckman's demand for experimental phototubes, National Technical Laboratories again had to design its own components in-house.[17]: 18 They developed a pair of phototubes, sensitive to the red and blue areas of the spectrum, capable of amplifying the signals they received.[28]: 230 With the incorporation of Beckman's UV-sensitive phototubes, the Model D became the Model DU UV–Vis spectrophotometer.[17]: 18 Its designation as a "UV–Vis" spectrophotometer indicates its ability to measure light in both the visible and ultraviolet spectra.[29]
The DU was the first commercially viable scientific instrument for measuring the amount of ultraviolet light absorbed by a substance.[2]: 148 [5]: 10 As he had done with the pH meter, Beckman had replaced an array of complicated equipment with a single, easy-to-use instrument. One of the first fully integrated instruments[17]: 11 or "black boxes" used in modern chemical laboratories,[30] it sold for $723 in 1941.[12]
It is generally assumed that the "DU" in the name was a combination of "D" for the Model D on which it was based, and "U" for the ultraviolet spectrum. However, it has been suggested that "DU" may also reference Beckman's fraternity at the University of Illinois, Delta Upsilon, whose members were called "DU"s.[31]
A publication in the scholarly literature compared the optical quality of the DU to the Cary 14 Spectrophotometer, another leading UV–Vis spectrophotometer of the time.[32]
Design
From 1941 until 1976, when it was discontinued, the Model DU spectrophotometer was built upon what was essentially the same design.[12] It was a single beam instrument.[16]: 11 [33]
The DU spectrophotometers used a quartz prism to separate light from a lamp into its absorption spectrum and a phototube to electrically measure the light energy across the spectrum. This allowed the user to plot the light absorption spectrum of a substance to obtain a standardized "fingerprint" characteristic of a compound.[2]: 151 [34][35] All modern UV–Vis spectrophotometer are built on the same basic principles as the DU spectrophotometer.[29]
"Light from the tungsten lamp is focused by the condensing mirror and directed in a beam to the diagonal slit entrance mirror. The entrance mirror deflects the light through the entrance slit and into the monochromator to the collimating mirror. Light falling on the collimating mirror is rendered parallel and reflected to the quartz prism where it undergoes refraction. The back surface of the prism is aluminized so that light refracted at the first surface is reflected back through the prism, undergoing further refraction as it emerges from the prism. The desired wavelength of light is selected by rotating the Wavelength Selector which adjusts the position of the prism. The spectrum is directed back to the collimating mirror which centers the chosen wavelength on the exit slit and sample. Light passing through the sample strikes the phototube, causing a current gain. The current gain is amplified and registered on the null meter." Model DU Optical System[36]: 3
Although the default light source for the instrument was tungsten, a hydrogen or mercury lamp could be substituted depending on the optimal range of measurement for which the instrument was to be used.[36] : 3 The tungsten lamp was suitable for transmittance of wavelengths between 320 and 1000 millimicrons; the hydrogen lamp for 220 to 320 millimicrons, and the mercury lamp for checking the calibration of the spectrophotometer.[36]: 6
As advertised in the 1941 News Edition of the American Chemical Society, the Beckman Spectrophotometer used an autocollimating quartz crystal prism for a monochromator, capable of covering a range from the ultraviolet (200 millimicrons) to the infrared (2000 millimicrons), with a nominal bandwidth of 2 millimicrons or less for most of its spectral range. The slit mechanism was continuously adjustable from .01 to 2.0 mm and claimed to have less than 1/10% of stray light over most of the spectral range. It featured an easy-to-read wavelength scale, simultaneously reporting % Transmission and Density information.[37]
The sample holder held up to 4 cells.
The user could also switch between phototubes without removing the sample holder. A 1941 advertisement indicates that three types of phototubes were available, with maximum sensitivity to red, blue and ultraviolet light ranges.[37]
The 1954 DU spectrophotometer differs in that it claims to be useful from 200 to 1000 millimicrons,[36]: 2 and does not mention the ultraviolet phototube.[36]: 3 The wavelength selector, however, still ranged from 200 to 2000 millimicrons.[36]: 4 and an "Ultraviolet accessory set" was available.[36]: 25 This shift away from using the DU for infrared measurement is understandable, since by 1954 Beckman Instruments was marketing a separate infrared spectrophotometer. Beckman developed the IR-1 infrared spectrophotometer during World War II, and redesigned it as the IR-4 between 1953 and 1956.[2]: 165 [39]: 6–7
Use
The Beckman spectrophotometer was the first easy-to-use single instrument containing both the optical and electronic components needed for ultraviolet-absorption spectrophotometry within a single housing.[2]: 153 The user could insert a cell tray with standard and sample cells, dial up the desired wavelength of light, confirm that the instrument was properly set by measuring the standard, and then measure the amount of absorption of the sample, reading the frequency from a simple meter.[40] A series of readings at different wavelengths could be taken without disturbing the sample.[41] The DU spectrophotometer's manual scanning method was extremely fast, reducing analysis times from weeks or hours to minutes.[39]: 6 [42] [43]
It was accurate in both the visible and ultraviolet ranges.[29]
Working in both the ultraviolet and the visible regions of the spectrum, the model DU produced accurate absorption spectra which could be obtained with relative ease and accurately replicated.
Other advantages included its high resolution and the minimization of stray light in the ultraviolet region.[12] Although it was not cheap, its initial price of $723[12] made it available to the average laboratory.[44]: 501 In comparison, in 1943, the GE Hardy Spectrophotometer cost $6,400.[39]: 6 Practical and reliable, the DU rapidly established itself as a standard for laboratory equipment.[35]: 141
Impact
Credited with having "brought about a breakthrough in optical spectroscopy",
The DU enabled researchers to perform easier analysis of substances by quickly taking measurements at more than one wavelength to produce an absorption spectrum describing the complete substance. For example, the standard method of analysis of the vitamin A content of shark liver oil, before the introduction of the DU spectrophotometer, involved feeding the oil to rats for 21 days, then cutting off the rats' tails and examining their bone structure. With the DU's UV technology, vitamin A content of shark liver oil could be determined directly in a matter of minutes.[39]: 6
The Scripps Research Institute and the Massachusetts Institute of Technology credit the DU with improving both accuracy and speed of chemical analysis. MIT states: "This device forever simplified and streamlined chemical analysis, by allowing researchers to perform a 99.9% accurate quantitative measurement of a substance within minutes, as opposed to the weeks required previously for results of only 25% accuracy."[42][43]
Inorganic chemist and philosopher of science
For the Beckman company, the DU was one of three foundational inventions – the pH meter, the DU spectrophotometer, and the helipot potentiometer – that established the company on a secure financial basis and enabled it to expand.[47]
Vitamins
Development of the spectrophotometer had direct relevance to World War II and the American war effort. The role of vitamins in health was of significant concern, as scientists wanted to identify
Penicillin
The DU spectrophotometer was also an important tool for scientists studying and producing the new wonder drug penicillin.[10]
The development of penicillin was a secret national mission, involving 17 drug companies, with the goal of providing penicillin to all U.S. Forces engaged in World War II.
At
Hydrocarbons
The DU spectrophotometer was also used for critical analysis of
Enzyme assays and DNA research
Another researcher who spent six months in 1947 at the Cori laboratory, "the most vibrant place in biochemistry" at that time, was Arthur Kornberg.[62] Kornberg was already familiar with the DU spectrophotometer, which he had used at Severo Ochoa's laboratory at New York University. The "new and scarce" Beckman DU, loaned to Ochoa by the American Philosophical Society, was highly prized and in constant use. Kornberg used it to purify aconitase, an enzyme in the citric acid cycle.[62][63]
"The enzyme could be assayed in a few minutes by coupling it to isocitrate dehydrogenase and in measuring the NADH formed using the Beckman DU spectrophotometer, an instrument that transformed biochemistry."[63]: 113
Kornberg and
The bases of DNA absorbed ultraviolet light near 260 nm.
Biotechnology
Ultraviolet spectroscopy has wide applicability in molecular biology, particularly the study of photosynthesis.[68] It has been used to study a wide variety of flowering plants and ferns[69] by researchers in departments of biology, plant physiology and agriculture science as well as molecular genetics.[70]
Particularly useful in detecting conjugated double bonds, the new technology made it possible for researchers like
Later models
The Beckman team eventually developed additional models, as well as a number of accessories or attachments which could be used to modify the DU for different types of work. One of the first accessories was a flame attachment with a more powerful photo multiplier to enable the user to examine flames such as
: 230In the 1950s, Beckman Instruments developed the DR and the DK, both of which were double-beam ultraviolet spectrophotometers. The DK was named for
The DR incorporated a "robot operator" which would reset the knobs on the DU to complete a sequence of measurements at different wavelengths, just like a human operator would to generate results for a full spectrum. It used a linear shuttle with four positions, and a superstructure to change the knobs. It had a moving chart recorder to plot results, with red, green and black dots.[16] The price of recording spectrophotometers was substantially higher than non-recording machines.[72]
The DK was ten times faster than the DR, but not quite as accurate.[16] It used a photomultiplier, which had introduced a source of error.[76]: 21 The DK's speed made it preferred to the DR.[16] Kaye eventually developed the DKU, combining infrared and ultraviolet features in one instrument, but it was more expensive than other models.[76]
The last DU spectrophotometer was produced on July 6, 1976.[77] By the 1980s, computers were being incorporated into scientific instruments such as Bausch & Lomb's Spectronic 2000 UV–Vis spectrophotometer, to improve data acquisition and provide instrument control.[29] Specialized spectrophotometers designed for specific tasks now tend to be used rather than general "all-purpose machines" like the DU.[5]: 1 [78]
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External links
- Jaehnig, Kenton G. Finding Aid to the Beckman historical collection, 1911–2011 (bulk 1934–2004). )