Introduction

More than any other technique discussed in this site, the advent of infrared spectroscopy was driven by a specific goal. Infrared spectroscopy was jump-started by the United States government, primarily as a tool for the manufacture of synthetic rubber but also for the production of aviation fuel and penicillin. Military demand for natural rubber far exceeded supply, and the Office of Rubber Reserve was founded to develop synthetic alternatives. A few World War II posters are shown in Figures 1, 2, 3, 4, and 5, emphasizing the vital need and role of rubber for the war effort.

Synthesis of wartime products is the very minimum of what is possible with infrared spectroscopy, and the technique has become one of the benchstones of any form of chemical analysis. The first commercial infrared (IR) instrument was the Beckman IR-1. However, the government restricted sales of the IR-1 until after the war, and the instrument never gained widespread acceptance. Perkin-Elmer, Inc. introduced the Model 12 infrared spectrometer in 1944, but it was too difficult to use for anyone but trained, unusually patient spectroscopists. The first widely successful IR spectrophotometer was the Perkin-Elmer 21. Its double-beam design and flexibility allowed non-specialists access to the IR for the first time. The technique quickly became popular in every branch of chemistry, and continues to be one of the most versatile and informative of modern analytical techniques.

During World War II, the United States needed massive quantities of rubber for gas masks, tires, and other wartime needs, but the Japanese invasion of Malaya and the Dutch East Indies had stopped the supply of natural rubber. President Roosevelt realized that America's small natural rubber stockpile and recycling programs were vastly inadequate, and formed the Office of Rubber Reserve to produce large quantities of synthetic rubber. The synthetic rubber of the time was formed from polymerizing butadiene, and after having arranged for a supply of butadiene from petroleum companies, the Office of Rubber Reserve set about procuring a method to determine the concentration and purity of butadiene at every step of rubber synthesis.

When Office of Rubber Reserve was formed, there was no good commercial method to distinguish between different hydrocarbons. Leading scientists in the field, including Arnold Beckman from National Technical Laboratories, Robert Brattain from Shell development, and Van Zandt Williams from Perkin-Elmer, were summoned to Detroit for a meeting to decide upon a technique for highly-precise, accurate measurements of the butadiene. Based on the known infrared properties of butadiene and other hydrocarbons, the committee unanimously decided to use infrared spectroscopy. Discussion shifted from what type of instrument to develop to which specific design to mass-produce. Brattain from Shell had developed a prototype single beam spectrophotometer that he brought with him to the meeting. Beckman and Williams both proposed alternative spectrophotometers based on dual-beam designs, but Brattain's design was tangible, a distinct advantage in times of crisis. The Office of Rubber Reserve approved Brattain's design and asked Beckman, based on the success and reputation of the DU, to make one hundred instruments from Brattain's plans. The instrument Beckman produced from Brattain's design was the Beckman IR-1, the first commercial infrared spectrophotometer. Unfortunately, the IR-1 never gained commercial acceptance because of government restricted sales of the IR-1 until after World War II..

At the same time but with less funding or experience in instrumental design, Perkin-Elmer was working on its first infrared spectrophotometer, the Perkin-Elmer Model 12, commonly referred to as the PE 12. Perkin-Elmer was founded by Richard Perkin and Charles Elmer, amateur astronomers who started their company to produce high quality American optics for astronomical and other applications. World War II would give Perkin-Elmer, Inc. a huge boost due to the need for quality optical components in everything from periscopes to cameras.

Even before the boom in wartime optics reached its peak, Perkin-Elmer's experience in producing demanding optics gave them an edge in designing the PE 12, shown in Figure 6. The first Beckman IR-1 shipped on September 18, 1942, six months after development began (Beckman et al., 1977). The IR-1 used spherical mirrors derived from the DU, making the instrument less complicated to design but reliant on long focal lengths and optical paths to effect dispersion. The design system of the Model 12, shown in Figure 7, was completely original, employing aspherical mirrors shaped and ground by Halley Mogey, Perkin-Elmer's talented optical craftsman. The use of aspherical optics not only allowed the Model 12 to be the more compact instrument, but smaller focal lengths and shorter beam paths resulted in more light at the detector and higher resolution.

The advanced optical design of the Model 12 gave Perkin-Elmer its entry into the analytical instrument business. Originally, the instrument that would become the PE 12 was to be made by American Cyanamid, but Cyanamid lacked the capability to make the required optics. Cyanamid contacted Perkin-Elmer, then a small manufacturer of optical components, to supply the optics for the first Cyanamid prototype. Perkin-Elmer was then "reluctantly induced" into building still more instruments," and was soon making infrared instruments for production under the Perkin-Elmer name (McDonnell, 1980). The first Perkin-Elmer Model 12A infrared spectrophotometer shipped in 1944 (Miller, 1992). At the time of its introduction, Richard Perkin had "expected to produce 25 instruments, and then the market would be completely saturated" (Wilks, 1992). By 1947, over 500 IR instruments had been produced (Miller, 1992).

Despite very good optical designs in both the Beckman IR-1 and PE 12A, technological limitations plagued each of these instruments. Because of the low energy of IR radiation, the Perkin-Elmer 12 and Beckman IR-1 both used sensitive DC thermocouples as detectors. These detectors were so responsive that "if one lit a match and held it near the instrument housing, the pen moved" (Miller, 1992). The original Perkin-Elmer Model 12 included a felt liners to stabilize the thermocouple response, but the liners often absorbed and gave off enough water, making the devices useless and even damaging to the optics. The high sensitivity of the thermocouples forced spectroscopists into seclusion from steam pipes and other heat sources, and indirectly other humans, giving spectroscopists "the reputation of recluses who spoke only to other spectroscopists" (Wilks, 1992).

Glass is traditionally used in optical components, but because of the high infrared absorption of glass, alkali halide crystals must be used for all refractive optics in infrared instruments. The most common crystals for infrared applications were made of sodium chloride, but several other types, including potassium bromide and calcium fluorite, were also used for different regions of the IR. Before grown alkali halide crystals became widely available, the only source of large, optically pure sodium chloride crystals was the Soviet Union. Foil Miller recalls an interesting story told by Van Zandt Williams about how American Cyanamid obtained the large crystals of rock salt used in its spectrophotometers.

In about 1939 [Williams] and one or two others went toÉ [the Soviet Union's business agents in] New York CityÉ to inquire about buying natural sodium chloride crystals mined in the USSR. The salesman said they could be provided and asked what size pieces were wanted. After a brief consultation among the Cyanamid people, one of them held out his forefingers to indicate two size of blocks that would be appropriate.ÉThe salesman took out a ruler, measured the spacings carefully, and noted them on the order.

Nothing was heard for a very long time. Finally, more than a year later, an urgent message was received. If they wanted their rock salt, they should come immediately to New York City, take a small boat out to a certain ship in the harbor, and pick up their purchase. The reason for the haste was that World War II was about to start in Europe, and the Soviets wanted to get their ships backÉ as quickly as possible. In fact, this was the last Soviet ship to leave New York. The packages, Éabout one foot in each dimension, were lowered over the ship's rail to theÉsmall boat below.

When they were opened back in the laboratory, each box contained a number of neatly wrapped blocks of rock salt. Each block had been cleaved to the exact size that had been indicated in such an impromptu fashion. The buyers realized then that they could have asked for any size within their wildest dreams and would have received it. This supply lasted Cyanamid laboratories for many years. (Miller, 1992)

The sodium chloride prisms used in most early infrared spectrometers had their own special set of problems. After the initial step of cutting with a wet string saw, sodium chloride crystals had to be diligently protected from water. Cleaning the crystals became a problem; water dissolved the prism faces and ethanol evaporated, scratching the prism. Even moisture from breath was enough to fog a sodium chloride prism being routinely adjusted in the instrument. Halley Mogey, the man responsible for the parabloid mirrors of the Perkin-Elmer 12, realized that water saturated with NaCl could not dissolve any more from the prism face, and used salt water as a lubricant for buffing. For minor touch ups, it was enough to breathe on a cloth and lay it on one's arm to pick up salt before polishing. When not in use, sodium chloride crystals were often kept in dessicators to prevent further degradation. (Wilks, 1992)

Single beam instruments were well suited to reliable quantitative use, but required tedious manual replotting to obtain a standardized spectrum. Dual beam instruments avoided this work by constantly comparing sample and reference beams, but required balancing the beams with an optical wedge attenuation device, resulting in varied percent transmittances from run to run. The imprecision of dual beam instruments made single-beam instruments better for quantitative work while dual-beam models were employed for high-resolution qualitative studies. Single beam infrared instruments were most prevalent until 1949, but trained spectroscopists were necessary to operate the instruments and plot spectra, severely limiting the usefulness of the technique for the average chemist.

In 1950, Perkin-Elmer introduced the first commercially successful dual beam infrared spectrophotometer, the Perkin-Elmer 21, shown in Figure 8 and Figure 9. The optical design of the Perkin-Elmer 21, shown in Figure 10, is essentially the same as the Perkin-Elmer 12, shown in Figure 19, with the addition of a reference beam and chopper. Scrutiny of the housings of the Perkin-Elmer 12 and 21 (Figure 6 and Figure 8, respectively) reveal that the housings of the PE 12 look much like the base sections of the PE 21, but reversed. In other words, light generally travels from left to right in the Model 12 and right to left in the Model 21. This is not coincidence. Paul Wilks recounts why the change was made:

John White was intrigued by the fact that I seemed to be the only one who could easily align the slits on the Model 12. While watching me workÉ, he suddenly exclaimed, 'You're left-handed, aren't you?' With that observation, he immediately went back to his drawing board and reversed the optical design on the Model 21. (Wilks, 1992)

The official introduction of the Model 21 was made at the 1950 Optical Society of America meeting in Detroit (Wilks, 1992). The instrument was greeted enthusiastically, and was soon the best selling infrared instrument in the nation. The Model 21 was used heavily in rubber manufacture, as well as the perfume industry, early synthetic pharmaceutical manufacturing, and other areas. The popularity of the Perkin-Elmer 21 arose from its much greater ease of use and speed compared to its single-beam counterparts. The Model 21 also helped establish Perkin-Elmer as a leader in analytical instrumentation, and a "significant expansion" of the company was a direct result of the success of the Model 21 (McDonnell, 1980).

Sales of the Model 21 were high because of several factors. Foremost were the accessibility and ease of use offered by the Model 21. IR spectroscopy was suddenly unhindered by a need for trained spectroscopists and large blocks of time. With a PE 21, anyone with minimal training could be taught how to take infrared spectra. Despite its ease of use, the a major factor contributing to the success of the Model 21 was its flexibility. The design of the Model 21 included complex slit-control programs, wavelength speed controls, and accommodated different prism materials, making the instrument applicable to a variety of chemical problems. Finally, Perkin-Elmer realized that an instrument with no market was useless. Van Zandt Williams was convinced of the market for IR spectrophotometers, but knew that public knowledge of infrared spectroscopy was minimal. To remedy this problem, Perkin-Elmer created a series of short courses and meetings to educate chemists on the applications and operation of a infrared instrumentation.

While the introduction of the Perkin-Elmer 21 may not have had the impact on instrumentation of the Beckman DU, the introduction of infrared spectroscopy as a tool for non-spectroscopists began a gradual revolution in almost every area of chemistry. At its introduction, IR spectroscopy was used both as a qualitative and quantitative tool, but the advent of commercial gas chromatography (GC) shifted the focus of IR spectroscopy firmly to the qualitative (Miller, 1992). Today, infrared spectroscopy is used almost exclusively to ascertain functionality without precisely knowing concentration.

Early advertisements for the Model 21 stressed ease of use and speed, as shown by the sales brochure reproduced in Figure 11. Compared to the Model 12, which could easily use half of a day to take a spectrum, the speed of the Model 21 was almost miraculous. Equally stressed by Perkin-Elmer, Inc. was the line of accessories that could be used with the Model 21. A sales brochure reads, "Accessories extend its versatility: 5 different prism materialsÉ7 quick interchange assembliesÉgas cellsÉliquid cells, both micro and macro, high and low pressure,Évariable path lengthÉpolarizers, heatable cells, [and] reflectance." (Perkin-Elmer Corporation, 1953).

The Perkin-Elmer Model 21 made the foundational technique of infrared spectroscopy accessible to thousands of chemists and gave Perkin-Elmer, Inc. a national level for producing fine analytical instrumentation. Van Zandt Williams, a lifelong proponent of IR spectroscopy, was once asked why Perkin-Elmer did not make UV instruments. He replied by holding up his hand with thumb and forefinger about an inch apart and replied, "This is the UV wavelength region." He then spread his arms and said, "and this is the IR region. It is much broader and contains much more useful information" (Wilks, 1992). History has proven him right: What began as a tool for making rubber has become one of the essential tools of modern chemistry, and a descendant of the PE 12 or 21 can be found in almost every modern chemical laboratory.

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.

Griffiths, P.R. "Strong-men, Connes-men, and Block-busters or how Hertz raised the Mertz." Anal. Chem. 1992, 64(18), 868A-875A.

McDonnell, H. G., Jr. "The evolution of analytical instrumentation at Perkin-Elmer." Amer. Lab. 1980, 21, 95-101.

Miller, F. A. "The infrastructure of IR spectrometry: Reminisces of pioneers and early IR instruments." Anal. Chem. 1992, 64(17), 824A-831A.

Wilks, P.A., Jr. "The evolution of commercial IR spectrometers and the people who made it happen." Anal. Chem. 1992, 64(17), 833A-838A.

Perkin-Elmer Corporation. "Instruction Manual: Perkin-Elmer infrared Equipment: Volume 3B: Model 21 infrared spectrophotometer operating and maintenance instructions" (instrument manual). The Perkin-Elmer Corporation: Norwalk, CT. 1952.

Perkin-Elmer Corporation. "The Model 21 Double Beam Infrared Spectrophotometer: The standard instrument for infrared analysis" (sales brochure). The Perkin-Elmer Corporation: Norwalk, CT. 1953.