The Early History of Spectroscopy

Nicholas C. Thomas Auburn University at Montgomery, Montgomery, AL 361 17

The techniques of visible, ultraviolet (UV), infrared (IR), and nuclear magnetic resonance (NMR) spectrometry are familiar to all chemists. They are, in fact, all analytical tech- niuues that measure both auantitativelv and uualitativelv the interaction of electroma&etic radiation with matter. 1; each case exnerimental data is presented as a plot or spec- trum of emitted or absorbed energy versus thi energyfre- quency, or wavelength of the radiation. Although mass spec- trometry does not involve a similar interaction between radi- ant enerm and matter, data is nonetheless presented in the same manner, and it is therefore also described as a spectro- scopic technique.

since the iitroduction of commercial spectrometers earli- er this century, the interpretation of spectra has provided fundamental information of atomic and molecular levels, moLculargeumetrJ., chemical bonding, and the mechanisms of chemical reactions (I). At the practical level, spectral analysis has been an invaluable technique for synthetic chemists allowing the characterization and analysis of new compounds. While most chemists and students of chemistry are familiar with the theories, practical applications, and interpretation of spectrosropic data, rarely is the historical development of spectroscopic techniques discussed. This ar- ticle examines the early development of spectroscopy from its origins in the 17th century through to the commercializa- tion a i d widespread applic&on of the four main spectro- scopic techniques earlier this century.

The Development ot Vlslble and Ultraviolet Spectrometry For thousands of vears n e o ~ l e marveled a t the multicol-

ored arc of visible light t h i t f;equently appeared after rain showers. This rainbow effect was also observed as unwanted color fringes in the early telescopes of the 17th century, but the colors in both phenomena remained unexplained curios- ities. In 1666 Sir Isaac Newton tackled the problem by studying the nature of light itself, hoping to determine the origin of the colors. Newton allowed sunlight to enter a small hole in a window shutter and passed it through a glass prism. He observed that the white light from the sun was separated into a regular series of colors, a spectrum, which he projected onto a screen. Bv as sine the individual colors throueh a prism in reverse p o h m i e observed the restoration oi the white light. From these exoeriments Newton concluded that white l g h t was actually composed of colors and that each one was bent differentlv as i t passed throunh the nrism (2). Newton's simple experiments-marked the beginning of the science of spectroscopy.

In 1802 William Wollaston (3) improved Newton's proce- dure by using a narrow slit instead of a round aperture and produced a series of visible spectral lines, each one an image of the slit and representing a different color of the visible spectrum (4). Wollaston also observed that this continuous line spectrum from sunlight was irregularly interrupted by a number of dark lines that were narallel to the slit. About a decade later a German optician, Joseph von Fraunhofer, studied these dark lines in more detail (5). Usine a convex lens between the slit and the prism, hd obtain2 a better defined series of images. To make more precise measure- ments of the angular positions of the lines, he connected a telescope to view the spectrum and by so doing developed

the spectroscope. Using this instrument, Fraunhofer ob- served over 500 dark lines in the solar soectrum and assiened the stronger ones the letters Ato H, A geing in the red and H in the violet renion. He also examined light from bright stars noting that thgpatterns of lines were dii'ferent fromihose of thesun. Usinga crudediffractiongrating by stretch~nn wires between the threads of two screws, Fraunhofer was a&ually able to measure the wavelength of the stronger D lines. In 1868 Anders J. Angstrom measured the wavelengths of about 1,000 Fraunhofer lines (6) and expressed them in units of 10-'0 m-a unit that now bears his name. Some time was to elapse however before a satisfactory explanation to the origin of the dark lines was proposed.

In addition to analyzing solar and stellar light, terrestrial sources of lieht such as flames were soon examined with Fraunhofer's spectroscope, and these were found also to emit bright spectral lines. In 1822 the Britishastronomer Sir John Herschel studied the visible spectra of colored flames and noted: "The colours thus contributed by different 011-

jects to flame afford in many cases a ready &d neat way of detecting extremely minute quantities of them" (7). This observation laid the foundation for spectral analysis, which would be further developed 40 years later by Kirchhoff and Bunsen (8).

In 1859 Gustav Kirchhoff proposed a theory of absorption and emission that attempted to explain the so-called Fraun- hofer lines in the sun's continuous spectrum (9). Kirchhoffs theorv stated that anv substance that was a eood light emit- .. ~ ,. ter oisome particul& wavelength would also absorh light of thesame waveleneth. Heconcluded that thedark lines in the - solar spectrum were caused by the cooler outer layer of gases in the sun's atmosphereabsorbing from the continuous spec- trum only those lines that these gases would emit if excited. Armed with this theory and the fact that each atom and molecule produced a unique and characteristic line spec- trum, Kirchhoff was able to exploit the enormous potential of visible spectrometry in chemical analysis. For indtance, in 1861, while studying the visihle spectra of the alkali metals, Kirchhoff and Bunsen discovered the new metals cesium and rubidium after observing new colored lines in the line spectra (10). In the same year Sir William Crookes discov- ered thallium when he noticed its characteristic green spec- tral lines. All three elements were named for the colors ob- served in their linespectra (11). A few years later the French astronomer PierreJules-Cesar Janssen visited India to ob- serve atotal eclipse of thesun. With the aid of aspectroscope he observed a new yellow line in the solar spectrum. The English astronomer Sir Norman Lockyer and chemist Ed- ward Frankland later attributed this line to a new element that they named helium (Gk. helios, meaning "the sun") (12). Thus, with the aid of spectroscopy an element had been discovered in another part of the solar system before being detected on earth.

Using a Bunsen burner flame to examine the visihle emis- sion spectra of elements was a limited technique since the heat of aBunsen flame could vaporize only a limited number of elements. However, with the development of the electric arc lamp, with temperatures of over 5000 OC. the atomic spectrabf all elements could be studied. ~ o r ' e x a m ~ l e , in 1885 J. J. Balmer observed the spectrum of hydrogen, the

Volume 68 Number 6 August 1991 631

spectra for over 50 compounds and correlated absomtion lightest element, which showed the simplest visible line spectrum with emission wavelengths a t 6563, 4861, 4341, 4102 and 3970 A. These early spectroscopic experiments clearlv showed that atoms did not emit a continuous distri- "~ ~

bution of frequencies. Rather, they emitted light a t discrete freouencies that were characteristic for each atom. The idea thaf, atomic spectra could provide a key to the structure of the atom and that electrons were somehow responsible for producing the emitted light had become ge~terally accepted, hut as vet the precise connection between the two had not been made. As a result of theoretical calculations by Balmer (13) and subseauentlv J. Rvdhera (14). a mathematical rela- tionship was d&elo&d th& accounted for these lines in the visible emission spectrum of hydrogen. Subsequently, in 1913, Niels Bohr (15) initiated anew era inspectial interpre- tation by linking line spectra to the quantum ideas recently proposed by Planck and Einstein. Put simply (163, Bohr proposed that electrons exist in states of constant energy and only change energy by undergoing a transition from one state to another. During the transition they either absorb or emit an amount of energy that is exactly equal to the energy difference between the two states. This behavior produced the characteristic soectral lines.

The significanci of visible emission spectroscopy as a method to study the transitions of outer electrons in atoms and molecules provided a convenient tool to examine the electronic structure of matter. Like emission spectra, ab- sorption spectra were classified as either continuous or line spectra and were also used to identify elements or study electronic transitions. Aueust Beer. a German Dhvsicist and professor of mathematicsat the university of '~onn, recog- nized the relationship between the absorption of light and concentration (17). One of the first instruments to use the absorption of light to determine concentration was the color comparator (18), which relied on Beer's law. The user visual- ly compared the transmitted light from the sample and a standard solution and adjusted the path length until the transmitted light from both solutions appeared to have the same intensit;. Eventually photodetect& replaced the in- accurate human ere, and in the 1930's a new instrument called the wlorimeter or spectrophotometer was developed that used a grating or prism to isolate a specific wavelength for ahsorotion sn&trd analvsis (18).

Like vkihle ~ i ' ~ h t , the emikon and absorption of UVradi- ation has provided useful information on electronic interac- tions. UV radiation was first detected in 1801 by the German ~hvsicist J. W. Ritter (19). Ritter was studvine the effect of the visible spectrum on the light-sensitivecompound silver chloride. which turned dark when exposed to lieht. He passed white light through a prism to produce the"various colors and irradiated a sample of silver chloride with each specific color. He ohserved that violet light caused the most dramaticdarkeningof the salt. Hut when Hitter placed some silver chloride in the lightless space just beyond the violet region, the compound was darkened even more. Ritter there- foreextended theelectromagneticspectrum beyond the visi- ble region, and this new invisible part of the solar spectrum was called the ultraviolet meanine "bevond the violet". The - first combined UV-visible absorption spectrometer, the Cary 11, was produced by Varian in 1947 (20).

The Development ol Infrared Spectrometry The discovery of the IR region of the electromagnetic

spectrum was made by Herschel in 1800 (21). Using a glass prism with blackened thermometers, Herschel detected the existence of radiant heat beyond the visible region near the red end of the solar spectrum. However, since Herschel's main interest was astronomy, he did not investigate this phenomenon further, and nearly a century elapsed before interest in the infrared region arose again.

In 1882 Ahney and Festing (22) obtained IR absorption

hands with the presence of certain organic groups in the molecules. Using sodium chloride plates, Julius (23) record- ed the spectra of 20 organic compounds and noted that methyl groups absorbed at characteristic wavelengths. By the turn~of the century interest in IR spectroscopy &as rap- idly growing. Beginning in 1903, W. W. Coblentz conducted a series of measurements over several years, during which time he studied the IR spectra of hundreds of organic and inorganic compounds (24). However, early workers studying IR encountered many experimental problems. They had to desien. construct, and calibrate their own instruments and com~onents, and measurements were generally recorded at night to minimize the effect of vibrations on the sensitive instruments. In addition, it took 3 4 hours to record a single spectrum. Due to the difficulty in measuring IR spectra, chemical aoulicationa were verv limited until the 1940's.

During ice late 1930's, two-men who shared a common interest in astronomy decided to establish a company in the United States that specialized in optics. To finance the pro- ject, Richard S. Perkin, a 30-year-old investment hanker, raised $15,000 from relatives, and his partner Charles W. Elmer. a court stenoera~her who was ~ lann ine to retire. contributed $5,000. 'I&e;her, on April 19, 1937. ;hey found: ed the Perkin-Elmer corporation. Initiallv thev ooerated an optics design and cons;lting business from a small New York office and within a year began fabricating precision optical components in Jersey City, New Jersey. During World War I1 the US. government was interested in produc- ing synthetic rubber by the polymerization of butadiene. This process required the analysis of Ca hydrocarbon iso- mers for which there was no commonly accepted method of analysis. The government offered support to two industrial research labs to desien such an instrument: thev were the Shell Development company in California and the Cyana- mid Com~anv in Connecticut. Cvanamid convinced the tinv perkin-Elmer optical shop to c o h r u c t optical elements idr a prototype IR. The instrument Perkin-Elmer huilt (2.5) was one of the first operating infrared spectrometers, the Model 12. At about the same time the Beckman Company, through a similar arrangement with Shell, developed their Model IR- 1 instrument. Once available commercially, these and successive instruments greatly enhanced the popularity of the IR technique, which, with the exception of optical iso- mers, was able to provide a unique "fingerprint" for any molecule.

The Development of Mass Spectrometry Around 1858 attempts to pass an electric current throueh

a vacuum led to the-discovery of cathode rays by ~ u l & s Plucker (26). Toward the end of the century these rays were found to be composed of fast-moving negative charges called electrons. In 1886 Eugen Goldstein conducted similar ex- periments with evacuated discharge tubes, but he used a perforatedcathode (27). In addition toseeina the usual cath- ode rays, Goldstein observed a second beam that passed through the perforated cathode and traveled onward in a straight line to the walls of the discharge tube. A few years later Wilhelm Wein observed that, like cathode rays, these new rays were also deflected by magnetic and electric fields but that the direction of deflection was opposite to that of the cathode rays, indicatina the new ravs were comnosed of positively charged particle; (28). ~ h e l e positive ions were formed when cathode raw encountered and ionized residual gas molecules in the discharge tube.

Beginning around 1907, J. J. Thomson studied the posi- tive rays described above. By 1912 he had constructed a positive ray analyzer, or parabola mass spectrograph, which was the forerunner of today's mass spectrometer (29). In Thomson's apparatus, a beam of positive ions emerging through the cathode was passed through an analyzer consist-

632 Journal of Chemical Education

21. Herschel, W. Phil. Transact. Roy. Soe. l8W.90. 284. 22. Abnory. W.deW.: Fertine. E. R.Phl1. Tronx. 18SZ.172.887.

28. Wein, W. Vrrhonol. Phyrik CPI. 1898,17. 29. Thornson, J. J. Rays o/ Posiriue Elacfrieiiy and Their Application to c h a m i d

Analysis; Longrnsns, Green: London, 1913. 30. M a w . J. R. T h e M m Spsclromofsr; Wykeham: London, 1977: p4. 31. Encycloprdio Riitonniro, 15th ed.; 1987;vol. 13, p 599. 32. Art0n.F. W. Phil. Mag. 1919.38.709. 33. Aston, F W. I8OIopepe;Arnold: &dm. 192%

34. Aston. F. W. Mars Speclro and Isotopes; Arnold: London, 1933. BS nemnat~r. A Phv* Rsll 191R l i 316

inaton. DC, 1982.

634 Journal of Chemical Education