Waters Symposium: Atomic Emission Spectroscopy
A Brief History of Atomic Emission Spectrochemical Analysis, 1666–1950 Richard F. Jarrell 173 Allen Ave., Waban, MA 02468-1734;
[email protected] Emission spectrochemical analysis is the oldest technique for performing a chemical analysis without the traditional chemical procedure. Foundations Isaac Newton was the first person to discover the true nature of white light. In 1666, Newton inserted a glass prism in a beam of sunlight that was shining into his darkened room through a hole in the shutter. This produced vivid and intense colors on a wall in the same order as in a rainbow. Newton used a mirror to reflect the colors back through the prism, producing white light again. He deduced that “white light” was made up of the spectral colors (1). Newton found that the “refrangibility” increased from red to violet. Newton believed that the light was transmitted by minute corpuscles (particles) traveling at high speed (2). Blue corpuscles were smaller than red and so were diverted more by a prism. In 1678 a Dutch physician, Christian Huygens, postulated that light was composed of minute waves in the atmosphere. Huygens’s wave theory of light eventually triumphed over Newton’s particle theory (3). Invention of the diffraction grating is ascribed to the American astronomer David Rittenhouse. In 1786 Rittenhouse made a “grating” of parallel hairs laid across two fine screws. When he looked through this at a small opening in the window shutter of a darkened room, he saw three images of approximately equal brightness and several others on either side “fainter and growing more faint, colored and indistinct, the further they were from the main line”. He noted that the blue light was bent more than red light and ascribed these effects to diffraction (4 ). Between 1814 and 1824 a German optician, Joseph Fraunhofer, built the first spectroscope by placing a prism before the eyepiece of a telescope to study the dim light of heavenly objects (5). Fraunhofer repeated Rittenhouse’s experiments with fine wire gratings. He placed two very fine wires on a framework of coarse wires. Next he carefully a
unwound one wire. When he placed this grating in a beam of white light it produced a spectrum, with blue light dispersed more than the red light (6 ). Later Fraunhofer produced reflection gratings by ruling grooves with a diamond point on a mirror surface. His finest grating was only 12 mm wide and contained 9600 grooves, but this enabled him to measure the wavelengths of light for the first time. Fraunhofer explained the phenomenon of diffracted orders and derived the grating equation (6 ) n λ = b(sin i ± sin r) where n is the order number and λ is the wavelength, b is the grating spacing in angstroms, and i and r are the angles of incidence and reflection. Spectrochemical Analysis Emission spectrochemical analysis dates from the work of the German physicist Gustav Kirchhoff and his collaborator, the German chemist Robert Bunsen. In 1860, they demonstrated to a group of geologists in Geneva, Switzerland, how they could identify useful elements such as iron, copper, and lead, or sodium and potassium, in potential ores, by the colors that powdered specimens sprinkled into a Bunsen burner flame produced. They could estimate concentrations by the intensities of the colors compared to those from chemically analyzed standards to within about ±40–50%, enough to decide whether a chemical analysis was worthwhile. Later, Kirchhoff and Bunsen built a visual spectroscope (Fig. 1) consisting of (a) an “entrance slit” of two narrow “jaws” separated by 1 mm or less, (b) a glass “collimating” lens to render the light parallel as it entered, (c) a glass prism, 60° × 60° × 60°, to form a spectrum, (d) a “camera lens” to focus the separated colors into “spectral lines” at (e) the focal plane, and (f ) a movable eyepiece to examine the lines more closely. In 1859, Kirchhoff and Bunsen, after studying the emission spectrum of a sample, discovered spectral lines that did not correspond to those of any known element. They postub
Figure 1. (a) Schematic of the Kirchhoff–Bunsen, 1860, 20-cm quartz spectrometer. (b) Line drawing of the Kirchhoff–Bunsen spectrometer (reverse of schematic, reproduced in J. Chem. Educ. 1956, 33, 20, from an illustration provided by the Deutsches Museum, Munich).
JChemEd.chem.wisc.edu • Vol. 77 No. 5 May 2000 • Journal of Chemical Education
573
Waters Symposium: Atomic Emission Spectroscopy
lated the existence of a new element, which they named “cesium”. A year later they discovered another new element, which they named “rubidium”. These discoveries were confirmed by older chemical techniques (7). Initially, the eye and brain could distinguish only visible light; that is, light between 4000 and 7000 angstroms. (An angstrom, symbol Å, is 10᎑10 meters and is named after the Swedish astronomer Anders Ångstrom, who measured the wavelengths of many spectral lines.) All traditional wavelength tables, including the M.I.T. Wavelength Tables and those produced by National Institute of Standards and Technology (NIST), utilize the angstrom because it provides a four-digit number, the minimum essential to identify the spectral lines of each individual element. For example, it was found that sodium had two very intense yellow lines close together. By the 1890s they were designated as 5890 Å and 5896 Å. Potassium had a red doublet, 7665 Å and 7699 Å. (Iron had a noted triplet at 3100.30 Å, 3100.67 Å, and 3100.90/97 Å in the ultraviolet, as described later.) In the 1870s and 1880s other “spectrochemists” in England and Continental Europe introduced the concept of the “internal standard line”, comparing the intensity of an analytical line to that of a nearby line of the major matrix element. The analytical line could be much weaker than, weaker than, equal to, more intense than, or much more intense than the internal standard line in an unknown specimen. This could be compared to the intensity ratio observed previously in a set of chemically analyzed standards. This improved analytical accuracy to between 25% and 40%. In 1882, H. A. Rowland built the first successful grating “ruling machine” at Johns Hopkins University. In 1883, he discovered that gratings ruled on concave aluminized spherical surfaces yielded spectra excelling in sharpness. This important discovery enabled wavelengths to be measured with an accuracy not previously possible (8). (Gratings ruled at Johns Hopkins became the standard used throughout the world through World War II.) Three developments in the 1890s made it easier to explore the spectral lines in the ultraviolet range: 1. Using quartz prisms and lenses or gratings instead of glass as a dispersant. 2. Using photographic plates as a recording mechanism. 3. Replacing Bunsen burners with dc arcs as source units. (The dc arcs produced relatively intense spectral lines on a weak background, making them useful for measuring low concentrations)
Professors and their students in the physics and chemistry departments at universities began building simple spectrographs to study the ultraviolet lines, using natural quartz crystals for prisms. But there was a problem because natural quartz occurs in two forms, dextrorotatory and levorotatory, and a prism of either produced close double images. So in the 1890s Cornu developed a spectrograph using a 30° × 60° × 90° dextrorotatory natural crystalline quartz prism and a 30° × 60° × 90° levorotatory quartz prism, adhesed along their 90° sides. This combined 60° prism canceled the optical rotation and yielded single, instead of the previous double, images at the focal plane, and produced lines from 2300 to 7000 Å.
574
R
2300 Å
UV 4000 Å
170 cm
Figure 2. 1910 Hilger Littrow quartz spectrograph.
Frank Twyman at Adam Hilger in London produced the world’s first commercial spectrograph in 1900. It was built in a mahogany housing for stability and had an adjustablewidth “entrance slit”, a 20-cm focal length quartz collimating lens, a Cornu quartz prism, a 20-cm quartz “camera” lens, and a plate holder for a 31⁄4 × 4-in. photographic plate. The instrument was useful for analyzing nonferrous materials, but the 20-cm focal length produced an unresolved continuum with specimens containing heavy metals such as iron. In 1906 Hilger produced a Cornu spectrograph with a 60-cm focal length. This spectrograph utilized a camera with 4 ×10-in. photographic plate. It was better, but still not good for complex spectra. In 1910 Hilger introduced a 170-cm focal length Littrow quartz spectrograph to analyze powdered specimens in graphite cups burned in dc arcs (Fig. 2). A Littrow mount used a 30° × 60° × 90° natural quartz prism with the light reflected back from the 90° side and a single quartz lens acting as a collimator and objective. Littrow had demonstrated that this arrangement: 1. Canceled the double images due to the two indices of refraction. 2. Provided three times greater dispersion.
The 170-cm focal length provided sufficient dispersion in the ultraviolet to resolve the spectral lines of iron and other heavy metals. For specimens with useful wavelengths above 4000 Å, a glass prism and lens substituted for the quartz system provided greater dispersion in the visible spectrum and permitted analysis of nonferrous elements. Until 1912 spectrographs were purchased by universities to identify and measure spectral lines of newly discovered elements. The first industrial application in the United States is credited to W. H. Bassett at Anaconda Research Laboratories in Waterbury, Connecticut (9). In 1912 Bassett used a 170-cm Hilger Littrow spectrograph with a spark source to analyze brass and other copper alloys and to control their composition. Sparks produced an alternating discharge at 10,000 to 15,000 V. They are useful for measuring higher concentrations because they are more repeatable, but the continuous background is higher so detection limits are poor. Between 1912 and 1920 other companies began manufacturing spectrographs with quartz prisms—Leitz and Zeiss in Germany, Bausch & Lomb and Gaertner in the United States, and Shimadzu in Japan. In 1920, Hilger produced the first evacuated spectrograph. This spectrograph utilized a Cornu-quartz prism and a 60-cm focal length. It was developed to analyze the vacuum ultraviolet lines of sulfur 1807 Å and phosphorous 1782 Å, important trace elements that must be held at low levels in steels.
Journal of Chemical Education • Vol. 77 No. 5 May 2000 • JChemEd.chem.wisc.edu
Waters Symposium: Atomic Emission Spectroscopy
By the 1920s and 1930s the wavelengths of spectra of most common elements had been exactly measured and classified into atom and singly ionized lines. Their energy levels had been identified and evaluated (10). Determining relative intensities was less satisfactory because most measurements were based on a visual estimate of the relative blackness of the photographed spectra (spectral lines in the red range were much blacker than lines in the ultraviolet range). In 1930, there was a major development in spectrochemical analysis when Kipp and Zonen, in Holland, produced a recording microphotometer capable of measuring repeatably the transmissions of narrow spectral lines on a photographic plate or film. The instrument scanned the plate slowly and recorded the output on a strip chart recorder. In 1932, Hilger introduced a less expensive and faster nonrecording microphotometer. Microphotometers improved analytical accuracy but increased the time required for analysis. Analysis required the following steps (11): 1. Measuring on each plate or film for each wavelength range the transmissions of the steps of an iron calibration line in an exposure, through a “stepped sector disk” or a “stepped neutral filter”. The disk was preferred because the relative transmissions were known and were the same at all wavelengths. The transmissions of the filter had to be measured at each significantly different wavelength. 2. Plotting the measured transmission of each step against the known relative intensity to produce a “characteristic curve” for each wavelength region. The longer wavelengths have steeper gammas. 3. Converting the measured transmissions of the line of the element to be determined and a nearby “internal standard line” of the matrix element and its relative intensities, using the characteristic curve. 4. Plotting Ia/Is against concentrations in chemically analyzed standards to produce a “calibration or analytical curve” for each element. 5. Analyzing an unknown sample by measuring the relative intensities in step 3 and converting these into concentrations from the analytical curves obtained in step 4.
The procedure improved accuracy to ±2–3%, but the process took a single operator about 30–45 minutes for an analysis of 10–20 elements. It was expensive to hold a furnace of steel or cast iron for 30–45 minutes while a routine spectrographic analysis was being conducted to control the composition of cast irons. Sawyer and Vincent, at the University of Michigan, developed the following assembly-line approach for the Ford Motor Company in 1939, utilizing a Bausch and Lomb Littrow Quartz Spectrograph with its plate holder mounted in the darkroom and a four-person team (12). Team member 1: received the specimen by pneumatic tube from the melt foreman, ground it, mounted it on the Petrey Stand, sparked it, and notified member 2 that the exposure was complete. Then he cleaned the stand for the next specimen.
Team member 2: in the darkroom, developed, fixed, washed and dried the photographic plate and delivered it back through the wall to team member 3. Then he loaded the plate holder for the next sample. Team member 3: measured the transmissions of the stepped filter exposures at several wavelengths and reported these to member 4. Then he measured the transmissions of the analytical lines and their internal standards, and reported these to member 4. Team member 4: used a calculating board to prepare characteristics curves at the required wavelengths, then used these to determine intensity ratios; then from the appropriate calibration curves, reported the corresponding elemental concentration back to the melt foreman. This procedure reduced analytical times to 10–18 min for a cast iron sample. By 1932, Medium and Littrow quartz spectrographs were being manufactured in the United States by Bausch and Lomb Optical Company and by Gaertner Scientific Company. They were protected from lower-cost European instruments by a 60% duty. In the late 1930s, U.S. manufacturers began producing spectrographs using gratings instead of prisms as a dispersing element. In 1937, Maurice Hasler at Applied Research Laboratories produced the first grating spectrograph (1.5 m, 7 Å/mm) for the Geological Survey of California. In 1938, Baird produced its first Eagle 3-m grating spectrograph having 15,000 grooves per inch and Jarrell Ash delivered its first 21foot Wadsworth grating spectrograph to the General Electric Company’s River Works in Lynn, Massachusetts, for quality control in the production of aircraft engines. The Jarrell Ash Wadsworth “stigmatic” spectrograph combined high resolution and dispersion (5.4 Å/mm with a wide wavelength range on two 4 × 10-in. photographic plates). This instrument was the largest grating spectrograph produced. It was designed to perform exacting measurements on a nickel, chromium, and iron super alloy for aircraft engines (13 ). It was necessary to keep volatile elements such as bismuth, cadmium, and arsenic at a parts-per-million level to prevent corrosion and failure of the engines. The high dispersion and wide wavelength range of a Wadsworth spectrograph permitted this kind of quality control (14 ). Spectrographs also played an important role during World War II. For example, Jarrell Ash spectrographs were used by International Nickel to ensure the quality of the nickel being shipped to GE for aircraft engines, and by the National Bureau of Standards, Union Carbide, and Velmer Fassel’s laboratory at Iowa State to analyze uranium for harmful trace elements. The NBS spectrograph was later used for most of the experimental work that was published as Tables of Spectral-line Intensities, Arranged by Elements (15 ). After World War II, universities that had supplied gratings were unable to keep up with demand, so manufacturers built their own ruling machines. Jarrell Ash built a machine that lasted for three decades before it was modernized. In 1950, Jarrell Ash produced its first replica grating (16 ). In the 1940s, professors and students at various universities experimented with replacing photographic plates in spectrographs with exit slits and photomultipliers. These experiments led to the development of three different types of
JChemEd.chem.wisc.edu • Vol. 77 No. 5 May 2000 • Journal of Chemical Education
575
Waters Symposium: Atomic Emission Spectroscopy
“direct readers” (17). Applied Research Laboratories produced the first commercial direct reader in 1947. The direct reader still had to be standardized frequently, but a single operator could report a complete analysis for an unknown in a few minutes with an accuracy to ±1–3%. Baird and Jarrell Ash introduced other types of direct readers. Literature Cited 1. Richtmyer, F. K. Introduction to Modern Physics; McGraw-Hill: New York, 1934; pp 36–40. 2. Wood, R. W. Physical Optics, 3rd ed.; MacMillan: New York, 1934; p 1. 3. Richtmyer, F. K. Op. cit., p 49. 4. Hutley, M. C. Diffraction Gratings; Academic: London, 1982; p 3. 5. Asimov, I. The History of Physics; Walker: New York, 1984; p 298. 6. Hutley, M. C. Op. cit.; p 4. 7. Asimov, I. Op. cit.; pp 298–299. 8. Wood, R. W. Op. cit.; p 260. 9. ASTM E-2 Committee. Five Decades of Optical Emission Spectrochemical Instrumentation; American Society for Testing and Materials: Conshohocken, PA, 1982; p 2. 10. Ibid.; p 3. 11. Ibid.; p 7. 12. Jarrell, R. F. A Brief History of the Development of Optical Emission Spectrochemical Analysis; Thermo Jarrell Ash, 1993; pp 4a–5.
576
13. Gustafson, M. Good Company: A History of Thermo Jarrell Ash and Spectroscopist Richard F. Jarrell; Thermo Jarrell Ash, 1998; p 7. 14. Ibid.; p 9. 15. Ibid.; pp 9–10. 16. Ibid.; pp 13–14. 17. Jarrell, R. F. Op. cit.; p 5.
Further Reading Asimov, I. The History of Physics; Walker: New York, 1984. ASTM E-2 Committee. Five Decades of Optical Emission Spectrochemical Instrumentation; American Society for Testing and Materials: Conshohocken, PA, 1982. Gustafson, M. Good Company: A History of Thermo Jarrell Ash and Spectroscopist Richard F. Jarrell; Thermo Jarrell Ash, 1998. Hutley, M. C. Diffraction Gratings; Academic: London, 1982. Jarrell, R. F. A Brief History of the Development of Optical Emission Spectrochemical Analysis; Thermo Jarrell Ash, 1993. Jenkins, F; White, H. Fundamentals of Physical Optics; McGrawHill: New York, 1937. Richtmyer, F. K. Introduction to Modern Physics; McGraw-Hill: New York, 1934. Robertson, J. K. Introduction to Physical Optics, 2nd ed.; Van Nostrand: New York, 1935. Slickers, K. Automatic Emission Spectroscopy, English Transl.; Applied Research Laboratories: Lausanne, 1980. Wood, R. W. Physical Optics, 3rd ed.; MacMillan: New York, 1934.
Journal of Chemical Education • Vol. 77 No. 5 May 2000 • JChemEd.chem.wisc.edu
