Physics: Spectroscopy

Introduction

Spectroscopy is the process of exposing a substance to some form of radiation, or gathering radiation emitted by the object, and then analyzing that radiation to gather information about the properties of the substance. Different kinds of radiation provide different kinds of information and are used to analyze different kinds of substances.

Spectroscopy's many guises have proved indispensable for research and analysis in all areas of chemistry as well as a variety of other scientific and technical areas. In analytical applications, spectroscopy can fingerprint (uniquely identify) a material by disclosing what elements it contains, and in what proportions. Usually it is not necessary to touch the object being studied, giving spectroscopy wide use in astronomy and astrophysics. At the same time, this remarkable scientific technique can also provide information on atoms and molecules and on even the tinier particles that make up the atom.

Historical Background and Scientific Foundations

Early Approaches

Although the fame of English physicist and mathematician Isaac Newton (1642–1727) rests on his contributions to calculus and the theory of gravitation, it was his work in optics that first made his reputation. In one of his early experiments, done in the mid-1660s, he cut a small hole in a window shade and held a glass prism in front of the beam of sunlight that streamed through it. Instead of a simple white spot on the opposite wall, he saw a narrow band of rainbow color—a spectrum.

Newton was not the first to produce a spectrum with a prism. Sixteen centuries earlier, Roman statesman and philosopher Seneca (4 BC–AD 65) observed that sunlight passed through an angular piece of glass revealed all the colors of the rainbow. But Seneca thought the glass had added the colors. Newton was the first to understand that the colors were contained within the light itself. He showed this by placing a second prism opposite the first and recombining the colors to produce white light.

The relationship of these facts to spectroscopy may be shown by a musical analogy. A person with a trained ear can listen to a chord (several notes played at once) and say immediately what notes it consists of. Each note in the chord can be compared with a color. If we mix several colors together, we can make another color, which we might call a color chord. But while the musical version is easily perceived using the human senses to be a mixture of specific notes, the color version is not: it just looks like a new color. The most experienced color expert can't tell what colors went into it simply by looking. What is needed is an instrument that has the capacity to analyze or separate out the different colors in the mixture (elements in the color's spectrum). The spectroscope is an instrument that does this job.

The Basic Optical Spectroscope

A spectroscope permits the visual observation of spectra. Developed in the late nineteenth century, the term derives from two root words: the Latin spectrum, meaning “image” or “appearance,” and the Greek skopein, “to view.”

In a simple spectroscope, light rays enter through a slit and are forced to be parallel by a collimating lens. A prism then disperses the light, which is then focused by a lens. In this system red is dispersed the least, violet most. Instruments that record spectral images on photographic plates or in computer memory, spectrographs, are found on most large telescopes.

If, instead of using an eyepiece or recording device, we use a photoelectric cell to measure how much of each color appears, the instrument is called a spectrometer (from the Greek metron, “measure”). In this case, however, the prism is rotated so that only a narrow portion of the spectrum is received by the detector at any time. In some cases, the whole operation is done automatically, and the results are fed to a recording spectrophotometer, which prints a complete record of the spectrum. Such devices are used widely in analytical labs to both identify and quantify substances in solution. The first spectrophotometers appeared in the 1870s and were used to measure hemoglobin and other substances in blood and urine.

The Science

Like all waves, light has both a frequency and a wavelength. Wavelength is the distance from one crest of a wave to the next. Frequency is the number of waves that pass a given point in a certain length of time.

The rates of vibration of visible light waves are all near 100 trillion (10 14) per second. Visible light's

wavelength is very small, in the range of 0.00002 inches (0.508 µm). When working with visible light, scientists use the nanometer (nm, 10 -9 m) or the angstrom (Å 10 -10 m). The wavelength of violet light, for example, ranges from about 380 to 440 nm, while red light ranges from about 620 to 760 nm.

Continuous Spectra and Dark Lines

Any substance will emit visible light when heated to a high enough temperature (if it does not evaporate first). For example, the heating coils in an electric heater, when heated to about 1,400°F (760°C), emit light containing a range of colors that combine into a reddish glow. The tungsten filament of an electric bulb incandesces at about 4,200°F (2,316°C), emitting an intense yellowish-white light. An electric arc, which is hotter still—about 6,300°F (3,482°C)—is even more intense and is bluish in color.

All substances at temperatures above absolute zero, in fact, emit electromagnetic radiation (light) at all times, but objects that have not been raised to the relatively high temperatures mentioned in the previous paragraph do not emit visible light. Rather, they emit infrared light—literally, “below-red” light, so-called because its frequencies are lower than that of the lowest-frequency red light visible to the human eye. Spectroscopy can be performed on infrared light, but substances are often heated for spectroscopy because they emit radiation across a wider range of frequencies at higher temperatures, supplying more information. Also, astronomical objects such as stars are generally at high temperatures by their very nature. On the other hand, heating to high temperature may break up the very molecules that the spectroscopist wishes to analyze (e.g., ink-soaked paper fibers in a criminal investigation). In these cases, the use of infrared spectroscopy is necessary.

When the light from a substance is passed through a spectroscope, the result is a continuous spectrum—a rainbow, in effect, whether it is a rainbow of visible colors or not. The detailed structure of this spectrum or rainbow—made apparent by using appropriate instruments—reveals much information about the substance that emitted the light. The insights gained from the unique qualities of heated substances are the basis for spectroscopy's explanatory power.

In 1802, well over a century after Newton's experiments, the English chemist and physicist William Hyde Wollaston (1766–1828) noticed that the sun's apparently continuous spectrum was crossed by a number of black lines, but he did not know what to make of them. About twelve years later, these dark lines were rediscovered by German physicist Joseph von Fraunhofer (1787–1826). Using better equipment than Wollaston's, he found that the spectrum was interrupted by “a large number of strong and weak lines.” He realized that, contrary to Wollaston's theory, these could not be divisions between colors, since some of them clustered near the centers of colors. He still didn't understand their cause, but he put them to work as landmarks or points of reference that could be used to describe precisely any desired part of the spectrum.

Diffraction Grating

What made this discovery possible was Fraunhofer's development of a different kind of dispersion device, the diffraction grating, a transparent or opaque plate

inscribed with thousands of fine lines per inch. These lines break up the light rays as a prism does, but much more evenly. The result is a spectrum that displays a definite geometrical relationship between the position of the spectral line and its corresponding wavelength (e.g., a separation of 1 nm per mm on a photographic plate). Grating spectroscopes made direct determinations of absolute wavelength possible. This proved particularly useful later in the century, when scientists began to use spectroscopy to unravel the properties of atoms and molecules.

Also, diffraction gratings allow much greater dispersion of light; that is, the wavelengths are spread further apart. This facilitates observation and was perhaps the most important factor in Fraunhofer's work. Prism spectroscopes are commonly used in schools and demonstrations because of their simplicity, low cost, and impressively bright spectra.

Introduction

Chemical Analysis

Fraunhofer had an even greater scientific discovery within his grasp, but did not perceive that under the proper circumstances each element produced its own characteristic set of lines and that these lines are as unique to it as fingerprints are to humans. That insight would be achieved by others.

Correlating spectral lines with distinct elements was finally accomplished by the German physicist Gustav R. Kirchhoff (1824–1887) and German chemist Robert W. Bunsen (inventor of the Bunsen burner, 1811–1899;) in 1859, with the spectroscope once again playing a major role. When gases are heated, the light they emit is quite different from that of a heated solid. If one introduces into the barely visible flame of a Bunsen burner a small amount of table salt, for instance, the flame turns a brilliant yellow. And if we look at this flame through a spectroscope, we do not see the usual rainbow-colored band, but a pair of closely spaced narrow yellow lines on a generally dark background. These bright emission lines, called D-lines, are produced by the sodium in the salt. Appearing at 588.99 nm and 589.59 nm, they are the same lines that Fraunhofer had seen.

In an related experiment, Kirchoff shone sunlight through a sodium flame and, using his spectroscope, observed two dark lines on a bright background exactly where the bright sodium lines were in the earlier experiment. He concluded that the gases in the sodium flame absorbed the D-line radiation from the sun, producing what is now known as an absorption spectrum. Kirchoff realized after further experiments that all the other Fraunhofer lines were also absorption lines. That is, gases in the sun's atmosphere absorbed some of the radiation coming from the sun's interior. He concluded from certain Fraunhofer lines in sunlight that sodium and calcium were present in the sun's atmosphere, while lithium, another alkali metal, appeared to be absent.

JOSEF VON FRAUNHOFER (1787–1826)

Josef von Fraunhofer (1787–1826) was born in Straubing, Bavaria, now part of Germany. The son of a poor glazier (a craftsman who makes and works with glass), Fraunhofer exemplified the ideal combination of artisan and mathematically inclined theoretician. He acquired this expertise mainly through apprenticeships with glass and mirror makers, combined with independent study. By 1809 he had become a manager in an optical firm in Munich; he was made director of the glassmaking section in 1811; and in 1819, thanks to his optical research, he was elected to the Bavarian Academy of Sciences.

While trying to produce an improved telescope for his firm, Fraunhofer came across the lines seen earlier by Wollaston. But where Wollaston had detected only seven lines, Fraunhofer found hundreds. Using his new equipment, he went on to measure and map the position of the most prominent lines, which he indicated by capital letters, still known as Fraunhofer lines.

He also performed the following experiment: He separated the opening of his spectroscope into two halves. One was lighted by the sun and the other with a sodium flame. A dark pair of lines in the sun's spectrum matched a brilliant pair of lines in the sodium flame.

Fraunhofer did not pursue what now seems such a significant match, and so the origin of the lines remained a puzzle. Nevertheless, his pioneering work with the spectroscope was of great importance. After his untimely death from tuberculosis at 39, his tombstone was engraved with Approximavit sidera, Latin for “He approached the stars.”

Carrying this line of reasoning further, Kirchoff and Bunsen compared the solar lines with the spectra of known elements and found a number present in the sun, the most abundant of which was hydrogen. Thus was born an exciting new form of chemical analysis. In 1868 the English astronomer Sir Joseph Norman Lockyer (1836–1920) found a group of lines in the sun's spectrum that did not match those of any element found on Earth. Lockyer correctly attributed his discovery to an unknown solar element, which he called helium (from the Greek helios, meaning “sun”). His fellow scientists dismissed the idea, which was hardly surprising, since spectroscopy was still a new science; it seemed presumptuous to posit the existence of a new element merely on the basis of some lines in a spectroscope.

It took almost 30 years for helium to be found on Earth, but Lockyer lived long enough to see himself proved right, and, as is often the case in science, his discovery had other ramifications. The helium line was a prominent one because the sun contains a good deal of helium. In other words, there is a relationship between the amount of an element in a substance and the density of its spectral lines. Very accurate instruments, called densitometers, can measure the density of these lines. Comparing these results leads to quantitative analysis, that is, the determination of how much of a substance is present in a material.

Putting Spectroscopy to Work

During this early period of spectroscopy, interest was largely confined to the chemical and physical properties of matter and with correlating and charting the lines of various elements. Some 30 new elements were discovered in this way. A few were even named for the brilliant spectral lines they provided: thallium (green), rubidium (red), cesium (blue), and indium (indigo).

The Electromagnetic Spectrum

Once the spectrum of visible light was understood, it became reasonable to ask, “What happens at the two ends of the visible spectrum?” In 1800 the English astronomer Sir William Herschel (1738–1822) performed a most intriguing experiment: He took the temperature of the visible spectrum by placing a sensitive thermometer in the colors of light spread out by a prism. He found that the temperature rose as the wavelength grew longer (toward the red); even more interestingly, it continued to rise beyond the red, where no color could be seen.

Although the real meaning of this experiment only became apparent later, Herschel was the first to find a relationship between light and radiant heat (the kind we get from the sun or an outdoor fire). Both are examples of electromagnetic radiation. These heat rays came to be called infrared, from the Latin infra, meaning “below”; their wavelength varies from the red end of the spectrum at 760 nm all the way up to 10 6 nm.

Only a year later the German physicist Johann Riter (1776–1810) found waves that were shorter than visible light. These were named ultraviolet (from the Latin ultra, for “beyond”) and range from the violet down to about 2.5 nm. As with visible light, there are no sharp divisions between the various forms.

Since then, many kinds of electromagnetic radiation have been observed, ranging from the tiny high-frequency gamma rays at about 0.1 nm to extremely long low-frequency radio waves that range up to 10 11 nm. Correlations between wavelengths and the matter that absorbs them make them useful as experimental tools to probe the properties of matter. Gamma rays, for instance, are absorbed by atomic nuclei, so they are used in nuclear physics; x rays are absorbed by the individual atoms of a gas or solid, so they are used to study atomic and solid-state phenomena, as well as for medical imaging; and wavelengths in the ultraviolet range (about 380 nm) are useful for studying viruses.

Astronomers use a variety of wavelengths for their studies, e.g., longer-wavelength studies are useful for probing the cooler structures in the universe, such as dark cold molecular clouds, while high-energy gamma-and x-ray instruments are used to study remnants from massive dying stars and accretion of matter around black holes.

The Atom

Beginning around 1890 the power of spectroscopy helped unlock the secrets of the atom itself. We know now that atoms consist of a nucleus made up of protons and neutrons, plus surrounding electrons. The line spectrum of a heavy element like iron has many lines and at first glance would suggest that they are randomly arranged. This was found not to be the case.

Ironically, the path to understanding the atom was opened by hydrogen, the simplest element's line spectrum. It showed relatively few lines, arranged in an ordered pattern. Careful study revealed mathematical relationships for the lines in all the elements; materials scientists used this knowledge to learn more about how electrons act inside their atoms.

Scientists knew that a substance can only give off a spectrum when it has been given energy in some way, as by the hot flame of the Bunsen burner or the electrical input of a neon lamp. The excited substance then “de-excites” itself, but in a very special way. Because electrons' energy can only drop to a set of specific energy levels, only discrete amounts of energy can be lost. This loss corresponds to the set of sharp lines that appear in a line spectrum. This stunning discovery led to the mathematical formulas that accurately predicted their positions.

In 1913 the Danish physicist Niels Bohr (1885–1962) used one of these formulas to deduce the first really useful picture of the atom. It's probably safe to say that most of the information that atomic scientists have obtained about the nature of the atom has been obtained using spectroscopy.

Band Spectra

In addition to continuous and line spectra, there is a third type: band spectra. High-powered instruments indicate that these bands actually consist of many narrow lines, numerous at the darker end and gradually fading away at the other. Band spectra are caused by changes in molecules, rather than individual atoms. In other words, by using spectroscopic techniques with other kinds of radiation, scientists have been able to learn about the remarkable flexibility of the molecular world—the vibrations, rotations, and other motions that make up the vast majority of the substances.

Because different molecules absorb energy at different wavelengths, different forms of radiation are used, depending on what kind of information is being sought. Infrared spectroscopy, for instance, is particularly useful for showing how the molecules in a sample vibrate; ultraviolet radiation, which has more energy, can provide information on the chemical bonds in a molecule.

Spectroscopy also refers to several fields in which electromagnetic radiation is not involved. Separation or dispersion spectroscopy separates a beam of particles by energy or some other property. Beta-ray (electron) spectroscopy, for example, measures the energies of electrons emitted from nuclei. Mass spectrometers sort charged particles by mass; mass spectrometry played an important part in the discovery of isotopes, which are used in medicine, industry, and various other branches of science and technology.

Other types of spectroscopy are fluorescence, microwave, magnetic, alpha scattering, activation analysis, Raman, and resonance. All differ, but utilize the same basic idea. For example, nuclear magnetic resonance spectroscopy, which depends on small energy transitions in molecules in a magnetic field, has become an important method of analysis in chemical laboratories.

Modern Cultural Connections

Various forms of spectroscopy have been basic to modern advances in science and technology, with all that the changes imply for cultural and economic activity. Geologists, including petroleum geologists, have found spectroscopy invaluable for studying earth and rock samples. Metallurgists use it to study the purity of metals and alloys. It has been used in medicine to determine the presence or lack of certain metals in blood. The spectroscope has been employed extensively in astronomy, where it provides information about the velocities, composition, magnetic characteristics, and temperatures of stars; the motions of stars orbiting in pairs; and so on. The twin rovers that were landed on Mars by NASA in 2004, and were still exploring that planet as of early 2008, are each equipped with two spectro meters to gather data on the composition of minerals on that planet, a miniature thermal-emission spectro meter that can image whole landscapes at once, and a Mossbaüer spectroscope for close-up work on individual objects.

The development of lasers in the 1960s had a powerful effect on the field, because the special qualities of laser light can greatly increase the resolution and sensitivity of conventional spectroscopic techniques. In the early 2000s, a 785-nm infrared laser was being tested for use in a spectroscopic system for possible rapid diagnosis of pancreatic cancer, for example.

In forensics (crime investigation), infrared spectroscopy advanced greatly from the early 1990s to the early 2000s with the growing availability of cheap optics and fast computers for data processing. Infrared spectroscopy is now routinely applied by forensics laboratories to drugs, paints, fibers, explosives, inks, document materials, blood, and soil. Materials from questionable documents, crime scenes, and many other sources are analyzed to determine their unique spectroscopic (and thus chemical) identities, producing hard scientific evidence that can stand up in court. In 1989, for example, an instant-win lottery scratch-card that appeared to be the winning card for a large sum of money was challenged because its number did not appear in computer records as a winning number. Infrared spectroscopy of the card proved that it was authentic, not a forgery—the materials of which it was made were the precise materials of all other authentic cards—and the winnings were paid out to the card's owner.

Primary Source Connection

In 2007, an Oklahoma teen gained national recognition for developing an accurate, low-cost spectrograph. Mary Masterman's invention may improve global access to inexpensive spectrograph technology and garnered her a coveted Intel Science Talent Search prize.

OKLA. TEEN WINS $100,000 SCIENCE PRIZE

OKLAHOMA CITY—A 17-year-old girl won a scholarship worth $100,000 for building an inexpensive yet accurate spectrograph that identifies the “fingerprints” of different molecules.

Mary Masterman, a senior at Westmoore High School in Oklahoma City, was named the winner Tuesday of the annual Intel Science Talent Search.

More than 1,700 high school seniors across the nation entered the contest, which is in its 66th year.

Spectrographs, which measure wave lengths, are used in research such as astronomy and medicine and in industry. For example, they can be used as a sensing device to look for explosives or drugs or to help determine how old an art work is through its pigments.

They can cost as much as $100,000, but Masterman's invention—made of lenses, a laser, aluminum tubing and a camera—cost less than $1,000, Intel said.

Masterman received the honor from Intel Corp. Chairman Craig Barrett during a banquet Tuesday night in Washington.

“It was a complete surprise,” Masterman said. “I wasn't expecting it.”

The 40 finalists spent the last week in Washington, where they exhibited their projects at the National Institute of Science and met government officials including Vice President Dick Cheney and U.S. Education Secretary Margaret Spellings.

Masterman said she has been interested in science “ever since I was little. I can't remember ever not being interested.” She credits her parents with encouraging her. She said she has not decided where she will attend college but would eventually like to become a physicist or chemist.

Among the former winners of the competition's top award are six Nobel Laureates, three National Medal of Science winners, 10 MacArthur Foundation Fellows and two Fields Medalists.

“You're not only dealing with the top young person in the science field in the country in Mary, but you're dealing with 40 finalists who are doing breaking-edge research in total,” said Brenda Musilli, Intel's director of education. “It's really something that's hard to imagine, how a young person like Mary could even achieve this level of capability at such a young age.”

evans, murr ay. “okla. teen wins $100,000 science prize.” a ssociated press newswire (march 14, 2007).

See Also Astronomy and Space Science: Pulsars, Quasars, and Distant Questions; Physics: Lasers.

bibliography

Books

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Duckworth, H.E., R.C. Barber, and V.S. Venkatasubramanian. 2nd ed. Mass Spectroscopy. Cambridge and New York: Cambridge University Press, 1986.

Hollas, J. Michael. Modern Spectroscopy. 4th ed. Hoboken, NJ: John Wiley & Sons, 2004.

Jackson, Myles W. Spectrum of Belief: Joseph von Fraunhofer and the Craft of Precision Optics. Cambridge, MA: MIT Press, 2000.

Jenkins, Reese V. “Fraunhofer, Joseph.” Dictionary of Scientific Biography, Vol. V. New York: Charles Scribner's Sons, 1972.

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Newton, Isaac. “A Letter of Mr. Isaac Newton.” Reprinted in Isaac Newton's Papers & Letters On Natural Philosophy. Edited by I. Bernard Cohen. Cambridge, MA: Harvard University Press, 1958.

Newton, Isaac. Opticks; or, A Treatise of the Reflections, Refractions, Inflections & Colours of Light. 4th ed. London, 1730. Reprinted. New York: Dover Publications, Inc., 1952, 1979.

Periodicals

Evans, Murray. “Okla. Teen Wins $100,000 Science Prize.” Associated Press Newswire (March 14, 2007).

Rinsler, M.G. “Spectroscopy, Colorimetry, and Biological Chemistry in the Nineteenth Century.” Journal of Clinical Pathology 34, no. 3 (March 1981): 287–291.

Wilkinson, T.J., et al. “Physics and Forensics Synchrotron Radiation and Infrared Spectroscopy Are Joining Forces to Fight Crime.” Physics World 15, part 3 (2002): 43–46.

Web Sites

College of Wooster (OH). “Spectroscopy. Introduction to Spectroscopy: What Is Spectroscopy?” http://www.wooster.edu/chemistry/is/brubaker/intro_spectroscopy.html (accessed April 15, 2008).

Imperial College, London. Department of Chemistry. “NMR Spectroscopy. Principles and Application.”http://www.ch.ic.ac.uk/local/organic/nmr.html (accessed March 28, 2008).

Spectroscopy Magazine. http://www.spectroscopymag.com/spectroscopy (accessed March 6, 2008).

University of California at Irvine, Department of History. “Spectroscopy and the Rise of Astrophysics” http://eee.uci.edu/clients/bjbecker/astrophysics.html (accessed March 28, 2008).

Hal Hellman