SPECTRAL ANALYSIS of ELEMENTS INTRODUCTION Every human being on Earth has a unique set of fingerprints. So far, no two humans have been found to have the exact same fingerprints, not even identical twins. Because fingerprints are so unique, doctors, police officers, and scientists can use them to identify people. For example, traces of fingerprints left at crime scenes are regularly used by the police to identify suspects.
In addition to actual fingerprints, there is a new type of identification tool called “DNA fingerprinting.” Modern technology allows scientists to produce a map of each human’s DNA which, like a fingerprint, is unique to every individual (with the exception of identical twins). DNA fingerprints can be used just like real fingerprints to identify suspects or victims of crimes.
During the early 19th century, scientists slowly discovered that like humans, each element has its own “fingerprint.” Of course, elements and atoms have no fingers, so what exactly is an element’s fingerprint and how do scientists see it? An element’s fingerprint is called its spectrum. A spectrum is a sequence of colors that appears when light passes through a glass prism. In fact, when you see a rainbow, you are really seeing a spectrum; instead of light passing through a prism, it passes through tiny droplets of water in the atmosphere.
Scientists used to think that a prism or a drop of water somehow added different colors to regular white light. Through a series of careful experiments, Sir Isaac Newton discovered that white light is really a combination of different colors of light. He concluded that all a prism does is separate the colors by refracting (or bending) them at different angles. Later, scientists found that this happened because each color has a different wavelength, frequency, and energy.
To see an element’s spectrum, or spectral fingerprint, it must be heated enough to make it glow. There are two good ways to do this: you can burn the element in a flame, or you can turn the element into a gas, seal it in a glass tube, and pass a large electrical current through the tube. Either way, as the element heats up, it gives off light that contains a variety of wavelengths. If that light passes through a prism, the different wavelengths are separated from each other. Since each wavelength has its own specific color, a spectrum of colors appears. Different elements are made up of different atoms, and different atoms give off light containing different wavelengths, so different elements will have different spectra (plural of spectrum). This important fact allows scientists to identify the composition of unknown substances or objects too distant to be directly sampled.
The process of studying the spectra of elements is called spectral analysis. Today, instead of using just a prism, scientists perform spectral analysis with a tool called a spectroscope. A spectroscope, which was invented in 1859 by Gustav Robert Kirchhoff and Robert Wilhelm Bunsen (of Bunsen burner fame), is like a telescope designed especially to look at spectra. It is composed of four main parts: a tube, a cover with a slit in it, an eyepiece with a round hole in it, and a diffraction grating. Light enters the slit on the cover and is focused into a narrow beam. The beam of light passes through the tube until it reaches the hole in the eyepiece. When the light reaches the hole, it passes through a diffraction grating. A diffraction grating is a very thin sheet of transparent plastic with thousands of straight, thin, parallel, closely and equally spaced grooves etched into it. As light passes through a diffraction grating, each of the different wavelengths it contains is refracted at a different angle. In other words, like a prism, a diffraction grating separates light into its component wavelengths and a spectrum of colors is formed. A compact disc (CD) is, in a way, a diffraction grating. Tiny grooves etched by laser on to the surface of the disc reflect different wavelengths of light at different angles, producing a visible rainbow of colors—a spectrum.
OBJECTIVES To use a spectroscope to
-identify samples of “unknown” elements
-determine the composition of various light sources
MATERIALS •one set of emission tubes:
A good collection of emission spectra. Includes H, He, Ne, Ar, and Kr but no Hg. (Also has N, O, S, Fe, Xe)
http://www.colorado.edu/physics/2000/quantumzone/ - “Pick an element from the menu to see its spectral signature.”
PROCEDURE 1. Observe: Use a spectroscope to observe the spectrum of one of the light emitting objects. Put your eye up to the end with the round opening and point the end with the slit toward the light emitting object. Make sure the slit is vertical, or pointing straight up and down. The object’s spectrum should appear inside the tube, to the far left and to the far right. If it does not, you may need to rotate the tube slightly, keeping the slit in the vertical position.
2. Record: Record your observations in the data section. In one of the boxes provided, draw, label, and color a diagram of the spectrum you observe. Be careful to accurately show the position, thickness, and color of each line in the spectrum. Be sure to indicate the name or “code” number of the object you are observing.
3. Analyze: a. Which element is which? Try to figure out the identities of the unknown elements. To do this, match the spectra you observed and recorded to one of the standard spectra on the websites. Record your conclusions on the analysis table. b. What are the known objects made of? Try to figure out the composition of the known objects. To do this, match the spectra you observed and recorded to one (or more) of the standard spectra on the websites. Record your conclusions on the analysis table.
4. Communicate: Answer each of the questions that appear at the end of the lab activity.
DATA Record your observations here. In each box, draw, label, and color a diagram of the spectrum you observe Object name:
ANALYSIS Try to figure out the identities of the unknown elements and the compositions of the light sources.
Record your conclusions in these analysis tables. Analysis Table 1: Unknown Elements
element atomic number
Analysis Table 2: Light Sources
Introduction (Use the reading to answer these questions.) 1. Explain how and why fingerprints can be used to identify humans.
2. What is a spectrum?
3. Sometimes, the spectrum of an element is called its “fingerprint.” a. Describe what an element’s spectral fingerprint is.
b. Compare and contrast an element’s spectral fingerprint to a human fingerprint.
4. a. What is spectral analysis? b. What do scientists use it for? c. Identify the tool used in spectral analysis.
5. a. Name the parts of a spectroscope. b. Identify the purpose of using a spectroscope.
6. a. Describe what diffraction grating is. b. Explain what a diffraction grating does.
Conclusions (Use your observations and analysis to answer these questions.) 1. a. Is it possible to determine the composition of an object without collecting a direct sample of it? _________________
b. If so, briefly explain how it can be done.
2. a. Do all of the elements have the same spectrum? ______ b. Do any of the elements have the same spectrum? ______
c. What does this tell you about those elements? ________________________________________________________
3. Use the periodic table to look up the atomic numbers of the elements you observed. Consider what an element’s atomic number tells you about the structure of the element’s atoms. a. Based on the periodic table and on your observations, is there a correlation between the spectrum of an element and the structure of the atoms that make up the element? _______ b. If there is a correlation, describe it. c. If there is a correlation, use it to make a prediction about other elements.
4. a. Do all of the light sources have the same spectrum? ___ b. Do any of the light sources have the same spectrum? ___
c. What does this tell you about those light sources? ______________________________________________________ut other light sources?
d. Based on this evidence, what could you infer about other light sources?
5. Not all of the spectral lines in the light sources match the spectral lines in the elements you observed. What do you think the “extra” spectral lines in some of light sources represent?
6. a. When you think about neon lights, like those used in signs, what color(s) come to mind? _______________________
b. What color(s) is (are) suggested by the spectrum (or “fingerprint”) of the element neon? _______________________
c. Based on this observation, what can you infer about the neon lights you have seen?
MISC STUFF… and sources
-hydrogen H 1
-helium He 2
-neon Ne 10
-argon Ar 18
-krypton Kr 36
-mercury Hg 80
Every now and then, on a warm, wet, and sunny day, you can see a rainbow reaching across the sky. The arc of the rainbow extends from side to side and includes a collection of bright colors: red, orange, yellow, green, blue, indigo (deep blue), and violet (purple). The name of a fictional character called “Roy G. Biv” is used as an abbreviation to help remember the names of the colors and the order in which they appear.
For years, people were mystified by rainbows. They did not know exactly what they were or what caused them.
His discoveries were so great, he was the first scientist to be knighted.
Newton was the first man ever to understand the mystery of rainbows. Until his time, people were aware of the fact that when light enters a prism, then you can see all the different colors exiting it. However, it was believed that the prism somehow colors the white light.
Newton's idea is drawn by himself in this picture. He put a second prism after the first one, and let only one color pass through it. Then, the color remained unaltered, disproving the common belief. He then suggested that the prism just divides the light and does not color it; that "light consists of rays differently refrangible".
Newton was a very esoteric man; this was the first discovery that he made known to the public, at the age of 29 (even though he did the experiment 6 years earlier). Yet many scientists disputed him, scientists like Christian Huygens and (his great rival) Robert Hooke.
Diffraction gratings are hand held gratings in which there are thousands of tiny slits, designed to help determine the wavelength of the light passing through it.
A series of thousands of microscopic lines drawn on a transparent surface. These lines are so tiny they divide light into a spectrum.
An optical device containing thousands of very fine parallel grooves which produce interference patterns in a way which separates out all the components of the light into a spectrum.
a device- usually made of glass, plastic or metal- with tiny parallel lines etched into its surface. The parallel lines cause different wavelengths of light to be separated by different angles of refraction or reflection, producing a spectrum.
an array of fine, equally spaced reflecting or transmitting lines, which diffracts light in a direction characteristic of the wavelength of the light
A series of closely spaced parallel slits or grooves that are used to separate colors of light by interference.
A device used to break light into its component wavelengths. It is usually composed of a material with tiny grooves cut into it which disperses the light as it passes through or bounces off the grating (depending on the type of grating). Physicists and astronomers often use diffraction gratings to study the nature of light. See also disperson, spectrum, electromagnetic spectrum
A device used to break light into its component wavelengths. It is usually composed of a material with tiny grooves cut into it which disperses the light as it passes through or bounces off the grating (depending on the type of grating). Physicists and astronomers often use diffraction gratings to study the nature of light. See also disperson, spectrum
An optical surface, either transmitting or reflecting, with several thousand equally spaced and parallel grooves ruled in it.
An array of fine, parallel, equally spaced grooves (“rulings”) on a reflecting or transparent substrate, which grooves result in diffractive and mutual interference effects that concentrate reflected or transmitted electromagnetic energy in discrete directions, called “orders,” or “spectral orders.” Note 1: The groove dimensions and spacings are on the order of the wavelength in question. In the optical regime, in which the use of diffraction gratings is most common, there are many hundreds, or thousands, of grooves per millimeter. Note 2: Order zero corresponds to direct transmission or specular reflection. Higher orders result in deviation of the incident beam from the direction predicted by geometric (ray) optics. With a normal angle of incidence, the angle θ, the deviation of the diffracted ray from the direction predicted by geometric optics, is given by
White light is composed of all the visible colors in the electromagnetic spectrum, a fact that can be easily proven through the use of a prism. As light passes through a prism, it is bent, or refracted, by the angles and plane faces of the prism and each wavelength of light is refracted by a slightly different amount. Violet has the highest frequency and is refracted the most. Red has the lowest frequency and is refracted the least. Because each color is refracted differently, each bends at a different angle, resulting in a fanning out and separation of white light into the colors of the spectrum.
Water droplets in the air can act in a manner similar to that of a prism, separating the colors of sunlight to produce a spectrum known as a rainbow. To be able to see a rainbow, you must be standing with the sun behind you. The sunlight shines into the water droplets in the air, bending as it moves from the air into the water, reflecting off the sides the drops, and bending again as it exits the drops. As a result, all of the colors in the white light of the sun separate into the individual bands of color characteristic of a rainbow.
http://micro.magnet.fsu.edu/optics/activities/teachers/prisms.html Newton argued that white light is really a mixture of many different types of rays, that the different types of rays are refracted at slightly different angles, and that each different type of ray is responsible for producing a given spectral color. A so-called crucial experiment confirmed the theory. Newton selected out of the spectrum a narrow band of light of one color. He sent it through a second prism and observed that no further elongation occurred. All the selected rays of one color were refracted at the same angle.
http://www.phy.hr/~dpaar/fizicari/xnewton.html shows the emission spectra you should observe
http://scidiv.bcc.ctc.edu/wv/spect/emission-flame-exp.html Spectroscope Lab
Spectroscopy, in physics and physical chemistry, the study of spectra (see Spectrum). The basis of spectroscopy is that each chemical element has its own characteristic spectrum (see Elements, Chemical). This fact was recognized in 1859 by the German scientists Gustav Robert Kirchhoff and Robert Wilhelm Bunsen. They developed the prism spectroscope in its modern form and applied it to chemical analysis. One of two principal spectroscope types, this instrument consists of a slit for admitting light from an external source, a group of lenses, a prism, and an eyepiece. Light that is to be analyzed passes through a collimating lens, which makes the light rays parallel, and the prism; then the image of the slit is focused at the eyepiece. One actually sees a series of images of the slit, each a different color, because the light has been separated into its component colors by the prism. The German scientists were the tint to recognize that characteristic color of light, or the spectra, are emitted and absorbed by particular elements.
http://www.eaglesci.com/8thscienceold/astro/spectroscopelab.asp If you give it enough thermal energy, any element can turn into a gas. If an element is in the state of a gas and receives enough energy, it will start to emit (give off) light waves. A scientist can use a spectroscope to separate the emitted light waves into a spectrum.
The photograph of the spectrum of a star, sorted by color across a plate, will reveal spectral lines upon close examination. The lines are produced by elements in a star at high temperature. These lines represent the chemical composition of the star. Each element has its own “fingerprint.” To analyze the spectra of stars, scientists collected spectra of all the known elements. If we compare the spectral lines of an unknown star with the spectral lines of elements, we can determine the chemical composition of the star. More recently, we have discovered not only the composition of the stars but also their temperatures, their rotational rate, and their relative motion with regard to Earth.
A star is basically an extremely hot and extremely large ball of gas. Different stars are made of different gases. So they emit different types of light waves. These light waves look almost the same to human eyes. However, a spectrometer can separate these light waves into different spectra.
Background (high level)
http://www.sciencelives.com/spectroscopy.html Chronology of Spectroscopy
1666 - Isaac Newton let sunlight pass through a prism, producing a spectrum of color.
1790 - Johann Wolfgang von Goethe said, "The idea of white light being composed of colored lights is quite inconceivable, mere twaddle, admirable for children in a go-cart."
1802 - William Wollaston, an English chemist, discovered that light from the sun did not form a perfect spectrum. The spectrum was slashed by dark lines.
1814 - Joseph Fraunhofer made one of the earliest studies of absorption lines. He discovered that the spectra of various stars had different black lines. He hypothesized that the dark lines were caused by the absence of certain wavelengths of light.
1872 - Henry Draper was the first researcher to successfully photograph the spectrum of a star. It was the star Vega. He went on to record the spectra of over eighty other stars.
1900 - Max Planck, a German physicist, theorized that electromagnetic radiation is emitted from an atom in certain quantities, called photons.
Experiments beginning in the 1800's indicated that the spectral emission lines were caused by the excitation of atoms of specific elements in specific elevated temperature ranges. Similar experiments showed that absorption lines were caused by radiation being absorbed by atoms of specific elements at specific temperatures.
Sodium, for example, has two prominent yellow lines (the so-called D lines) at 589.0 and 589.6 nm --- any sample that contains sodium (such as table salt) can be easily recognized using these pair of lines.