Atomic Emission Spectroscopy

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1 Background Atomic Emission Spectroscopy In the 1850s Robert Bunsen pioneered the development of flame emission spectroscopy for elemental analysis. He is credited with discovering the heavier alkali elements rubidium and cesium. This technique is still commonly used in qualitative flame-color tests, and atomic emission spectroscopy (AES) instruments 1 for quantitative analysis have been commercially available for decades. Flames can vaporize samples and reduce cations to elemental form. Electronic excitation of atoms is mainly thermal at flame temperatures of 1600 to 2800 K, which depends on the type of fuel and oxidizer. The relative population of excited states of atoms at a given equilibrium temperature may be estimated using the Boltzmann relation N upper N lower = g upper g lower e ΔE kb T where N is the number of atoms in the upper or lower energy level, g is the corresponding degeneracy, ΔE is the term-energy difference in cm -1, and k B is Boltzmann's constant (0.695 cm -1 /K). For example, the lowest excited states of atomic sodium are at about 17,000 cm -1 and have three times the degeneracy of the ground state, so in a methane-air flame at 1800 K the relative population of excited atoms is approx. 4 ppm. Other alkali elements also have rather low-lying excited states and likewise can be excited efficiently in flames. Highly excited states are usually not formed at the temperatures achieved in a methane-air flame. Because of their simple atomic structure, spectra of alkali elements are relatively easy to assign. When powdered alkali salts are introduced into a flame, the resulting strong emission is characteristically colored and readily identifies the element. Each element's emission is primarily due to the p s transition of the lone valence-shell electron. These strong transitions are often called the resonance lines of the respective elements. The difference in energy between the ground and first excited states of atoms generally decreases as the principle quantum number of the valence shell increases. As a result, in any group of the periodic table the resonance lines of the heavier elements usually appear at longer wavelengths. This observed trend supports the idea that penetration and shielding by electrons in s orbitals are less effective for larger shells. Since the late 1800s electric discharges or plasmas in various forms (arc, spark, glow) have been used to excite atomic emission spectra. More recently, micro plasmas formed using focused laser pulses (typical energy 1 to 10 mj/pulse, 100 fs to 10 ns duration) have proven to be convenient, spatially resolved, and efficient at exciting most elements. Laser-induced plasmas produce very high temperatures, and nonequilibrium conditions are usually present initially, which causes emission spectra to be broad and complex. Spectra of ions are often observed together with neutral atoms. Commercial laser-induced breakdown spectroscopy (LIBS) instruments often include time-gated detection of the emission with a time delay after the laser pulse (e.g. 20 µs) when emission spectra highlight just a few characteristic transitions for each element. LIBS plasmas may also formed in an inert atmosphere (e.g. He) to aid in simplifying the spectra. Compared with some other methods for elemental analysis such as ICP-AES 1 which use digested (i.e. dissolved) samples, LIBS requires practically no sample preparation and enables non-destructive, spatially selective testing of heterogeneous mixtures such as contaminated soils, pigments on art work, historical artifacts, or even in non-terrestrial locations (e.g. Mars rover). Spin-orbit coupling is a magnetic, relativistic interaction between the spin and orbital motion of each electron. The total electronic angular momentum is the vector sum of spin and orbital angular momenta: J = L + S. A given atomic term is denoted with the symbol 2S+1 L J and represents a group of J states having different energies. The value of 2S+1 is called the multiplicity (i.e. singlet, doublet, etc.) The appearance of multiplets in a spectrum is called fine structure. As atomic mass increases, spin-orbit coupling increases, causing larger splittings in the multiplets. For very heavy atoms (and especially on the right side of the periodic table), the splitting is so large that L and S are not good quantum numbers, and the atomic states are said to follow j - j coupling.

2 Ordinary one-photon electric-dipole transitions obey the following optical selection rules: Δn unrestricted, Δl = ±1, and Δs = 0. For alkali atoms, this simply means that all s p, p d, d f, etc. transitions are allowed, and all s s, p p, s d, etc. are not allowed. Likewise, singlet singlet or doublet doublet, etc. transitions are allowed, while singlet triplet, etc. are not allowed. These rules are relaxed as spin-orbit coupling increases causing l and s to be coupled. The fine structure in some alkali emission spectra may be resolved using a medium-resolution grating spectrograph. 2 Photomultiplier tubes (PMT s) have long been used as light detectors in UV/vis/NIR spectrometers. 3 Although PMT s are very sensitive, a spectrum must be recorded by scanning different wavelengths of the dispersed light across a slit in front of the PMT. This means that at any given time, most of the spectrum is not being detected. Many modern spectroscopic instruments have been built with charge-coupled device (CCD) detectors. CCDs are arrays of small semiconductor light sensors, also commonly used in digital cameras. When used as a multichannel spectral detector, a CCD array allows a wide range of wavelengths to be recorded at one time. This so called multiplex advantage is also exploited in FTIR and FTNMR instruments. 4 CCD control electronics usually enable variable exposure times (analogous to different shutter speeds on a camera) and digitize the output. In this experiment, we will use three different Ocean Optics fiber-optic multi-channel instruments; the USB4000 and UV-VIS4000 have a resolution of 1.5 nm and the HR4000 has a resolution of 0.2 nm. Before coming to lab, determine the term symbols for the ground and first excited states of the alkali atoms. List the possible values of J for each term and then compute the degeneracy for each of these J states. Draw a generic energy level diagram showing the resonance lines of alkali atoms. Experiment Part I: Flame Emission Spectroscopy With help from your instructor or TA, boot up OceanView or SpectraSuite software for the spectrometers and survey the menus to learn the operation. Connect the USB4000 and try setting the integration time to 200 ms, dynamic average to 1 sample, the boxcar average to 1 pixel, the wavelength range from 350 to 950 nm, and the intensity range up to 1000 counts. Attach the fiber probe to a lens and adjust the lens to collect light from 1 cm above the burner surface. Position the lens at least 5 cm from the burner to avoid overheating it. Molecular or atomic emission from the overhead lights (e.g. 546 nm) may be subtracted as part of the background. Light a methane-air flame on a clean Fisher burner. Identify the flame emission bands from CH molecules (431 nm) and C 2 molecules (460 to 520 nm). You may need to increase the integration time to observe the broad, structured band contours from these molecules. Introduce powdered salts into the flame one at time by shaking the sample powder inside its container and then quickly removing the lid near the air intake to the burner. Pause the acquisition to capture the spectrum when the atomic emission is nearly full scale. Identify new any new spectral lines and measure their wavelengths using the cursor. Zoom in and note the width of each recorded line. Reduce the integration time if necessary and make sure to keep the peaks on scale so that line broadening and splittings are not overlooked. When the signal-to-noise ratio is good (> 100), line positions can usually be measured with a precision approximately 10 % of the width. For example, a single line measured with the USB4000 might be recorded as ± 0.1 nm (0.2 % relative uncertainty). Use the HR4000 to study emission features at higher resolution within its operating range (520 to 720 nm). Convert your measured wavelengths in nm units to wavenumbers in cm -1 units. Pay attention to precision. Determine the spin-orbit splitting for each atom. Use the average energy of the spin-orbit states to calculate the difference in energy between the ground and excited electronic terms for each atom. Prepare to answer the questions and complete the table in the brief report below. Experiment Part II: Laser-Induced Breakdown Spectroscopy (LIBS) Go to the laser spectroscopy lab where an Ocean Optics UV-VIS array spectrometer is set up near a Q- switched Nd:YAG laser. The second-harmonic output of this laser produces 10 ns pulses with energies of 1 to 10 mj/pulse at a repetition rate of 10 or 20 pulses per second. Unlike in a commercial LIBS 2

3 instrument, the timing of this laser and spectrometer is not synchronized. A lens focuses the laser beam onto sample targets that you select. A 532-nm mirror serves as a filter to prevent most of the scattered laser light from reaching the collection lens for the spectrometer; unfortunately this also blocks any emission lines between approximately 500 and 560 nm, which are useful for a few elements. SAFETY GLASSES MUST BE WORN AT ALL TIMES WHEN THE LASER IS OPERATING DURING THIS EXPERIMENT. Launch OceanView or SpectraSuite software for the spectrometer and set the integration time to 500 ms. Set the wavelength range to display from 200 to 600 nm and the intensity scale to a maximum of 10,000 counts. Record and subtract the background, which may include atomic and molecular emission bands from the overhead lights. Position a beam block in the path of the laser beam and turn on the laser at low power. Mount a sample and adjust its position using the laser beam. Switch the laser to high power and optimize the angle of the collection lens with the fiber optic cable. Pause the spectrum acquisition when a well resolved emission pattern is obtained, then block the laser beam and set it back to low power. Use the cursor to measure and record the most prominent emission lines for your sample. Save the spectrum for later reference. Repeat data collection with at least five other metal samples chosen from among Zn, Cd, Cr, Cu, Ag, Au, Al, or In. Obtain reference data on atomic energy levels 5, 6 and transition wavelengths ("lines") for the elements listed above. Traditionally, levels are tabulated starting with the ground state (term energy = 0 cm -1 ) for each element with the "spectrum" of the neutral species listed first (e.g. Cu I), followed by the singly ionized form (Cu II), doubly ionized (Cu III), etc. Note that these numerals are not the oxidation numbers. Assign upper and lower states for at least two observed transitions for four different elements. Consider selection rules to check that your assignments are actually allowed. Complete the table and answer the questions about LIBS in the brief report below. References 1. D. A. Skoog, F. J. Holler, and T. A. Nieman, "Principles of Instrumental Analysis", Saunders College Pub., Orlando, Fla., C. W. Garland, J. W. Nibler, and D. P. Shoemaker, "Experiments in Physical Chemistry", 8 th Edition, McGraw-Hill, New York, ibid., pp ibid., pp Charlotte E. Moore, Atomic Energy Levels as Derived from the Analyses of Optical Spectra, U.S. National Bureau of Standards, Washington, D.C., National Institute for Standards and Technology (NIST) Atomic Spectral Database (ASD), a free internet resource. SAFETY ISSUES An open flame is a fire hazard. Do not allow flammable materials such as clothing near the flame. Do not leave the flame unattended. Use proper gas flow rates to avoid flashback. Pulsed Nd:YAG lasers produce Class IV beams capable of skin damage and permanent blindness. A qualified laser operator must be present during this experiment. NEVER work without appropriate eye protection. Remove reflective jewelry, watches, etc. to prevent unexpected beam reflections. The focal region of the laser beam is particularly hazardous. 3

4 Brief Report: Atomic Emission Spectroscopy Name: Partner(s): Date: 1. Complete the following table to summarize your flame emission data. Pay attention to precision. element Li observed wavelength (nm) transition wavenumber (cm -1 ) spin-orbit splitting (cm -1 ) Na K Rb Cs 2. Do any of the above transition wavenumbers or spin-orbit splittings not follow the predicted general periodic trends? Explain. 3. Consider the structure of potassium and the excited state for the transition you observed. Using electric-dipole selection rules, predict the valence configuration and term symbol for the next higher excited state that can emit light in a transition to the ground state. 4. The aufbau principle (with some noteworthy exceptions) allows us to predict the ground-state electron configurations for all of the elements. List two elements not from group IA whose ground states have the same term symbol as the alkali elements. and 5. Assign J quantum numbers to the lines in each doublet. For each alkali element, why is the resonance line at lower wavenumber generally less intense?

5 6. Draw a generic energy-level diagram for the resonance lines of the alkali elements: 7. Complete the following table summarizing your LIBS assignments: upper lower transition lit. obs. spectrum config. term J config. term J ν / cm -1 λ / nm λ / nm 2

6 8. Do any of your LIBS assignments violate selection rules? Explain. 9. Suppose that we could synchronize the laser pulses with a variable-delay shutter for the spectrometer; explain how this would eliminate the need for the filter and thus enable emission wavelengths near 532 nm to be observed: 10. Of the elements you studied using LIBS, identify two from the same group of the periodic table, and compare the spin-orbit splitting for levels in the valence shells. Is this the expected trend? 11. What is the highest energy level you observed using LIBS? term symbol cm -1 element: Assuming thermal equilibrium and a population ratio of 1 ppm for this excited state relative to the ground state at 0 cm -1, estimate the plasma temperature using Boltzmann's relation. (show calculation) K 3

7 TA s Instructions Atomic Emission Spectroscopy 1. Prepare a few grams of dry, finely ground powders each of the following salts in small screw-top vials: a) LiCl b) NaCl c) KCl d) RbNO 3 e) CsNO 3 2. Locate and test a clean (unused) Fisher burner and matches or a striker. 3. Set up Ocean Optics USB4000 and HR4000 spectrometers with lens/fiber-optic collection. Connect to a laptop running OceanView or SpectraSuite. Test each alkali sample for good emission intensity (>10,000 counts) in a flame. Store the samples in a desiccator to keep them dry. 4. Obtain metallic samples (wire, foil, rod, turnings, random objects) of Mg, Ca, Cr, Cu, Ag, Al, and In. Place these in a tray in the laser spectroscopy lab. 5. Set up an Ocean Optics UV-VIS4000 spectrometers with UV lens/fiber-optic collection in the laser spectroscopy lab. Connect to a laptop running OceanView or SpectraSuite. 6. Turn off all equipment when students are finished each day.