Atomic Emission Spectroscopy Index

Inductively Coupled Plasma Atomic Emission Spectroscopy

ICP - AES Introduction	Atomic Emission Spectroscopy with an ICP	Atomic Excitation and Spectral Line Production	Inductive Heating and Plasma Production	ICP - AES Classroom Application
Atomic Spectroscopy Atomic Spectroscopy in it broadest sense is the study of the interaction of electromagnetic radiation with matter. Some might argue that atomic spectroscopy was born in 1966 when Sir Issac Newton discovered that light can be separated into a continuous series of colors by passing light through a prism. He subsequently called this separation of light into its component colors a "spectrum" (3). Although he is not credited with its invention, Newton's experimentation with light essentially involved the creation of a very basic spectroscope - the fundamental tool used in atomic emission spectroscopy. Atomic Emission Spectroscopy Atomic emission spectroscopy is the study of the electromagnetic radiation emitted from matter (elements, compounds, molecules, etc...) when matter is excited by an excitation source. Atomic emission spectroscopy has its origins in the flame test in that a burning flame was one of the first excitation sources used to generate the emission of light from matter (2, 4). The flame test involves placing a sample to be tested into a burning flame and observing the light emitted from the sample. The burning flame is referred to as an excitation source because the energy being released from the chemical reaction occurring in the region of the flame is what causes the excitation of the atoms and/or molecules that are being introduced into the flame (the test sample). There are a number of different types of excitation sources used in atomic emission spectroscopy. The primary emission sources used in atomic emission spectroscopy today are the following (2): 1. AC or DC arcs 2. High, medium or low voltage sparks 3. Glow discharges with flat or hollow cathode 4. Inductively coupled plasmas 5. Lasers 6. Direct Current Plasmas 7. Capacitively coupled microwave plasmas 8. Microwave induced plasmas 9. Furnaces 10. Exploding wires or foils. Inductively coupled plasmas have grown in popularity in analytical spectroscopy because the inductively coupled plasma offers advantages over other excitation source options. Attempting to explain all of the advantages that ICP offers over each of the other excitation sources listed above would take away from the main focus of this site which is to explain how ICP - AES works, so it will not be attempted. A simple explanation as to why ICP is advantageous to use is that the design of the excitation source and the temperatures achieved in the plasma allow for a more efficient interaction between the sample being studied and the excitation source which leads to greater sensitivity, greater accuracy, greater precision, and greater atomic excitation. The basic sequence in the process of atomic emission spectroscopy is illustrated in Figure 1. Figure 1: Stepwise Sequence in Atomic Emission Spectroscopy Step 1: Atomic emission spectroscopy is used for the determination of the elemental composition of substances. The sample to be tested could come from any number of sources. Common applications of atomic emission spectroscopy are in the analysis of trace elements in soils, water, metals, biological specimens, clinical specimens, food, physical evidence such as glass, and other solids. Step 2: Each sample to be tested in ICP-AES must be prepared for analysis and this most commonly involves dissolving the sample into an aqueous or organic solvent. It is possible to analyze solid samples with ICP-AES and the technique used to do so depends on the solid being analyzed. Some of the techniques used in the analysis of solid samples include mechanical agitation of powders, arcs and sparks, and laser ablation. Step 3 & 4: When analyzing a liquid solution it is necessary to convert the solution into an aerosol before introducing the sample into the excitation source. This aids in the atomization process as well as increases the intensity of spectral emissions. The aerosol is produced immediately before the sample is forced into the excitation source such as a plasma for the efficient production of atoms and spectral emissions. Step 5: The sample typically enters the ICP in the form of a fine liquid aerosol. There are various stages involved in the formation of atoms. To illustrate the stages of atomization consider a solution of lithium chloride. Lithium chloride (LiCl) will enter the plasma and rapidly form a solid aerosol as illustrated below: The solid aerosol will then vaporize to form gaseous "molecules" of lithium chloride as illustrated below: The gaseous lithium chloride will then decompose into lithium and chlorine atoms as illustrated below. At this point it is possible for the lithium and chlorine atoms to become excited and/or ionized. Once the atoms become excited it is them possible for them to emit light. Step 6: Once the atoms are emitting light the light must be collected and detected. There are quite a variety of ways to accomplish this task and the basic system required to accomplish these goals involves an entrance slit; a collimator to produce a parallel beam of light; a dispersing element such as a diffraction grating or prism; a focusing element that reforms the specific dispersed bands of light; and one or more exit slits to isolate the desired spectral bands or band (Montaser and Golightly). Figure 2: Schematic Diagram of Collimator Producing Parallel Beam of Light Courtesy of Wikipedia Images A Paschen-Rung mount illustrated in Figure 3 below is presently the most commonly used optical configuration for ICP spectrometers (5). The Paschen-Rung mount is a diffraction grating mounting in which the entrance slit and the grating are fixed, and the photographic plates are clamped to a fixed track running along what is referred to as a Rowland Circle. Figure 3: Schematic Diagram of Paschen-Runge Mount Taken from Inductively Coupled Plasmas in Analytical Atomic Spectrometry 2nd Edition by A. Montasar & D.W. Golightly After passing through the exit slit the light is detected by the electronic components of the spectrophotometer. There are a number of different detection systems that can be used in the spectrophotometer such as the photo multiplier, vidicons, image dissectors, photodiode arrays (PDA), and charge-coupled and charge-injection detectors (CCD & CID). The photomultiplier contains a photoemissive cathode coated with an easily ionized material such as a cesium-antimony alloy (4). A photon striking the surface of the alloy will cause an electron to be emitted. This is essentially the basic premise behind the detector - converting the light emitted from the atoms into an electrical signal via the flow of electrons. The long wavelength detection limit of the photomultiplier is determined by the photon energy required to ionize the cathode in the tube. The short wavelength limit is determined by the material used to construct the light transmitting window of the photomultiplier tube. If quartz glass is used the shortest wavelength that can be detected is ~185 nm (5). Inductively Coupled Plasma Atomic Emission Spectrophotometer The instrument used in inductively coupled plasma atomic emission spectroscopy is the ICP spectrophotometer. The Environmental Science Department at the University of Pennsylvania have their very own ICP spectrophotometer various members of the department were kind enough to let me come in and photograph the instrument. James Bedison - a 3rd year PhD student at Penn - was kind enough to take some time out of his day to give me some background information on the instrument and explain how the ICP spectrophotometer pictured below works. Spectro-Genesis ICP Spectrophotometer at University of Pennsylvania The instrument used at the University of Pennsylvania is called the Spectro-Genesis ICP AES and has its own dedicated room. The room is small so I could only photograph the instrument as a whole from the doorway (what you see above is a side view of the instrument). In the picture above you can see the auto sampler in the foreground. This is where the analyst can line up trays of unknown samples to be analyzed as well as known reference standards. The auto sampler is controlled by the computer. The analyst must tell the computer how many samples are in the trays as well as configure the instrument so that it will recognize what it is that is being sampled. There are a number of parameters that must be set before the sample is introduced into the instrument. Once the instrument has been configured, the samples to be analyzed can be introduced to the ICP instrument. The auto sampler removes the liquid samples from the sample tray via a thin capillary tube. The auto sampler and tube are pictured below. ICP Auto Sampler The liquid sample is forced through a thin tube by a pneumatic pump into a spray chamber pictured below. The spray created in the spray chamber is forced up into the nebulizer where it is converted into an aerosol. The aerosol is introduced into the plasma via the glass plasma torch. The nebulizer used in the instrument at the University of Pennsylvania is a pneumatic concentric glass nebulizer. Sample Introduction Components of Spectro-Genesis ICP There are a number of different nebulizers available and the choice of nebulizers is made based on the type of sample being analyzed. Figure 6.1 below illustrates the methods of sample introduction into the ICP - AES (6) Figure 6.1: Sample Introduction Methods in ICP-AES Taken from Inductively Coupled Plasma Emission Spectroscopy-Part I, Chapter 5, by J. Broekart & P. Boumans Liquids are introduced into the plasma via the nebulizer. The purpose of using the nebulizer is to break the liquid sample into an aerosol so that the sample to analyzed is more rapidly analyzed. If the liquid sample were injected directly into the plasma the atomization process would not be as efficient and the sample may not have enough time to heat up to the point where atomization occurs. The aerosol created by the nebulizer is introduced to the plasma where the solvent is evaporated. The molecules comprising the solvent are thermally dissociated and the atoms comprising the molecules become partially ionized (6). Before a sample is analyzed the ICP-AES must be calibrated with standard solutions so that a zero emission reading can be obtained by the spectrophotometer. The Plasma Torch The picture below is the housing for the inductively coupled plasma torch. Spectro-Genesis ICP Plasma Torch Housing If you were to open up the housing and look inside you would see the plasma torch and the hollow, water cooled, copper tubing responsible for generating and sustaining the inductively coupled plasma. The is illustrated in the picture below. Spectro-Genesis ICP Torch This is the location of atomic excitation in the inductively coupled plasma atomic emission spectrophotometer. This is where the plasma is generated and where atomization and atomic excitation occurs. The next section describes the details of atomic excitation and spectral line production. <- BACK NEXT ->
References 1. Tetsuya, H. Masao, U., Haraguchi, H. Hsiech, C, Montasar, A. Fundamental Properties of Inductively Coupled Plasmas. In Inductively Coupled Plasmas in Analytical Atomic Spectrometry 2nd Edition; Montaser, A. and Golightly, D.W. Ed.;VCH Publishers Inc.: New York, 1992; pp 373-440. 2. Boumans, P. W. J. M. Introduction to Atomic Emission Spectrometery. In Inductively Coupled Plasma Emission Spectroscopy - Part I; Boumans P.W.J.M. Ed.;Wiley: New York, 1987; Vol. 90; pp 1-34. 3. MIT Spectroscopy. The Era of Classical Spectroscopy. The Spectral Nature of Light. http://web.mit.edu/spectroscopy/history/history-classical.html (accessed April 2008). 4. Metcalfe, E. Atomic Absorption and Emission Spectroscopy; Wiley: New York, 1991, Chapter 1. 5. Strasheim, A. and Montaser, A. Instrumentation fo Optical Emission Spectrometry. In Inductively Coupled Plasmas in Analytical Atomic Spectrometry 2nd Edition; Montaser, A. and Golightly, D.W. Ed.;VCH Publishers Inc.: New York, 1992; pp 109-185. 6. Broekaert, J. A. C. and Boumans, P. W. J. M. Sample Introduction Techniques in ICP-AES. In Inductively Coupled Plasma Emission Spectroscopy - Part I; Boumans P.W.J.M. Ed.;Wiley: New York, 1987; Vol. 90; pp 296 - 357. 7. Zumdahl, S. Chemical Principles 5th Edition; Houghton Mifflin: Boston, 2005, Chapter 12. 8. Electronic Structure and Atomic Orbitals. Jim Clark. www.chemguide.co.uk/basicorg/bonding/orbitals.html (accessed April 2008). 9. Ralchenko, Yu., Kramida, A.E., Reader, J., and NIST ASD Team (2008). NIST Atomic Spectra Database (version 3.1.5), [Online]. Available: http://physics.nist.gov/asd3 [2008, May 2]. National Institute of Standards and Technology, Gaithersburg, MD. NIST Atomic Spectra Database. Levels Data. http://physics.nist.gov/cgi-bin/ASD/energy1.pl (accessed April 2008). 10. Winge, R.K. et al. Inductively Coupled Plasma-Atomic Emission Spectroscopy: An Atlas of Spectral Information; Elsevier Science Publishers: New York, 1985, Appendix A-C. 11. Manning, T. and Grow, W. Inductively Coupled Plasma - Atomic Emission Spectroscopy. The Chemical Educator. [Online] 1997, vol 2, #1. http://www.mrl.ucsb.edu/mrl/centralfacilities/chemistry/icp.pdf (accessed April 2008). 12. Hyper Physics. Solenoid. Nave, R. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html (accessed April 2008).
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