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FIU CHM 4130 - CHAPTER 10_Xiao_AES_2018

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Chapter 10: Atomic Emission Spectroscopy Using Plasmas, Arcs or SparksEmission'of'radia,on'upon'relaxa,on'from'excited'state'2 / VOL. 2, NO. 1 ISSN 1430-4171THE CHEMICAL EDUCATOR http://journals.springer-ny.com/chedr© 1997 SPRINGER-VERLAG NEW YORK, INC. S 1430-4171 (97)01103-5EXCITED STATEGROUND STATE h!FIGURE 1. EMISSION OF RADIATION UPON RELAXATION FROM AN EXCITED STATE.As illustrated in Figure 1, atoms emit electromagnetic radiation (h!)as they relax froman excited state to their ground state. The emitted radiation can be easily detectedwhen it is in the vacuum ultraviolet (VUV, 120–185 nm), ultraviolet (UV, 185–400nm), visible (VIS, 400–700 nm), and near infrared regions (NIR, 700–850 nm).Although atoms emit electromagnetic radiation in the infrared, microwave, andradiowave regions, the detection systems are less sensitive in these regions; therefore,the VUV, UV, VIS, and NIR regions are preferred. Of these only the VUV needs aspecial environment devoid of air. Nevertheless, a portion of the VUV spectrum isused by analytical spectroscopists.The basic aim of analytical atomic spectroscopy is to identify elements and quantifytheir concentrations in various media. The procedure consists of three general steps:atom formation, excitation, and emission. Before excitation, an element that is boundin a specific matrix must be separated from that matrix so that its atomic emissionspectra is free from interferences. For UV and visible spectroscopy, the input energymust be sufficient to raise an electron from the ground state to the excited state. Oncethe electron is in the excited state, the atom emits light, which is characteristic of thatparticular element. This tutorial will compare the Inductively Coupled Plasma (ICP)Excitation and Atomization Traditionally based on Flame and Electrothermal But Plasma Arc and Spark Why 1. Higher temperature 2. Excitation conditionsICP7AES'• Increased'atomiza,on/excita,on'7'Inductively coupled plasmas are at least 2X as hot as flames or furnaces. 6000-10000 K are common. Higher temperature to prevent the formation of most interferences, break down oxides and eliminate most molecular spectral interferences. • Wider'range'of'elements'7'Good emission spectra for most elements with same experimental setup (can record all at the same time). • Can determine low concentrations of refractory compounds (i.e. oxides) • Can determine non-metal (i.e. Cl, Br, I, S) • Wide'dynamic'range'7'Larger linear range. Advantages• Spectra are highly complex (increase probability of interferences for quantitative work) • Need more expensive optical equipment (more difficult to maintain) Disadvantages ICP7AES'What is a Plasma? - electrically conducting gaseous mixture - contains high concentration of cations and electrons (zero net charge) - very high temperatures (10,000 K)Types of Plasma • Inductively coupled plasma (ICP) • Direct current plasma (DCP) • Microwave induced plasma (MIP)Inductively coupled plasma (ICP) ICP source is called a torch. Consist of - three concentric quartz tubes - water cooled induction coil7 / VOL. 2, NO. 1 ISSN 1430-4171THE CHEMICAL EDUCATOR http://journals.springer-ny.com/chedr© 1997 SPRINGER-VERLAG NEW YORK, INC. S 1430-4171 (97)01103-5FIGURE 3. SCHEMATIC OF AN ICP TORCH.Wavelength200 400 600 800 nmI FIGURE 4. THE INTENSITY (I) OF AN ARGON PLASMA VERSUS WAVELENGTH OF EMISSION.• Three concentric tubes: torch up to 2.5 cm diameter • Three gas streams: cools outer tube, defines plasma shape and delivery sample • Tesal coil: ionize Ar. • RF power: provide radio-frequency (RF) up to 2 kW ICP torchSchematic of an ICP torch and load coil showing how the inductively coupled plasma is formed.• Brilliant white core - Ar continuum lines • Flame - like tail up to 2 cm • Transparent region - measurements made • Hotter than flame (10,000K) – more complete atomization/excitation • Atomized in “inert” atmosphere • Little ionization – too many electrons in plasma Core produces the atomic spectrum of Ar superimposed on a continuum. Above the core (15-30 mm), continuum fades and plasma is optically transparent.7 / VOL. 2, NO. 1 ISSN 1430-4171THE CHEMICAL EDUCATOR http://journals.springer-ny.com/chedr© 1997 SPRINGER-VERLAG NEW YORK, INC. S 1430-4171 (97)01103-5FIGURE 3. SCHEMATIC OF AN ICP TORCH.Wavelength200 400 600 800 nmI FIGURE 4. THE INTENSITY (I) OF AN ARGON PLASMA VERSUS WAVELENGTH OF EMISSION.Temperature in a typical ICP source Emission spectrum of Argon plasmaSEPTEMBER 2003 TODAY’S CHEMIST AT WORK 51©2003 AMERICAN CHEMICAL SOCIETYInductively coupled plasma (ICP) spec-trometers have been successfully usedfor years to determine major and minorconstituents in a variety of samples (seebox, “Plasma Science”). The sample maybe any liquid or suspension of solid parti-cles that can be atomized through a nebu-lizer. The markets served include aerospace,automotive, chemical, environmental, food,geological, metallurgical, paints and coat-ings, petrochemical, pharmaceutical,precious metals, and semiconductors.The ICP instrument includes a powersource (radio frequency generator), sampleintroduction system, spectrometer, detec-tor, electronics, and software. Each portionof the instrument can be designed in vari-ous ways, each of which has distinct advan-tages and disadvantages. A common sampleintroduction system features a nebulizerand spray chamber that provide a sampleaerosol traveling to the plasma via atorch. The relative advantages and disad-vantages of the position of this torch—horizontal or vertical—and the viewing ofthe plasma—axial or radial—have prompt-ed much discussion (Figure 1). When the sample is something otherthan a relatively clean water sample,dissolved solids, organic constituents, andother contaminants can complicate sampledescription. These problems are sometimesintensified when the ICP instrument usesan axial plasma with a low- to mid-reso-lution spectrometer.Depending on the sample type, howev-er, the advantages of high-end ICP unitscan be fully utilized. As the level ofdissolved solids in a solution increases, sodoes the possibility of difficulties withinstrument detection limits, accuracy,and stability. This problem is magnifiedin ICP


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