FIU CHM 4130 - CHAPTER 10_Xiao_AES_2018 (59 pages)

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CHAPTER 10_Xiao_AES_2018



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Chapter 10 Atomic Emission Spectroscopy Using Plasmas Arcs or Sparks 2 VOL 2 NO 1 ISSN THE CHEMICAL EDUCATOR http journals springer ny 430 4171 9 Emission of radia on upon relaxa on from excited S s1 tate 1997 SPRINGER VERLAG NEW YORK INC EXCITED STATE h GROUND STATE Excitation and Atomization Traditionally based on Flame and Electrothermal But Plasma Arc and Spark Why 1 Higher temperature 2 Excitation conditions ICP AES Advantages Increased atomiza on excita on 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 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 Larger linear range ICP AES Disadvantages Spectra are highly complex increase probability of interferences for quantitative work Need more expensive optical equipment more difficult to maintain 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 coil OF AN ICP TORCH 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 torch Schematic 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 FIGURE 3 SCHEMATIC OF AN ICP TORCH Emission spectrum of Argon plasma I Temperature in a typical ICP source 200 400 600 800 nm Wavelength ments of axial ICP needed to improve the instrument s performance A primary need was to eliminate the fireball Radially region of the plasma while allowing researchers to view A equivalent concentration which has a direct effect on the detection limit This is one of the limiting factorsAxially for spectrometers using solid state detectors because of the B NAZ IRZ FIGURE 1 A The plasma flame is composed of several regions including the initial radiation zone IRZ and the normal analytical zone NAZ B Depending on the ICP unit the plasma can be studied from either a radial view left or an axial view just the central channel This was largely accomplished with improved optical baffling The new optical baffling and better resolution improved performance with respect to required echelle optical system which typically produces high stray light in part from the use of very high optical orders of light 4 The decision as to which viewing arrangement to use depends on The chemical behavior of the anlyte in the plasma The spectral line chosen for the analysis The quality of the data required The detailed nature of the experiment Sample Introduction Samples can be aerosols vapor or solid 1 Solution Sample Introduction Nebulizer Convert solution to fine spray or aerosol use high pressure gas to entrain solution Only for introduction not atomization VOL 2 NO 1 ISSN 1430 4 HE CHEMICAL EDUCATOR http journals springer ny com ch Spray Chamber 1997 SPRINGER VERLAG NEW YORK INC To Plasma S 1430 4171 97 0110 Aerosol Liquid sample Spray To Plasma Chamber Aerosol Liquid sample Argon Argon Drain Drain FIGURE 5 A PNEUMATIC NEBULIZER THAT USES THE BERNOULLI EFFECT FOR SAMPLE UPTAKE EUMATIC NEBULIZER IN WHICH THE SAMPLE AND NEBULIZING GAS COMBINE AT RIGHT A pneumatic nebulizer in which the sample and nebulizing gas combine at right RM AN AEROSOL angles to form an areosol ICP Torch with sample introduction system nebulizer and spray chamber Fig 10 2 2 Solution liquid and solid Sample Introduction Electrothermal vaporizer electric current rapidly heats sample Sample carried to atomizer by gas Ar furnace is used only for introduction atomization occurs in the plasma opened Device for electrothermal vaporization 3 Sample Introduction For solid samples can use laser ablation How it works 1 A laser beam is focused and fired at the sample surface 2 As the laser drills into the sample the material removed is picked up in a flow of gas 3 This is transported to an ICP torch where the material is atomized and ionized 4 Sample Introduction Hydride generation Job of the hydride generation system 1 Suck up aspirate liquid sample at a controlled rate 2 Mix liquid sample with sodium borohydride and HCl 3 Create a volatile hydride of the analyte metalloid from that reaction 4 Flow that gaseous hydride into the ICP torch Hydride generation Hydride generation ICP torch 3BH4 3H 4H3AsO3 3H3BO3 4AsH3 3H20 Direct Current Plasma DCP Torch Direct Current Plasma DCP Torch DC current 10 15 A flows between C anodes and W cathode Plasma core at 10 000 K viewing region at 5 000K Simpler less Ar than ICP less expensive Sample measurement region DCP vs ICP Good Multielement analysis Simple spectra and high sensitivity Less argon and simple auxiliary power supply Better performance for organic aqueous solutions with high solids content Bad Short residence time Incomplete sample volatilization Small optimum viewing region Careful alignment High maintenance graphite electrodes Plasma Sources Atomization of Sample Samples reside in the plasma for 2 ms before being measured Temperatures range from 5500 to 8000 K Plasma Sources Advantages More complete atomization in plasma Fewer chemical interferences Less oxide formation Plasma temperature is more uniform Larger linear range for calibration curves Plasma produces significant ionization a plus for ICPMS ICP AES Performance characteristics Elements determined 60 elements Li K Rb Cs strongest lines in IR Line selec9on Most elements have several lines that can be selected Linear range Be er than AAS Interferences Chemical interferences lowered Spectral interferences s9ll


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