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1. ATOMIC ABSORPTION SPECTROSCOPY Dr Kiran Atomic Emission Spectroscopy (AES, OES)

2. I n tr o d u c t i o n Atomic emission spectroscopy (AES or OES) uses quantitative measurement of the optical emission from excited atoms to determine analyte concentration. Analyte atoms in solution are aspirated into the excitation region where they are desolvated, vaporized, and atomized by a flame, discharge, or plasma. These high-temperature atomization sources provide sufficient energy to promote the atoms into high energy levels. The atoms decay back to lower levels by emitting light. Since the transitions are between distinct atomic energy levels, the emission lines in the spectra are narrow. The spectra of multi-elemental samples can be very congested, and spectral separation of nearby atomic transitions requires a high-resolution spectrometer. Since all atoms in a sample are excited simultaneously, they can be detected simultaneously, and is the major advantage of AES compared to atomic-absorption (AA) spectroscopy .

3. Principle of AES Study of radiation emitted by excited atoms and monoatomic ions. Excited atoms and ion relax to the ground state. Relaxation often result in the emission of light, producing the spectra in the visible and UV regions of the spectrum, including vacuum UV region. These emitted atoms or ions lines can be used for the qualitative identification of elements present in sample and quantitative analysis of such elements at concentration ranging from low parts per billion(ppb) to percent.

4. Sample Introduction: Liquid samples are nebulized and carried into the excitation source by a flowing gas. Solid samples can be introduced into the source by a slurry or by laser ablation of the solid sample in a gas stream. Solids can also be directly vaporized and excited by a spark between electrodes or by a laser pulse. As in AA spectroscopy, the sample must be converted to free atoms, usually in a high- temperature excita t ion sou r ce

5. The excitation source must desolvate, atomize, and excite the analyte atoms. A variety of excitation sources are described in separate documents: Flame Arc / Spark Plasma – Inductively-coupled plasma (ICP) – Direct-current plasma (DCP) – Microwave-induced plasma (MIP) – Laser-induced plasma , Laser-induced breakdown (LIBS)

6. AES based on Plasma Sources: Plasma is an electrical conducting gaseous mixture containing significant amounts of cations and electrons ( net charge approaches zero) 1) increased atomization/excitation wider range of elements simultaneous multielement analysis wide dynamic range A+ A e -

7. ICP-AES OR ICP-OES (OPTICAL EMISSON SPECTROSCOPY) Used to detect trace metals, AES used ICP to produce excited atoms and ions that emit electromagnetic radiation at wavelength characteristics of particular elements.

8. 1 ) IC P -OES: Ind u ction coil -A typical ICP consists of three concentric quartz tubes through which streams of argon gas flow at a rate in the range from 5-20 L/min. -The outer tube is about 2.5 cm in diameter and the top of this tube is surrounded by a radiofrequency powered induction coil producing a power of about 2 kW at a frequency in the range from 27-41 MHz. This coil produces a strong magnetic field as well. -Ionization of flowing argon is achieved by a spark where ionized argon interacts with the strong magnetic field and is thus forced to move within the vicinity of the induction coil at a very high speed. -A very high temperature is obtained as a result of the very high resistance experienced by circulating argon (ohmic heating). -The top of the quartz tube will experience very high temperatures and should, therefore, be isolated and cooled. This can be accomplished by passing argon tangentially around the walls of the tube

9. ICP Plasma Structure The viewing region used in elemental analysis is usually about 6000 o C, which is about 1.5-2.5 cm above the top of the tube. It should also be indicated that argon consumption is relatively high which makes the running cost of the ICP torch high as well. Argon is a unique inert gas for plasma torches since it has few emission lines. This decreases possibility of interferences with other analyte lines. A plasma torch looks very much like a flame but with a very intense nontransparent brilliant white color at the core (less than 1 cm above the top). In the region from 1-3 cm above the top of the tube, the plasma becomes transparent. The temperatures used are at least two to three orders of magnitude higher than that achieved by flames which may suggest efficient atomization and fewer chemical interferences.

10. Sample preparation Sample are usually introduces into plasma in solution form but solid, finely divided can also be used. Final solution obtained that depend upon nature of sample and concentration of element to be determined. Two main type of sample preparation: acid digestion Dry attack Acid digestion: acids, weather singly or in mixture use their oxidizing or reducing properties. Ex- loss of As.Se,Sn in the form of chloride in the presence of HCl. Creation of PPTs of Ca, Ba,Pb also represent a error in the presence of H2SO4, Open and closed microwave system are increasingly used in labs. Dry attack: alkaline fusion is used as well as high temperature calcination (450C-600c) with acid recovery of ashes. Losses by votalization and insolubilization can be more than negligible

11. Applications Determination of metals in wine, arsenic in food and trace element bound in protein. Used in minerals to provide the data on grades in various streams, for construction of mass balances. Trace element in soil

12. Principle of AAS

13. Introduction AAS was developed in the 1950 by Alan Walsh. Atomic absorption spectrometry (AAS) is a technique in which free gaseous atoms absorb electromagnetic radiation at a specific wavelength to produce a corresponding measurable signal. The  absorption signal is proportional to the concentration of the free atoms present in the optical path . AAS is elemental analysis technique capable of providing quantitative information.

14. Instrumentation THE BASIC COMPONENTS (1) a light source ; (2) a sample cell; and (3) a means of specific light measurement

15. LIGHT SOURCES A light source which emits the sharp atomic lines of the element to be determined is required. Hollow cathode lamp (HCL) Electrodes Discharge Lamp (EDL)

16. Hollow Cathode Lamp The cathode of the lamp frequently is a hollowed-out cylinder of the metal whose spectrum is to be produced. The anode and cathode are sealed in a glass cylinder normally filled with either neon or argon at low pressure. At the end of the glass cylinder is a window transparent to the emitted radiation. When an electrical potential is applied between the anode and cathode, some of the fill gas atoms are ionized.

17. The positively charged fill gas ions accelerate through the electrical field to collide with the negatively charged cathode and dislodge individual metal atoms in a process called ‘‘sputtering’’. Sputtered metal atoms are then excited to an emission state through a kinetic energy transfer by impact with fill gas ions.

18. OPTICAL CONSIDERATIONS Photometers The portion of an AAS optical system which conveys the light from the source to the monochromator is referred to as the photometer . Three types of photometers are typically used in atomic absorption instruments: single-beam , double-beam and what might be called compensated single-beam or pseudo double-beam .

19. Double Beam AA spectrometer

20. Compensated single beam system

21. Electrothermal Atomizer (Graphite Furnace atomizer)

22. Nebulizers , Burner Heads and Mounting Systems Stainless steel has been the most common material used for construction of the nebulizer. Burner heads typically are constructed of stainless steel or titanium. All-titanium heads are preferred as they provide extreme resistance to heat and corrosion.

23. Monochromator and Spectrometer optics Amount of light energy reaching the photomultiplier (PMT). Lamp intensity is optimized to be as bright as possible while avoiding line broadening problems. The impact of single-beam and double beam photometer systems. Light from the source must be focused on the sample cell and directed to the monochromator , where the wavelengths of light are dispersed and the analytical line of interest is focused onto the detector

24. Monochromator

25. Light from the source enters the monochromator at the entrance slit and is directed to the grating where dispersion takes place . The diverging wavelengths of light are directed toward the exit slit. By adjusting the angle of the grating, a selected emission line from the source can be allowed to pass through the exit slit and fall onto the detector . All other lines are blocked from exiting.

26. The angle of dispersion at the grating can be controlled by the density of lines on the grating. Higher dispersion will result from greater line density, i.e., more lines/mm . High dispersion is important to good energy efficiency of the monochromator

27. DETECTORS

28. Calibration of the Spectrometer Most modern atomic absorption instruments include microcomputer-based electronics. The microcomputer provides atomic absorption instruments with advanced calculation capabilities, including the ability to calibrate and compute concentrations from absorbance data conveniently and accurately. In the linear region, data on as little as one standard and a blank may be sufficient for defining the relationship between concentration and absorbance . The accuracy of a calibration computed for a nonlinear relationship depends on the number of standards and the equations used for calibration . To obtain reliable quantitative data the following are required to ample and control. Same solvent system (water,5%nitric acid) Same anion at same concentration (sulfate, chloride) Same type of flame Stable pressure Background correction in each sample.

29. Analysis of samples Liquid samples: Both flame and furnance atomizer. Solid sample: sample dissolved solvent to form liquid solution and introduce in to flame or furnance . Gas sample: Gas is bubbled through an absorbing solution.

30. Forensic application Qualitative analysis Quantitative analysis soil sample Biological material Water sample Cement Oil Paint Food Wires Gold and silver

31. Advantages of AAS Advantages Determination of 70 metals Ability to make ppb determination. Little interference. Easy to use and operate. Disadvantages: only for metals, provide information's about chemical form of the metal.

32. https://www.iitk.ac.in/che/pdf/resources/AAS-GTA-reading-material.pdf https://www.youtube.com/watch?v=5fvWhCk7x6U