Atomic absorption spectroscopy

Sample handling and Sample Preparation in Atomic Absorption Spectroscopy

1. Atomic Absorption Spectroscopy Sample preparation and sample handling Prepared by Ayesha Rasty University of Education BS-Chemistry

2. Contents Introduction Principle of AAS Instrumentation Block Diagram Radiation Source Sample handling Nebulizer Pneumatic Ultra-sonic Atomizer Flame Graphite tube Sample Preparation Sample selection criteria Sample phase Wet Decomposition Use of Organic Solvents Micro Digestion References Submitted to: Dr. Farhat Nosheen 2

3. Introduction This technique was introduced in 1955 by Sir Alan Walsh in Australlia . Atomic Absorption Spectroscopy(AAS) is a quantitative method of analysis of metals and some non-metals. Concentrations are found in g/mL ranges(ppm and ppb). 3

4. Principle of AAS The element being determined must be reduced to the elemental state, Vaporized and imposed in the beam of the radiation in the source. Absorption of Electromagnetic Radiation(UV-Vis) by neutral atoms in gaseous state. Same as Molecular Electronic Spectroscopy but sample holding, equipment and spectra are different. 4

5. Instrumentation There are five basic components of an atomic absorption instrument: The light source that emits the spectrum of the element of interest An "absorption cell" in which atoms of the sample are produced (flame, graphite furnace) A monochromator for light dispersion A detector, which measures the light intensity and amplifies the signal A display that shows the reading after it has been processed by the instrument electronics 5

6. Block Diagram Wave Selector Radiation Source Detector Amplifier Signal Processor Sample Atomizer Focusing lenses 6

7. 1. Radiation Sources Hollow Cathode lamp is most common radiation source in AAS. It contains a tungsten anode and a hollow cylindrical cathode made up of element to be determined. These are sealed in a glass tube filled with inert gas(Ne or Ar ). Each element has its own unique lamp which must be used for that analysis. 7

8. Sample Handling Atomizer & Nebulizer 8

9. 2. nebulizer Suck up liquid samples at controlled rate. Create a fine aerosol spray. Mix the aerosol and fuel and oxidant thoroughly for introduction into flame. Most common types of nebulizer in AAS are: Pneumatic Nebulizers Ultrasonic Nebulizers 9

10. 2.1 Pneumatic Nebulizers Simplest, for clear or non-turbid solutions. Converts sample solution into an aerosol of tiny droplets using a jet of compressed gas. Flow of inert gas carries the droplets to an atomizer. Efficiency depends upon flow rate, viscosity, surface tension of solvent. There are several versions of pneumatic nebulizers several are as follows: 2.1.1 Concentric tube Nebulizer 2.1.2 Cross-flow Nebulizer 2.1.3 Fitted disk Nebulizer 2.1.4 Babington Nebulizer 10

11. 2.1 pneumatic Nebulizer types Concentric tube Nebulizer Cross-flow Nebulizer 11

12. Fritted disk Nebulizer Babington Nebulizer 12

13. 2.2 Ultrasonic Nebulizers Creates aerosol of tiny droplets by pumping a sample solution onto the surface of a piezoelectric crystal . That vibrates at a frequency of 20KHz to several MHz. These vibrations convert the sample into a dense and homogenous aerosol. Viscous liquids and particulates lower its efficiency. The aerosol is then carried to an atomizer by an inert gas. 13

14. 3. Atomizer Atomic Absorption process + = Elements to be analyzed need to be in atomic state. To attain atomic state analyte is exposed to high temperature in a flame or graphite furnace. These are of two types: Flame atomizer Graphite tube atomizer Light energy Ground State Excited State 14

15. 3.1 Flame Atomizer Flame is used to atomize the sample. High temperature of flame causes excitation. The amount of energy absorbed is specific for a particular element. Sequence of steps in Flame Atomizer 15

16. 3.2 Graphite tube atomizer Uses a graphite coated furnace to vaporize the sample. Samples are deposited in a small graphite coated tube which can then be heated to vaporize and atomize the analyte. Graphite tubes are heated using a high current power supply. 16

17. Construction Made of Quartz allow light to pass through the tube Protect graphite tube from oxidation Heating of graphite tube 17

18. Sample preparation Wet Decomposition, Use of Organic Solvents & Microwave digestion 18

19. Selection criteria for sample preparation Selection of preparation method is dependent upon: The analyte The analyte concentration The sample matrix The required sample size Instrumental measurement technique Instrument operation, conditions, costs Environmental conditions 19

20. Sample phase The sample in solid, liquid & gas phase can be analyzed by Flame AAS. However, in most of the cases sample analyzed by AAS is in the solution form. Therefore the solid sample is first dissolved and converted into a solution. Solids can directly analyzed directly by using an electrothermal furnace. The gaseous sample on the other hand generally are preheated by scrubbing before the resultant solution. Alternatively the gas may be adsorbed on the solid surface and then leached into the solution with suitable reagent. The dissolution of the solid sample is an important step in AAS. 20

21. Preparation of a sample The choice of reagent and techniques of decomposition and dissolution of the sample in critical step for the success of AAS determination. This is often done by acid digestion which produces a clear solution without ions of any of element to be determined. It is therefore essential that all the reagent and solvents used in the wet decomposition should be of highest purity as an impurity may raise the blank value. Common acid used for dissolution are HCl, HNO 3 , aqua regia and HClO 4 which dissolve most of the inorganic materials. A combination of Nitric acid and Perchloric acid is useful for the complete destruction of fats and proteins in biological samples. 21

22. Methods for sample preparation Sample for Atomic Absorption Spectroscopy can be prepared by following methods. Wet decomposition Use of Organic Solvents Microwave digestion 22

23. i . Wet Decomposition Involves chemical decomposition of sample matrices using the oxidizing and dehydrating power of concentrated mineral acids into aqueous solution which is ideal to be introduced in analytical instrument. Sample digestion is carried out in open or closed vessel using conventional heating or microwave radiation. Wet decomposition in open vessel may give rise to systematic errors so in order to avoid the error closed vessel system have been developed. 23

24. Advantages of Closed Vessel System There are no volatilization losses. These have shorter reaction time and improved decomposition due to high temperature. The blank values are less. They do not have contaminants from external source. Concentration of element to be determined If the concentration of element is too high then the solution must be diluted before taking the absorbance measurments . Conversely if the concentration of element in test solution is too low a concentration procedure such as solvent extraction or ion exchange must be followed. 24

25. ii. Use of organic solvents In the early development stages of AAS it was observed that analyte solution containing organic solvent of lower molar mass e.g. alcohols, ethers, ketones and esters enhances absorption peaks. In favorable cases, up to three fold increase in sensitivity could be obtained by adding a miscible organic solvents such as acetone to the solution. To obtain the increased sensitivity, the technique of solvent extraction is usually employed. The metal extracted into the organic phase is directly aspirated into the flame. 25

26. This method has following Advantages Analyte element is separated from the bulk matrix of the sample thereby eliminating chemical interference. Complexation of Metal with Organic solvent increases atomization efficiency causing up to ten fold signal enhancement. The analyte element may be extracted into a smaller volume of organic solvent with 10 to 100 fold gain in concentration. Methyl iso butyl ketone(MIBK) is an ideal solvent which is easily aspirated into the flame. 26

27. Adjustment of flame While using an Organic solvent, flame should be adjusted before aspirating the solvent which must be burned along with the fuel. If the flame is too rich in fuel the solvent will not be burnt, resulting in smoky flame. Thus fuel oxidant ratio must be adjusted while using Organic solvent to offset the presence of Organic solvent. Solvent should be aspirated between samples because the hot lean flame will heat up burner. However, the lean mixture results in lower flame temperature, thus increasing the possibility of chemical interferences. Therefore suitable safety procedure must be followed while using Organic solvents. 27

28. Microwave digestion A Microwave Digestion System(MDS) offers more rapid and efficient decomposition of complex matrices of geological and biological samples. The concept of microwave oven for the decomposition of inorganic and organic samples was first prepared during 1970s. The advantage of this sample is that it takes less time than conventional method because of rapid heating ability of microwave. In contrast to conventional flame/hot plate heating method based on conduction microwave energy of 2450MHz, which is directly transferred to all the molecules of the solution almost simultaneously without heating the vessel. As MDS greatly reduces the operator time to prepare sample for analysis. Now a days multi vessels with 4, 6 or 8 vessels are commercially available where more number of sample can simultaneously dissolved. 28

29. Microwave Digestion vessels are constructed from low loss materials that are transparent to microwave radiations. Teflon is the ideal material for acids including FH commonly used for dissolution. It has low melting and boiling points of 300 o C which is off course lower than H 2 SO 4 and H 3 PO 4 . For these Quartz and borosilicate glass vessels are used. A typical close vessel MDS consisting of Teflon body cap and safety relief valve. The maximum recommended temperature obtained with this device is 250 o C. When over pressurization occurs safety valve gets astorted , similarly to home pressure cookers and excess pressure is released. MDS Vessels 29

30. references Atomic Absorption, Methods and Instrumentation Steve J Hill and Andy S Fisher, University of Plymouth, UK Sample Introduction Techniques for Atomic Spectroscopy Richard F. Browner School of Chemistry Georgia Institute of Technology Atlanta, Ga. 30332 Andrew W. Boorn Sciex 55 Glen Cameron Road, #202 Thornhill, Ontario L3T 1P2, Canada Analytical Methods for Atomic Absorption Spectroscopy By Perklin -Elmer Sample Preparation of Solid Samples for Metal Determination by Atomic Spectroscopy—An Overview and Selected Recent Applications Joseph Sneddon a , Carey Hardaway a , Kishore K. Bobbadi a & Avanthi K. Reddy a a Department of Chemistry , McNeese State University , Lake Charles, Louisiana, USA

31. THANK YOU 31