9-Atomic Absorption Spectrophotometry (AAS)


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Atomic Absorption Spectrophotometry (AAS): 

Atomic Absorption Spectrophotometry (AAS) U.A. Deokate Copyright © by U. A. Deokate, all rights reserved.

PowerPoint Presentation: 

2 Types of Atomic Spectrometry A class of spectroscopic methods in which the species examined in the spectrometer are in the form of ATOMS ( not molecules or ions as in solution Spectrophotometry & spectrofluorimetry) Three important methods based on spectroscopy of atomic species are: Flame Emission Photometry (FEP) Atomic Absorption Spectrophotometry (AAS) Inductively Coupled Plasma Atomic Emission Spectrometry (ICPAES )

Atomic Absorption Spectrophotometry (AAS): 

3 Atomic Absorption Spectrophotometry (AAS) Flame photometry has limitation in determining limited no of elements like Na + , K + , ca ++ etc In atomic absorption spectroscopy measurement is made of the radiation absorbed by the non excited atoms in the vapor state. In emission spectroscopy, measurement is made of energy emitted when atoms in the excited state return to the ground state


4 Introduction Atomic-absorption spectroscopy (AAS) uses the absorption of light to measure the concentration of gas-phase atoms. Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in a flame or graphite furnace. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption. Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration.


5 Introduction Atomic absorption is the absorption of light by free atoms. An atomic absorption spectrophotometer is an instrument that uses this principle to analyze the concentration of metals in solution. The substances in a solution are suctioned into an excited phase where they undergo vaporization, and are broken down into small fragmented atoms by discharge, flame or plasma.


6 Introduction By exposing these atoms to such temperatures they are able to “jump” to high energy levels and in return, emit light. The versatility of atomic absorption an analytical technique (Instrumental technique) has led to the development of commercial instruments. In all, a total of 68 metals can be analyzed by AAS.

Advantages of AA: 

7 Advantages of AA Determination of 68 metals Ability to make ppb determinations on major components of a sample Precision of measurements by flame are better than 1% rsd. AA analysis is subject to little interference. Most interference that occurs have been well studied and documented. Sample preparation is simple (often involving only dissolution in an acid) Instrument easy to tune and operate

PowerPoint Presentation: 

8 Atomic absorption spectroscopy is a quantitative method of analysis that is applicable to many metals and a few nonmetals. The signal from the detector is transferred to the computer, and the output registers on the monitor in a manner specified by the user.

Relationship Between Atomic Absorption and Flame Emission Spectroscopy : 

9 Relationship Between Atomic Absorption and Flame Emission Spectroscopy Flame Emission -> it measures the radiation emitted by the excited atoms that is related to concentration. Atomic Absorption -> it measures the radiation absorbed by the unexcited atoms that are determined. Atomic absorption depends only upon the number of unexcited atoms, the absorption intensity is not directly affected by the temperature of the flame. The flame emission intensity in contrast, being dependent upon the number of excited atoms, is greatly influenced by temperature variations.

PowerPoint Presentation: 

10 An atomic absorption spectrophotometer consists of a light source, a sample compartment and a detector. The source of light is a lamp whose cathode is composed of the element being measured. Each element requires a different lamp.

PowerPoint Presentation: 

11 Atomic Absorption Spectrometry (AAS)

Light source : 

12 Light source The light source is usually a hollow-cathode lamp of the element that is being measured. Lasers are also used in research instruments. Since lasers are intense enough to excite atoms to higher energy levels, they allow AAS and atomic fluorescence measurements in a single instrument. The disadvantage of these narrow-band light sources is that only one element is measurable at a time.

Hollow Cathod Lamps (HCL): 

Hollow Cathod Lamps (HCL) The cathode is coated with the metal of interest. Inert filler gas (Ne or Ar) is ionized by an electric current and these ions are then attracted to the cathode. The ions bombard the cathode and excite the metal ions coated on it. This excitation of the metal produces the emission of EMR with wavelengths characteristic of the analyte.

Hollow Cathode Lamps (HCL): 

14 Hollow Cathode Lamps (HCL) Hollow-cathode lamps are a type of discharge lamp that produce narrow emission from atomic species. They get their name from the cup-shaped cathode, which is made from the element of interest. The electric discharge ionizes rare gas atoms, which are accelerated into the cathode and sputter metal atoms into the gas phase. Collisions with gas atoms or electrons excite the metal atoms to higher energy levels, which decay to lower levels by emitting light. Hollow cathode lamps are available from several manufacturers either as single or multiple elements lamps.

Atomizer : 

15 Atomizer AAS requires that the analyte atoms be in the gas phase. Ions or atoms in a sample must undergo desolvation and vaporization in a high-temperature source such as a flame or graphite furnace. Flame AAS can only analyze solutions, while graphite furnace AA can accept solutions, slurries, or solid samples. Flame AAS uses a slot type burner to increase the path length, and therefore to increase the total absorbance. Sample solutions are usually aspirated with the gas flow into a nebulizing/mixing chamber to form small droplets before entering the flame.

Basic Principle : 

16 Basic Principle The technique of flame atomic absorption spectroscopy (FAAS) requires a liquid sample to be aspirated, aerosolized, and mixed with combustible gases, such as acetylene and air or acetylene and nitrous oxide. The mixture is ignited in a flame whose temperature ranges from 2100 to 2800 O C. During combustion, atoms of the element of interest in the sample are reduced to free, unexcited ground state atoms, which absorb light at characteristic wavelengths

AAS Nebulization Chamber : 

17 AAS Nebulization Chamber A solution of the sample was sprayed as an aerosol through a nebulizer into a flame. The nebulizer functions because the high velocity of the combustion gases of the fuel and oxidant rushing past a small orifice draws the liquid into the flow as small droplets. A simple burner head and nebulizer chamber are shown at the left. The design of the nebulizer is to limit the size of the atomized sample or droplets introduced to the flame to a very small size (~5-10 mm). Droplets larger than this are stopped by baffles or spoilers and end up flowing to waste.

The variables that control the size of droplets : 

18 The variables that control the size of droplets Difference in the velocity gas and the liquid, The density of the liquid, The viscosity of the liquid, and The volume flow rates of both the gas and the liquid.

Types of flame : 

19 Types of flame Different flames can be achieved using different mixtures of gases, depending on the desired temperature and burning velocity. Some elements can only be converted to atoms at high temperatures. Even at high temperatures, if excess oxygen is present, some metals form oxides that do not redissociate into atoms. To inhibit their formation, conditions of the flame may be modified to achieve a reducing, nonoxidizing flame.

Characteristics of different flames Source : 

20 Characteristics of different flames Source Max. flame speed (cm/s) Max. temp. ( o C) Air-Coal gas 55 1840 Air-propane 82 1925 Air-hydrogen 320 2050 Air-50% oxygen-acetylene 160 2300 Oxygen-nitrogen-acetylene 640 2815 Oxygen-acetylene 1130 3060 Oxygen-cyanogen 140 4640 Nitrous oxide-acetylene 180 2955 Nitric oxide-acetylene 90 3095 Nitrogen dioxyde-hydrogen 150 2660 Nitrous oxide-hydrogen 390 2650

Atomic Absorption Methods Other Than Flame : 

21 Atomic Absorption Methods Other Than Flame Electrothermal atomisation : This type of atomisation requires a graphite furnace, where after thermal pre-treatment the sample is rapidly atomized. To maintain a dense fraction of free ground state elements in the optical path, an inert gas atmosphere is used. Since the dilution and expansion effects of flame cells are avoided, and the atoms have a longer residence time in the optical path, a higher peak concentration of atoms is obtained. Carbon rod analyser: This device can be used to convert a powdered sample into atomic vapour. A current is applied to a very thin, heated carbon rod that contains the solid sample in order to vaporise it. Tantalum boat analyser: This is another technique that produces an atomic vapour from a solid sample. A Tantalum boat is electrically heated in a manner similar to the carbon rod system, within an inert atmosphere

Graphite furnace: 

22 Graphite furnace The graphite furnace has several advantages over a flame. It is a much more efficient atomizer than a flame It can directly accept very small absolute quantities of sample. It also provides a reducing environment for easily oxidized elements. Samples are placed directly in the graphite furnace and the furnace is electrically heated in several steps to dry the sample, ash organic matter, and vaporize the analyte atoms.

Three types of high-temperature plasmas : 

23 Three types of high-temperature plasmas The direct current plasma (DCP). The microwave induced plasma (MIP). Inductively coupled plasma (ICP).( it is the most important of these plasmas)

The Direct Current Plasma Technique : 

24 The Direct Current Plasma Technique The direct current plasma is created by the electronic release of the two electrodes. The samples are placed on an electrode. In the technique solid samples are placed near the discharge to encourage the emission of the sample by the converted gas atoms.

Light separation and detection : 

25 Light separation and detection AAS use monochromators and detectors for uv and visible light. The main purpose of the monochromator is to isolate the absorption line from background light due to interferences. Simple dedicated AAS instruments often replace the monochromator with a bandpass interference filter. Photomultiplier tubes are the most common detectors for AAS


26 Instrumentation

Applications of AAS: 

27 Applications of AAS Determination of 68 metal, only limitation on type of sample is hat it must be capable of giving solution. Metallurgical and inorganic analysis for determination of alloys as Co, Cr, Mg, Mn, Pb, and Zn. Analysis of Ores eg. Ag, Co, Cu, Fe, etc Biochemical Analysis of various element or determination of Fe level in blood. Pollution analysis Agriculture industry, Analysis of Wines, Oils, Petrochemicals, Pharmaceuticals etc.

Pharmaceutical application of AAS: 

28 Pharmaceutical application of AAS For analysis of Ca in calcium gluconate injection. Analysis of ORS for Na, K, ca Analysis of Zn in Zinc Insulin prep. Analysis of Fe in Haematonics Analysis of co in radiopharmaceuticals Analysis of Zn in dusting powders