Download - Atomic Absorption Spectroscopy
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ATOMIC ABSORPTION SPECTROSCOPY
ByK . Rakesh Gupta
Asst. ProfessorDept. of Pharmaceutical Analysis
GBN INSTITUTE OF PHARMACY
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CONTENTSIntroduction
Principle
Instrumentation
Interferences
Applications
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WHAT IS
ATOMIC ABSORPTION
SPECTROSCOPY?
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Introduction
In 1802 by Wollaston when he observed the "Fraunhofer lines" or absorption lines in the spectrum of the sun.
This principle was only applied in 1954 by an Australian physicist, Alan Walsh.
The principle states that "Matter absorbs light at the same wavelength at which it emits light".
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The original 1954 AAS instrument
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Principle
Gaseous molecule
s
Solid/Gas aerosol
Spray
Light Light Light source
Detector
Nebulization
Desolvation
Volatilization
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A light beam is passed through the flame, Radiation is absorbed, transforming the ground state atoms to an exited state.
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ATOMIZATION METHODS
Flame atomization Plasma source. Flame atomizer.
Electro thermal atomization Graphite furnace.
Specialized atomization procedures Glow discharge atomization. Hydride atomization. Cold vapor Atomization.
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Flame Atomization
A solution of a sample is nebulised by a flow of gaseous oxidant and gaseous fuel.
The nebulised liquid sample is converted into spray.
The spray on desolvation forms gas/solid aerosol.
The solid/gas aerosol by volatilization converted into gaseous molecule.
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Gaseous molecule
Atoms
Atomic ions
Dissociation (reversible)
Ionization (reversible)
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Types of Flames
The temperatures of 1700°C-2400°C occur with the various fuels when air is the oxidant.
At these temperatures only easily decomposed sample are atomized, so oxygen or nitrous oxide must be used as the oxidant for more refractory samples.
These oxidants produce temperatures of 2500°C-3100°C with the common fuels.
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The burning velocities are important because flames are stable only in certain range of gas fluorides.
Where the flow velocity and the burning velocity are equal, in this region the flame is stable.
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PROPERTIES OF FLAMES
FUEL OXIDANT TEMPERATURE °c
MAX. BURNING VELOCITY cmˉ¹
Natural gas Air 1700-1900 39-43
Natural gas Oxygen 2700-2800 370-390
Hydrogen Air 2000-2100 300-440
Hydrogen Oxygen 2550-2700 900-1400
Acetylene Air 2100-2400 158-266
Acetylene Oxygen 3050-3150 1100-2480
Acetylene Nitrous oxide 2600-2800 285
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Flame structure
Region in a flame.1. Primary combustion
zone.
2. The interzonal area.
3. Secondary combustion zone.
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Primary combustion zone:
The hydrocarbon flame is recognizable by its blue luminescence arising from the band emission of C, CH and other radicals.
The thermal equilibrium is usually not achieved in this region.
Therefore this region is rarely used for spectroscopy.
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The interzonal area:
Which is relatively narrow in hydrocarbon flames and may reach several centimeters in height in fuel rich acetylene-oxygen or acetylene-nitrous oxide sources.
Because free atoms or prevalent in this region, it is the most widely used part for the flame spectroscopy.
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Secondary combustion zone:
The products of the inner core are converted to stable molecular oxides that are then dispersed into the surroundings.
The flame profile provides useful information about the processes that go on in the different parts of a flame.
Regions of the flame that have similar values for a variable of interest.
Some of these variables include temperature, chemical composition, absorbance and radiant or fluorescence intensity
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Temperature Profiles
The maximum temperature is located in the flame about 2.5cm above the primary combustion zone.
It is important particularly for emission methods to focus the same part of the flame on the entrance slit for all calibrations and analytical measurements
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Flame Absorption profiles
In this graph we can observe the absorption of 3 different atoms viz.,
Magnesium Silver Chromium
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Magnesium It exhibits a maximum absorbance at about
the middle of the flame because of the two opposing effects.
1. The initial increase in absorbance as the distance from the base increases results from an increase in number of Mg atoms produced by the longer exposure to the heat of the flame.
2. As the secondary combustion zone approaches oxidation of Mg ions takes place, because of the oxide particle formation absorbance decreases.
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Silver: As it is not easily oxidised, so the increase in
the absorbance is observed.
Chromium: Chromium forms very stable oxides, shows a
continuous decrease in absorbance beginning close to the burner tip.
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Flame atomizer
A typical commercial laminar flow burner that uses a concentric-tube nebulizer.
Aerosol mixed with fuel and passes a series of baffles ( remove all the finest solution droplet).
The aerosol oxidant and fuel are burned in a slotted burner to provide a 5-10cm high flame.
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Flame atomizer
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FUEL AND OXIDANT REAGENT:• It is important to have a close control on
the flow rate of both oxidant and fuel.
• Fuel and oxidant are combined in a exact proportions.
• By using double diaphragm pressure regulators and needle valves, flow rates are adjusted.
• Rotameter is used to measure the flow rates.
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PERFORMANCE CHARACTERISTICS OF FLAME ATOMIZER
• Reproducibility
• There are two primary reasons for the lower sampling efficiency of the flame.
1. A large portion of the sample flows down the drain.
2. The residence time of individual atoms in the optical path of the flame is brief.
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Electro thermal atomization
Electro thermal atomizer which first appeared on the market in the early 1970s
It provides enhanced sensitivity, because entire sample is atomized in a short period.
Upto a second of time the atom will be in a optical path.
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Graphite furnace
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MECHANISM:
• A little is evaporated in low temperature and then ashed in a higher temperature in an electrically heated graphite tube.
• The ash is atomized at 2000-3000°C for a short period of time.
• The absorption or fluorescence of the atomic vapour is then measured.
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GRAPHITE FURNACE:
• It has a cylindrical graphite tube that opens at both ends and it has a central hole for sample introduction.
• The tube is 5cm long and has a internal diameter of less than 1cm.
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• The graphite tube is fitted into a pair of cylindrical graphite electrical contacts located at the two ends of the tube.
• These contacts are held in a water cooled metal housing .
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• Two inert gas streams are provided.1. The external streams prevents outside air
entering.2. The internal stream flows into the two ends
of the tube and out through the central sample port.
• The graphite furnace is having a platform is also made of graphite and is located beneath the sample entrance port.
• The sample is evaporated and ashed on this platform.
• By increasing the temperature gradually atomization occurs.
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OUT PUT SIGNAL
Typical output for the determination of lead from a 2µlt canned orange juice
At a wave length at which absorbance or fluoroscence occur s ,the output raises a maximum after a few seconds of ignition followed by a rapid decay back to zero as the atomization products escape into the surroundings.
The change is rapid enough (often <1 ) to require a moderately fast data acquisition system.
Quantitative determinations are usually based on peak height, although peak area is also used
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PRFOMANCE CHARECTERISTICS OF AN ELECTROTHERMAL ATOMIZER :
It has a high sensitivity.
Even small volumes can be atomized.
The sample volumes between 0.5-10µl are used.
The electro thermal atomization is the method of choice when flame or plasma atomization provides inadequate detection limit.
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Specialized atomization techniques
Glow discharge atomization
Hydride atomization
Cold vapour atomization
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Glow discharge atomization By using this device atomized vapour can be
swept into a absorption measurements
Sample is positioned on the sample table
The chamber is evacuated and the argon gas is injected through the sample surface
Current flowing from anode to the sample cathode ionizes the argon
The ionized argon bombards the surface causing the sample sputtering
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DIAGRAM OF GLOW DISCHARGE ATOMIZATION
ARGON JET SPUTTERING THE SAMPLE ATOMS
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The ionized argon bombards the surface causing the sample sputtering.
Where by the atoms are ejected from the sample cathode into a vapour phase.
Then the atoms are passed through the cell, where the light is passed from the source to detector.
This technique is applicable only when the sample is having electrical conductivity.
Eg.. Of samples : Cadmium, Selenium and Lead.
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Hydride atomization This technique provides a method for
samples containing Arsenic, Tin, Bismuth, Lead and selenium etc., into an atomizer as a gas.
Acidified aqueous
solution of sample
Aqueous solution of
sodium boro
hydride
Volatile hydride of
sample
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The volatile hydride is swept into the atomization chamber by an inert gas.
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Cold vapor atomization This method is applicable to the
determination of mercury because it is the only metallic element that has an appreciable vapour pressure at ambient at temperature.
The detection of mercury is important because it has toxic effects.
The mercury can be estimated at 253.7nm.
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COLD VAPOUR KIT
The mercury is converted to Hg²+ by oxidizing mixture of nitric acid and sulphuric acid followed by reduction of Hg²+ with Sncl2
The elemental mercury is then swept into long absorption tube by bubbling stream of inert gas
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Instrumentation
Radiation Source Hallow cathode lamp Electrodeless discharge lamp
Slits Atomizers Monochromators Detectors
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Radiation Source
Hallow cathode lamp: This is most common source for the atomic
absorption measurements.
Electrodeless discharge lamp:This is an alternative light source used in
AAS.
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Hallow cathode lamp 300 V applied between anode (+) and
metal cathode (-)
Ar ions bombard cathode and sputter cathode atoms
Fraction of sputtered atoms excited, then fluoresce
Cathode made of metal of interest (Na, Ca, K, Fe...)
Different lamp for each element
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Electrode less discharge lamp It provides a radiant intensities usually one to
two orders of magnitude. It consists of a sealed quartz tube containing a
small amounts of an inert gas (Argon) and a small quantity of the metal or its salt whose spectrum of interest.
DIAGRAM OF ELECTRODE LESS DISCHARGE LAMP
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The lamp has no electrode but instead is energized by an field of radio frequency or micro wave radiation.
Ionization of argon causes acceleration by the high frequency component of the field until they gain sufficient energy to excite the atoms of the metals whose spectrum will appear.
Elements like Selenium, Arsenic & Tin, EDLs exhibit better detection limits the HC lamps.
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AtomizersVarious atomizers are used in AAS.
Type of Atomizers Typical Atomization
Temperature ,°C
Flame 1700-3150
Electro thermal vaporization (ETV) 1200-3000
Inductive coupled argon plasma (ICP)
4000-6000
Direct current argon plasma (DCP) 4000-6000
Microwave-induced argon plasma (MIP)
2000-3000
Glow-discharge plasma (GD) Non thermal
Electric arc 4000-5000
Electric spark 40,000
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Monochromators
All monochromators contain the following component parts - -An entrance slit -A collimating lens -A dispersing device (usually a prism or a grating) -A focusing lens -An exit slit
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TYPES:
Diffraction grating Transmission grating
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Detectors
BARRIER LAYER CELL
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PHOTO MULTIPLIER TUBE
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The first commercial prototype
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Modern commercial AAS instrument
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Spectral Interferences The two line correction method The continuous source correction method Background correction based on the Zeeman Effect Background correction based on source self reversal
Chemical Interferences Formation of compounds of low volatility Dissociation equilibria Ionization equilibria
Interferences
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ATOMIC ABSORPTION ANALYTICAL TECHNIQUES
Sample Preparation Sample introduction by flow injection Organic solvents Calibration curves Standard addition method
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SAMPLE PREPARATION:Samples to be analysed are used in the
form of solution.
Aqueous solutions: These may be diluted with water and sprayed.
Plant and animal tissues: These are ashed by wet or dry ashing techniques and then a solution of ash is prepared in HCl.
Metals as well as alloys: These are first dissolved in acid or alkali and the resulting solution is diluted with water.
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SAMPLE INTRODUCTION BY FLOW INJECTION
In segmented-flow system, samples were carried through the system to a detector by a flowing aqueous solutions that contained closely spaced air bubbles.
The purpose of the air bubbles was to minimize sample dispersion, to promote mixing of samples and reagents and to prevent cross-contamination between successive samples.
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The air bubble had to be removed prior to detection using a debubbler or the effects of the bubbles had to be removed electronically.
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ORGANIC SOLVENTS:
• The absorbance can increase when the solution contains the low molecular weight alcohols, esters or ketones.
• The effects of the organic solvents is largely attributable to increased nebulizer efficiency.
• The lower the surface tension of such solutions results in smaller drop sizes and a resulting increase in amount of sample that reaches the flame.
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• The more rapid solvent evaporation may also contribute to the effect.
• Methyl iso butyl ketone is used in flame spectroscopy to extract chelates of metallic ions.
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CALIBRATION CURVES:
• The Atomic absorption should follow Beer’s law with absorbance being directly proportional to concentration.
• The calibration curves we get are non-linear.
• So it is counter productive to perform AA analysis without permanently confirming the linearity of the instrument response.
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• A calibration curve that covers the range of concentrations found in the sample should be prepared periodically.
• It is even better to use two standards that bracket the analyte concentrate.
• Any deviation off the standard from the original calibration curve can then be used to correct the analytical result.
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STANDARD ADDITION METHOD
• This is particularly used for analyting samples in which the like hood of matrix effects are substantial.
• This is one of the most common form involves adding one or more increments of a standard solution to sample a liquots containing identical volumes, this process is often called spiking the sample.
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• Each solution is then diluted to a fixed volume before measurement.
• Note that when the amount of sample is limited, standard additions can be carried out by successive introductions of increments of the standard to a single measured volume of un known.
• Measurements are made on the original sample on the sample plus the standard after each addition.
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Detection of a element
The element is detected by AAS, by the light intensity emitted by the sampleThis is a series of colored lines on a dark background, depending on the element, at different wavelengthsEach element has a unique spectrum.
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Applications
AAS has a various applications in every branch of chemical analysis.
The technique is already a firmly established procedures in analytical chemistry, ceramics, mineralogy, bio-chemistry, water supplies, metallurgy, soil analysis.
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1. QUALITATIVE ANALYSIS2. QUANTITATIVE ANALYSIS3. SIMULTANEOUS MULTICOMPONENT
ANALYSIS4. DETERMINATION OF METALLIC ELEMENTS
IN BIOLOGICAL MATERIALS5. DETERMINATION OF METALLIC ELEMENTS
IN FOOD INDUSTRY6. DETERMINATION OF CALCIUM,
MAGNESIUM, SODIUM AND POTASSIUM IN BLOOD SERUM
7. DETERMINETION IF LEAD IN PETROL
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REFERENCES
SKOOG, Instrumental analysis , Indian Edition , CENGAGE Learning,2007.
B.K. SHARMA , Instrumental methods of chemical analysis, third edition,GOEL publishing house,2004.
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