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.

http://www.shsu.edu/~chm_tgc/sounds/sound.html

www.bing.com. www.googleimagesearch.com

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