Analytical Instrumentation: Atomic Absorption Spectroscopy

Nikhilbinoy.CAssistant ProfessorICE DepartmentAnalytical Instrumentation:Atomic Absorption Spectroscopy

First observed in 1802 with Wollastons observation of dark bands in the emission spectrum of the sun.In 1859, Kirchoff and Bunsen correctly explained this by showing that the dark bands were due to the absorption of emission radiation by ground state gas phase atoms in the sun.Since 1953, AAS is used as a standard method for the analysis of both metallic and non-metallic elements.AAS is widely accepted because the technique is element selective and it provides analytical sensitivities at the part-per-million level and less.Applications: routine method of metal analysis include various forms of industrial manufacturing, geology, medicine and agriculture.

Origin of Atomic SpectroscopyTransition involves prompting an electron from a ground state to a higher empty atomic state orbitals (excited state).5P4P3P

3S

ExcitationEmission

A

Elements Detectable by AAS (highlighted in pink)

Basic Principle

Atomization

AtomizationAtomizer devices are either continuous or discrete.Continuous are in the form of electrothermal.In either of the two methods, thermal energy is used to vaporize the analyte material and to break the chemical bonds within the component molecule.Nebulizers are the methods to introduce samples into the atomizer.Create fine droplets.Fine droplets are turned into an atomic gas in three steps:Desolvation (drying): the liquid solvent is evaporated, and the dry sample remains.Vaporization (ashing): the solid sample vaporizes to a gas.Atomization: compounds making the sample are broken into free atoms.The essential requirements of a burner for atomic absorption are:The flame must extend over a reasonable length to improve sensitivity.More of the atoms in the long narrow band brought into the light beam.Inorder to avoid problems due to scattering of light by unburned droplets and large sample droplets, which cannot be burned, they must be removed before reaching the flame.

Flame Atomizers

Flame Atomizers

Nebulizers

1) Pneumatic NebulizerThe main practical requirements for pneumatic nebulizers are:A high velocity gas stream.A reasonable pressure drop of the liquid injection capillary for venturi-effect natural aspiration.Maximum interaction between the gas and liquid streams for fine aerosol production.Freedom from blockage.

2) Ultrasonic NebulizerThe sample is pumped to a transducer plate which vibrates at high frequencies of about 1.5KHz.Results in a very dense aerosol.Carried through heater and condenser to remove as much of the solvent as possible.For highly volatile solvent such petrol and naphtha, a secondary desolvation stage is required.Membrane dissolvator is used.

3) Pneumatic Aerosol Generator

4) Pneumatic Jet Nebulizer

Electro Thermal AtomizationConsists of a hollow graphite cylinder 28mm long, and 8mm in diameter.Positioned in such a manner that radiation from external source pass through the center of the cylinder.The interior of the cylinder is coated with pyroelectric graphite.Electrodes at the end of the cylinder are connected to a low voltage, high current power supply (3.6KW).Liquid samples are introduced with a micro syringe through the small opening in the top of the cylinder.Solid samples are introduced through the end of the tube with a special sampling spoon/microdish made of tungsten.Metal housing surrounding the furnace is water cooled to allow the entire atomizer unit to be rapidly restored to ambient temperature after each sample has been atomized.Inert gas (argon) ensures quick removal of vaporized matrix components during the ashing step; and ensures nothing is deposited on the inner wall of the tube.

Electro Thermal Atomization

Electro Thermal Atomization

Advantages:High sensitivity.Ability to handle small sample volumes.Ability to analyse solid samples.Low noise from furnace.

Plasma Excitation SourcesA plasma is a hot partially ionized gas. It contains relatively high concentration of ions and electrons.

Direct Current Plasma (DCP)Operates at DC and uses a multiple electrode system.Have three electrodes: two pyroelectric graphite anodes and one thoriated tungsten cathode.Has four electrical poles: two anodes and two cathodes; and uses two power supply.Single cathode electrode is connected to the negative pole of both power supplies.Each electrode is cooled by flowing argon.The electrode set keeps the plasma volume in position.Provides much stability.The sample excitation region and observation zones are centred in the crook of the Y, where the spectra contribution from the molecular bands of the plasma is minimal.

Direct Current Plasma (DCP)The arc is initiated by moving the three electrodes into contact using argon-driven piston and then withdrawing them. No spark is required in this process.Once ignited, the plasma is sustained by a low voltage (400V at 7.5A) and produces temperatures as high as 9000 to 10000K.The excitation region is at 6000K.Samples are nebulized and introduced into the excitation area.Entire cycle of desolvation, molecular dissociation, and excitation takes place in the extremely high temperature plasma during the residence time of the sample.

Direct Current Plasma (DCP)The current density and energy available for atomic excitation are independent of the arc current.If the current is increased, the crossectional area of the arc current path increases proportionally, with no increase in the arc temperature.To increase the current density and thus the plasma temperature, it is necessary to squeeze the plasma to decrease the current crossection.This is accomplished by cooling the edges of the plasma with a high velocity inert gas vortex.The cooler outer areas of the plasma cannot support significant ionization, and therefore the current density in the channel region is increased due to the reduction in size of the arc channel.Argon, nitrogen or helium is used.A limitation of DCP is their inability to be incorporated into totally automated system.

2 Electrode Direct Current Plasma (DCP2)

Inductively Coupled Plasma (ICP)Inductively Coupled Plasma (ICP) torch is a special type of plasma that derives its sustaining power by induction from high frequency magnetic field.Initially argon gas (auxiliary) passes through a 25m quartz tube, and upon emerging at the tip, is surrounded by an induction coil.An AC current flows through this coil at a frequency of around 30MHz and power levels of about 2KW.The argon gas is initially seeded with free electrons from a tesla discharge coil.These seed electrons quickly interact with the magnetic field of the coil and gain sufficient energy to initialize collision.Cations and anions are generated.

Inductively Coupled Plasma (ICP)The cations and anions (eddy current) generated are accelerated by the magnetic field, in a circular flow perpendicular to the stream that emerges from the tip of torch.Fast moving cations collide with more argon atoms to produce further ionization and intense thermal energy.Temperature is 6000K to 10000K.Second stream of gas (Argon) is required to provide a vortex flow of supporting gas to cool the inside quartz walls of the torch.Serves to center and stabilize the plasma.Introduction of sample is in aerosol form through the innermost concentric tube of the torch.

Inductively Coupled Plasma (ICP)

Inductively Coupled Plasma (ICP)ICP torch provides extremely high sensitivity, low matrix interference and multi-element analysis capability.Are expensive and less convenient to operate.There is no electrode contact.Therefore excitation and emission zones are spatially separated.

ICP DCP ComparisonDCP is less sensitive than ICP.Similar reproducibility.DCP requires less Argon.Auxiliary power less expensive in DCP.Graphite electrode must be replaced every couple of hours.

Comparison of Plasma to Flame AtomizationPlasma sources off significantly better quantitative data than flame atomization.High stability.Low noise.Low background.Freedom from interferences.

Comparison

Performance CriteriaFlame AAElectrothermal SamplingFlame AESDCPICPDynamic rangelimitedlimitedwidewideWideQualitative analysispoorpoorexcellentexcellentexcellent

Elemental rangeexcellentexcellentexcellentexcellentexcellent

Trace analysisexcellentexcellentexcellentexcellentexcellent

Performance CriteriaFlame AAElectrothermal SamplingFlame AESDCPICPMatrix interfernceshighhighhighlowlowSpectral Interferenceslowlowlowmoderatehigh

Precision & AccuracyBetter for unskilledSame skilledWorse for unskilled same skilledCosts$$$$$$$$$$$$$

Performance CriteriaFlame AAElectrothermal SamplingFlame AESDCPICPCosts Instrumentationlowlowlowmoderatehigh

Maintenancelowlowlowmoderatehigh

Sample PreparationmoderatemoderatelowmoderatemoderateOperator skilllowerhigherhigherhigherhigher

Excitation Sources

The full width at half-height of the absorption line produced by either flame or electro thermal atomization as measured by emission spectroscopy is approximately 0.002nm.Factors that modify (broader) this line width include natural, pressure, resonance, and Doppler.To provide both sensitivity and selectivity, the line sources used for excitation must be stable and narrower than the full width at half-height of the absorption of the analyte.

Hollow Cathode Lamps (HCL)Two types: single element and multielement sources.A potential of 350 to 500V (1 to 50mA) is placed across the cathode and anode terminals.The element selectivity of the HCL is achieved by fabricating the cathode from, or coating the interior of the cathode with, the element of interest.

Hollow Cathode Lamps (HCL)

Hollow Cathode Lamps (HCL)Atomic line radiation produced depends on the number of sputtered metal atoms, which depends on the kinetic energy of neon atoms, which in turn is dictated by lamp current.Higher lamp current produces a greater spectra line intensity (increases sensitivity).Higher lamp current produces excess metal atoms.This dense cloud will absorb the emitted radiation (this self absorption is known as self reversal, which dictates the maximum operating current).

Hollow Cathode Lamps (HCL)Following emission, metal atoms redeposit on cathode surface.Available for removal by other accelerated neon ions.Gaseous metal atoms may migrate and deposit on the Pyrex tube.Reduces the life time of lamp.

Hollow Cathode Lamps (HCL)For multi-element HCL, the cathode is fabricated from pressed alloys or powder of several elements.Cost is comparable with single-element.Provides detection of multiple element simultaneously.Has shorter life, because the metal atoms of the cathode which have highest volatability will redeposit over the surface of the other metals (in accessible during the collision of neon ions).

Hollow Cathode Lamps (HCL)

Electrodeless Discharge Lamps (EDL)Is also available in a multi-element format.EDL is fabricated by sealing a small amount of the metal or metal iodide of interest in a quartz tube filled with an inert gas, usually argon, at reduced pressure.Placed in a microwave, radio frequency field to ionize the inert gas.The metal or metal iodide species collide with the free electrons from ionized inert gas, producing excited state metal atoms.These excited-state metal atoms emit the characteristic atomic lines of the metal as they release to the ground state.EDL produces atomic line source of greater intensity.Requires a separate power supply.Increase the instrument cost.

Electrodeless Discharge Lamps (EDL)

Continuous Source

Instrumentation

Background Correction

Continuous Source

Pulsed Hollow Cathode LampBy pulsing the HCL between low (5 to 20mA) and high (100 to 500mA) current, a background corrected spectrum can be obtained.At low current, absorption by species in the flame again account for both atomic and non atomic absorption.At high currents, the lamp emission spectra are broadened owing to Doppler effects and over the bandpass of the instrument.Mimics a continuum source.The absorbance in the high-current mode is due to non atomic absorbance.These two signals are subtracted to get a corrected absorbance.

Interferences

Chemical InterferencesMajor interference.Radical species are produced that can react with the metal compound undergoing vaporization to produce metal hydroxide, metal hydrides, or metal oxide species.Can not be dissociated.Use fuel rich flame, or use releasing agent.

Ionization InterferencesTo some extent, all metallic atoms produced at the temperatures of the flame undergo ionization.Can be certainly minimized by cooler flames, or chemically alter the ionization equilibrium.

Matrix InterferencePhysical properties like viscosity, surface tension, and density will affect the nebulization.These properties must be matched with standard for analysis.

Spectral InterferencesArise when radiation of a wavelength different from that of the wavelength of the element of interest falls on the detector.

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