Transcript
ATOMIC ABSORPTION
SPECTROSCOPY
Atomic Absorption Spectroscopy (AAS) most widely used of all atomic methods
because of its simplicity, effectiveness, and low cost
Atomic absorption methods were not used before this because of problems due to very narrow widths of atomic absorption lines
Line Width Effects in Atomic Absorption The width of atomic absorption lines are very
narrow (0.002 – 0.005 nm) Even good – quality monochromators have
effective bandwidths greater than the width of atomic absorption lines
This results to instrumental departures from Beer’s law
non linear calibration curves are produced
Only a small fraction of the radiation from the monochromator slit is absorbed by the sample
- obtain small slopes of calibration curves
- poor sensitivity
Solution :Use of line sources with bandwidths narrower
than the absorption band widthsExample: use sodium vapor lamp
Sodium atoms are excited by an electrical discharge to produce a line
Other sodium lines produced are filtered by a monochromator
Source temperature and pressure are kept below that of the atomizer so that the Doppler broadening of the emitted lines is less than the broadening which occurs in the flame/atomizer
Hollow Cathode lamp
Application of about 300 V across the electrodes causes ionization of inert gas and generation of a current of 5 – 10 mA
Sputtering – gaseous cations acquire enough kinetic energy to dislodge some of the metal atoms from the cathode surface and produce an atomic cloud
Some of the sputtered metal atoms are in excited states and thus emit radiation as they return to their ground states
Metal atoms diffuse back to the cathode suface or glass walls of tube and are redeposited
Cylindrical configurationconcentrates the radiation in a limited region of
the metal tubeEnhances the probability that redeposition will
occur at the cathode rather than glass walls Efficiency depends on operating voltage
high voltages and thus high currents lead to greater intensities
BUT: greater currents produce an increased number of unexcited atoms in the cloud which are capable of self absorption and leads to lowered intensities
Electrodeless Discharge Lamp
The inert gas ionizes in this field The ions are accelerated by the high –
frequency component of the field until they gain sufficient energy to excite atoms of the metal whose spectrum is sought
Performance is not as reliable as hollow cathode lamps
Has better detection limits for Se, As, Cd, and Sb
Source Modulation In atomic absorption measurement, it is
necessary to eliminate interferences caused by emission of radiation by the flame
Much of this emitted radiation is removed by the monochromator
However, emitted radiation corresponding in wavelength to the monochromator setting is present in the flame because of excitation and emission of analyte atoms
The source is then modulated so that its intensity fluctuates at a constant frequency
The detector receives two types of signals:Source: alternatingFlame: continuous
These signals are converted into the corresponding types of current and an electronic system eliminates the unmodulated dc signal produced by the flame
The ac signal is passed from the source to the amplifier and to the readout device
Modulation can be accomplished by interposing a motor – driven circular chopper between the source and the flame
Rotation of the disk at a constant speed provides a beam that is chopped to the desired frequency
As an alternative, the power supply for the source can be designed to pulse the hollow – cathode lamps in an alternating manner
Spectrophotometers
Most AAS instruments are equipped with ultraviolet – visible monochromators, which are capable of achieving a very narrow bandwidth
Atomic absorption instruments use photomultiplier tubes as transducers
Single - Beam
Double - Beam
Atomic Absorption Analytical Techniques
nebulizers
Nebulization – to convert a liquid into a fine spray or mist
Aerosol – suspension of finely divided liquid or solid particles in a gas
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.
Types of nebulizersPneumatic Nebulizer
- liquid sample is introduced to a high velocity jet of the oxidant gas, and the mist produced is mixed with the fuel gas and passed via a mixing / expansion chamber to the burner
- fabricated from inert material, usually platinum-iridium alloy for capillary and tantalum or titanium or platinum for the nosepiece and annulus.
Ultrasonic Nebulizer
Babington and V-groove
Sample Preparation
Flame Spectroscopic methodsDisadvantages
○ The sample must be introduced in the form of a solution .
○ Decomposition of materials may lead to loss of analyte.
○ The reagents used can also introduce chemical and spectral interferences.
○ Moreover, the decomposition and solution steps are time-consuming and error-prone.
Sample Preparation
Treatment with hot mineral acids Oxidation with liquid reagents Combustion in an oxygen bomb Ashing at a high temperature High-temperature fusion
Sample Introduction by Flow Injection Flow injection analysis (FIA)
An excellent method of sample introduction especially when sample conservation is important
The carrier system of the FIA system (deionized water or dilute electrolyte) provides continuous flushing of the flame atomizer.
advantageous for samples with high level of salts or suspended solids.
Organic Solvents
Enhanced absorbances due to increased nebulizer efficiency .Lower surface tension results in smaller
drop sizes which eventually leads to an increase in the amount that reaches the flame.
Rapid solvent evaporation contributes to the enhanced absorbance.
The added organic material may be offset by leaner fuel-oxidant ratios.
Organic Solvents
Methyl butyl ketoneTo extract chelates of metallic ionsThe sensitivity of the method is enhanced
and interferences are reduced.
Calibration Curve A calibration curve that covers the range of
concentrations found in the sample should be prepared periodically.
In addition, the number of uncontrolled variables in atomization and absorbance measurements is large to allow measurement of one standard solution each time an analysis is performed.
Standard Addition Method Applied only when calib. Curve is linear
over region of interest and background correction is made
Advantage: Matrix matching is achieved automatically
Accuracy
the relative error is of the order of I% to 2%. With special precautions, this can be lowered to a few tenths of a percent.
Errors encountered with electrothermal atomization usuallv exceed those for flame atomization by a factor of 5 to 10
Detection Limits
Sample Atomization Techniques
Atomization – process in which a sample is converted into gas – phase atoms or elementary ions
Flame Atomization
Electrothermal Atomization
Special Atomization Techniques
Flame Atomization Analyte
Nebulization
Desolvation
Volatilization
Dissociation
Ionization
Excitation
Types of Flames
Temperature of 1700 °C to 2400 °C when air is the oxidant.
Temperature of 2500 °C to 3100 °C when oxygen or nitrous oxide is used as oxidant.
Flame Atomizers
Flame Atomizers Aerosol
Mixes with fuel
Runs through a series of baffles
Collects the finest solution droplets
Most of the sample solution
Mixing chamber
Waste container
Performance Characteristics of Flame Atomizers
Flame atomization – most reproducible Low sampling efficiency
A large portion of the sample flows down the drain.
Residence time of individual atoms in the optical path of the flame is brief.
Electrothermal Atomization
Enhanced sensitivity because the entire sample is atomized in a short period of time.
Long residence of atoms in the optical path
Used in atomic absorption or atomic fluorescence but not for atomic emission.
Electrothermal Atomization
A small volume of sample (in μL)
Evaporation
Ashing in an electrically heated
graphite tube
Current is rapidly increased.
Temperature is consequently increased.
Electrothermal Atomizers
Electrothermal Atomizers Graphite Tube
Where atomization occursOpen at both ends and has a central hole
for sample introductionFits into a pair of cylindrical graphite
contacts which are held in a water-cooled metal housing
Electrothermal Atomizers Two inert gas streams
External stream prevents outside air from entering and incinerating the tube.
Internal stream excludes air and carries away the vapors generated from the sample matrix during the initial heating stages.
Electrothermal Atomizers L’vov Platform
Made of graphite and located below the sample entrance.
Where the sample is evaporated and ashedImproves reproducibility of analytical signals
○ The sample is not in contact with the furnace wall so atomization occurs in an environment where the temperature is not rapidly changing.
Electrothermal Atomizers Two ways of heating
Longitudinal mode continuously varying temperature profile
Transverse mode uniform temperature profile
optimum condition for the formation of free atoms
Performance Characteristics of Electrothermal Atomizers Advantages
High sensitivity for small volumes of sample.
Detection limits lie in the range of 10-10 to 10-13 g of the analyte.
Disadvatages
The method is slow.The analytical range
is narrow, less than two orders of magnitude.
Specialized Atomization Techniques Glow-Discharge Atomization
Hydride Atomization
Cold Vapor Atomization
Interferences
Chemical Interferences from chemical processses during atomization,
altering absorption characteristics of analyte Physical Interferences
Any influence of present materials in the sample
Spectral Interferences occur when absorption/emission of interfering
species overlaps or lies close to analyte absorption
Dissociation Equilibria Dissociation and association reactions lead
to conversion of metallic constituents to the elemental state.
Dissociation reactions play an important part in determining the nature of the emission or absorption spectra for an element
Dissociation Equilibria
Dissociation equilibria that involve anions other than oxygen may also influence flame emission and absorption.
For example, the line intensity for sodium is markedly decreased by the presence of HCl.
Ionization Equilibria
Ionization is important in higher temperature flames where oxygen or nitrous oxide serves as the oxidant.
There is a significant concentration of free electrons produced by the equilibrium
Ionization Equilibria Increased temperatures cause an increase
in the population of excited atoms, according to the Boltzmann relationship .
Counteracting this effect, is a decrease in concentration of atoms
A decrease in emission or absorption may be observed in hotter flames. Thus lower excitation temperatures are usually specified for the determination of alkali metals.
Ionization Equilibria
The effects of shifts in ionization equilibria can be eliminated by addition of an ionization suppressor
Higher temperature with nitrous oxide enhances the degree of decomposition and volatilization of the compounds
Spectral Interferences
From combustion products which exhibit broadband absorption of particulate products that scatter radiation
Often occur with aspirated concentrated solutions with : Ti, Zr and W
Caused by scattering – due to carbonaceous particles from incomplete combustion of organic matrix.
Interferences due to overlapping lines are rare
Spectral Interferences
Avoided by varying analytical variables such as flame temp. And fuel-to-oxidant ratio
If source of interference is known, an excess of the interfering substance can be added to both sample and standards. The added substance is sometimes called a
radiation buffer.
Methods of Correction
Two-line Correction Method Continuum-Source Correction Method Background Correction Based on the
Zeeman Effect
Two-Line Correction Method Uses a line from the source as a reference,
This line should lie as close as possible to the analyte line but must not be absorbed
Any decrease in power of the reference line observed during calibration arises from absorption or scattering by the matrix products of the sample
This decrease in power is then used to correct the absorbance of the analyte line
Continuum-Source Correction Method
A background corrector compensates for the background absorption
Background Correction Based on the Zeeman Effect
Zeeman Background correction An applied magnetic field to the source or
atomizer is used to split the resonance line into its Zeeman components (π and ±σ).
The background is monitored by the polarizers
The analyte and signal background are monitored with the central component
Background Correction Based on the Zeeman Effect
Applicationbased on the differing response of the two
types of absorption lines to polarized radiation.
The π line absorbs only that radiation that is plane polarized in a direction parallel to the external magnetic field
the σ lines, absorb only radiation polarized at 90° to the field
Background Correction Based on the Zeeman Effect
Background Correction Based on the Zeeman Effect
A more accurate correction for background
Particularly useful for electrothermal atomizers and permit the direct determination of elements in samples such as urine and blood
The decomposition of organic material in these samples leads to large background corrections (background A > I)
Sources
[1] Skoog, et al. Principles of Instrumental Analysis 6th edition. Thomson Brooks/Cole. USA. 2007.
[2] Verma, A. CRC Handbook of Atomic Absorption Analysis Vol. 1. CRC Press Inc. USA.1987.
[3] Skoog, et al. Fundamentals of Analytical Chemistry 8th edition. Thomson Brooks/Cole. USA. 2004.
[4]http://weather.nmsu.edu/Teaching_Material/soil698/Student_Reports/Spectroscopy/report.htm
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