Atomic Spectros o71b

ATOMIC SPECTROSCOPY (Based on Flame Atomization)

LECTURE 5

Atomic Spectroscopy Three techniques (methods) included in atomic spectroscopy. 1. Atomic absorption spectroscopy

2. Atomic emission spectroscopy 3. Atomic fluorescence spectroscopy Focus on 1 and 2. 2

Atomic Spectroscopy In order to perform atomic spectroscopy, atoms of the analyte must first be formed, usually in the form of an atomic vapor. Atomization The process by which a sample is converted to an atomic vapor. Atomizer A device used to convert a sample to an atomic vapor. 3

Atomic Spectroscopy Three types of atomizers: 1.

2. 3.

Flame atomizer Plasma atomizer Electrothermal atomizer

Focus on 1.

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Atomization Process  Solution

of the analyte is evaporated rapidly at an elevated temperature to yield a finely divided solid.

 Further

heating will break down into gaseous atoms.

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FLAME ATOMIZATION

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Processes occurring during atomization Nebulization Conversion of the liquid sample to a fine spray.

Desolvation Solid atoms are mixed with the gaseous fuel.

Volatilization Solid atoms are converted to a vapor in the flame (molecules/atoms/ions.)

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What are the process involve to change the analyte from one state to another?

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• There are three types of particles that exist in the flame: I.

Atoms

II.

Ions

III.

Molecules

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Types of Flames Selection of flame type depends on the volatilization temperature of the atom of interest. Most common is Acetylene/air.

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Flame Structure 



Interzonal region is the hottest part of the flame and best for atomic absorption. Oxidation of the atoms occurs in the secondary combustion zone where the atoms will form molecular oxides and are dispersed into the surroundings.

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Temperature Profile

Temperature profile in °C for a natural gas-air flame

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Nebulizer

a) b) c) d)

Concentric tubes. Cross flow. Fritted disk. Babington.

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Burners Two types of burners in flame spectroscopy:

i. Turbulent flow (total consumption burner)

ii. Laminar flow (premix burner)

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Turbulent Flow Burner  Nebulizer & burner are combined into a single unit.  Sample is drawn up the capillary & nebulized.  Sample flow rate: 1 to 3 mL/min.

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Turbulent Flow Burner Advantage 1. introduce relatively large & representative sample into the flame. Disadvantages 1. A relatively short path length through flame. 2. Problems with clogging of the tip. 3. Burners noisy from electronic and auditory stand point. 18

Laminar Flow Burner







Sample is nebulized by the flow of oxidant which flow through a capillary tip. Resulting aerosol then mixed with fuel & flow through a series of baffles. Only finest droplets went through the baffels.

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Laminar Flow Burner





Bigger sample droplets is collected at the bottom of mixing chamber then drained to a waste container. Aerosol, oxidant & fuel are burned in a slotted burner that provides a flame of 5 – 10 cm in length.

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Laminar Flow Burner Advantages i. ii.

Provide quiet flame. Provide longer path length that enhance the sensitivity & reproducibility.

Disadvantages i. ii.

iii.

Lower rate of sample introduction. Possibility of selective evaporation of mixed solvents in the mixing chamber could create analytical uncertainties. Mixing chamber contains a potentially explosive mixture that can flash back if the flow rates are too low. 21

Atomic Absorption Spectra  The spectra result from the atomized sample absorbing photons of radiation of the appropriate energy (wavelength).  Energy of radiation absorbed by a vaporized atom is similar with energy needed for electron excitation transitions.  The excitation transition take place as the electrons jump from ground state to a higher energy level. 22

Atomic Absorption INSTRUMENTATION

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AA Spectrophotometers Single Beam Instrument

The modulated power source can be replaced by a chopper. 24

AA Spectrophotometers Double Beam Instrument

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Double Beam Instrument 

Radiation from HCL is split into 2 beams. 1. One es through the flame. 2. The other around the flame.



A half-silvered mirror returns both beams to a single path then through the monochromator then detector.



Monochromator is placed between sample and detector. It used to eliminates most of the radiation originating from the flame. 26

Radiation Sources for AAS Radiation source in AAS is a line source which provide narrow emission bands. Common radiation source used in AAS 1. Hollow cathode lamp (HCL) 2. Electrodeless discharge lamp (EDL)

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Hollow cathode lamp (HCL)

 

Light from this lamp exactly light required for the analysis, even no monochromator is used. Hollow cathode lamp MUST contain the element to be determined.

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How does the HCL works?

 When lamp is on, atoms are supplied with energy that causes electrons of the atoms elevate to the excited states.  Upon electrons returning to ground state, wavelength of the photon emitted are useful for the analysis.  The photon emitted will supply the exact amount of energy needed for the analyzed metal to 29 undergoes excitation.

Excitation Mechanisms in HCL

 The lamp is a sealed glass envelope filled with Argon or Neon gas.  When lamp is ON, Ar atoms are ionized with electrons at Anode (+ electrode).  The Ar ions, Ar+ bombard the surface of the Cathode (- electrode).  Metal atoms,M in the cathode are elevated to the excited state and are rejected from the surface as a result of this bombardment.

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Excitation Mechanisms in HCL

 When atoms return to ground state, line spectrum of that specific atom emitted.  This light is directed at the flame where unexcited atoms of the same element absorb the radiation and raised to the excited state.  Absorbance is measured and related to the concentration. 31

Excitation Mechanisms in HCL Ar + e-

Ar+ + 2e-

Reactions in the HCL Ionization of filler gas: Ar + e- Ar+ + 2 eExcitation of metal atoms: M + Ar+ M* + Ar Light emission: M* M + h

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Electrodeless Discharge Lamp (EDL)  Constructed of a metal or salt of interest sealed in a quartz tube filled with a noble gas (Ne or Ar) at low pressure (1 – 5 torr).

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EDL

 The noble gas is ionized and accelerated by a strong radio-frequency (RF) or microwave field and excite the metal or salt of interest.  EDL can provide radiant intensities usually one to two orders of magnitude greater than HCL. 34

Source Modulation

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Source Modulation Why source modulation is employed in AAS? 1. To eliminate interference caused by emission of the radiated flame from analyte atoms and flame gas species. 2. To distinguish between the component of radiation arising from the source and the component of radiation arising from the flame background.  Source modulator: Light chopper (circular rotating metal disk) 36

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Light Chopper

 The function of light chopper is to eliminate the effects of radiation from the flame.  Light is “chopped” with a rotating half-mirror so that detector could received two alternating signals. One from the radiation source and one from the flame.  At one moment (opaque), only light emitted by flame is read by the detector since the light from 38 the radiation source is cut off.

Light Chopper

 Next moment (transparent), light from both the flame emission and radiation source is read. Transmission from the source light is measured since the source light is allowed to .  Absorbance of the sample is determined by measuring the difference in radiant power between flame emission signal and signal from the radiation source. 39

a) b) c)

Signal provide by radiation source. Sample absorbance. Signal provide by the sample after absorbing part of the radiation power.

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Questions In flame atomic absorption spectroscopy, briefly describe the 'atomization' process which the analyte undergoes. 2. Why is source modulation employed in atomic absorption spectroscopy? Name the device used for this purpose. 3. Name the common line source used in AAS. 1.

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INTERFERENCES

Spectral interference

Chemical interference

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Spectral Interference Spectral interference occurs when the spectral line of the elements being determined are overlaps with a spectral line or band from another element present in the sample. The effect of the element will also be measured and therefore the results will be incorrect. 43

How it is occurs? 1. Scattering by combustion or particulate products. Arise from the combustion products or particulate matters from the atomization scatters the radiation from the source. Both products reduce the power of the transmitted beam. 2. Scattering by sample matrix interference . Arise when the emission or absorption of an interfering species overlaps or lies so close to the analyte absorption so that resolution by the monochromator becomes impossible. 44

How to solve the issues? 1. Tune the monochromator to a different spectral line for the element of interest so that there is no overlap. 2. Use the secondary lines for the element of interest (can be found in the literature).

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Chemical Interference Common types of chemical interferences that reduce the concentration of free gaseous atoms during analysis are:

1. Ionization. 2. Refractory Formation (formation of compound of low volatility). 3. Dissociation reactions.

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1. Ionization  Sample either in liquids or solid is vaporized and atomized at high temperature provide by the flame source.  This high temperature environment lead to ionization of the analyte atoms.  Ionization of analyte atoms reduced the concentration of of analyte atoms to be excited.  Analysis of AAS are interested in analyte that in atomic state not the analyte ionic state. 47

Example Determination of Ba in alkaline earth mixtures. Since the wavelength for Ba absorption appears in the center of broad absorption band of Ca.  Samples containing organics solvents where incomplete combustion of the organic matrix leaves carbonaceous particle scattered the beam.  In concentrated solutions where atomization products form refractory metal oxides with greater diameter scattered the beam. 

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How to cure ionization?  By adding ionization suppressors such as Cesium, Cs and potassium, K to analyte solutions. These atoms are easily ionized & produce a high concentration of free electrons in the flame.  These atoms suppressed the analyte ionization by adding a source of electrons which shifts the equilibrium of the analyte from the ionic back to the atomic form. Analyte ↔ Analyte+

+

e49

2. Refractory Formation  Anions that can form refractory compounds of volatility with analyte that are not atomized in flames.  Refractory compound formed reduced the fraction of analyte that is atomized.  For example, a decrease in Ca absorbance is observed with the increasing concentrations of phosphates,PO43- and sulphate,SO42- due to formation of calcium phosphate and calcium sulphate. 50

How to cure Refractory Formation?  Refractory compounds formation can be prevented by adding a releasing agent such as Lanthanum and Strontium ion.  Releasing agent are cations that react preferentially with the interferent and prevents its interactions with the analyte.  Lanthanum ions, La3+ is added to the sample and standard in Calcium measurement. The La3+ binds with phosphates presence as La(PO4)3 which is low volatility and not atomized.

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3. Dissociation Reactions  In the hot gaseous environment of a flame or furnace, dissociation and associations reactions lead to conversion of the metallic constituents to the elemental state. MO ↔ M+ O

M(OH)2 ↔ M + 2OH  Dissociation reactions involve metal oxides and hydroxide for an element and anions other than oxygen such as Cl-. MCl ↔ M + Cl 52

Example The emission spectra of Na is decreased with the presence of HCl. Cl atom from the added HCl formed NaCl which later decrease the atomic concentration of Na as well as the Na emission and absorption.  Alkaline earth oxides are relatively stable but at very high temperatures, oxides and hydroxides of the alkaline metal easily dissociated. These bands are at higher intensity compared to the band of atom or ions required for analysis. 

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How to cure dissociation reactions?  By the addition of Aluminium,Al and Titanium,Ti in the analysis of Vanadium,V.  Addition of Al and Ti change the concentration of Oxygen needed for the combustion of V causes an increased in Vanadium atoms concentration to be atomized and excited.

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Quantitative Analysis 1. Calibration curve 2. Standard Addition Method Quantitative analysis still follows Beer’s Lambert Law and the concentration of unknown are determined by the same way using the formula( A = εbc). Width of the flame is consider as the beam pathlength. 55

Calibration curve A general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of standard of known concentration.  Plot is a linear over a significant concentration range (the dynamic range).  Analysis should never be based on the measurement of a single standard with assumption that Beer’s law is being followed. 56

Standard Calibration Curve How to measure the concentration of unknown? Practically, you have measure the absorbance of your unknown. Once you know the absorbance value, you can just read the corresponding concentration from the graph .

Standard Addition Method  Extensively used in AAS. This process is often called spiking the sample.  Compensate for variation caused by physical and chemical interferences in analyte solution.  Involves adding one or more increments of a standard solution to sample aliquots of the same size.  Each solution is then diluted to a fixed volume before measurement.

Standard Addition Methods If Beer’s law is obeyed,

A = εbVstdCstd Vt = kVstdCstd A= (kCstd)Vstd

+

εbVxCx Vt + kVxCx + kCxVx

k is a constant equal to εb/Vt

Standard Addition Methods Plot a graph of A vs Vstd

A = (kCstd)Vstd Y = mX

+ kCxVx + C

where the slope m and intercept C are

m = kCstd C = kCxVx

Cx can be obtained from the ratio of these two quantities m and C

C m Cx

=

kVxCx kCstd

= CCstd mVx

Standard Addition Methods For single-point standard addition Dividing the 2nd equation by the first & then rearrange it will give.

Cx

=

A1 Cs Vs (A2 – A1 ) Vx

Example The chromium in an aqueous sample was determined by pipetting 10.0mL of the unknown into each of five 50.0mL volumetric flasks. Various volumes of a standard containing 12.2 ppm Cr were added to the flasks, following which the solutions were diluted to the volume.

1. 2.

Plot the data. Calculate the concentration of Cr in the sample.

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ADVANTAGES (Flame) 1. Inexpensive in term of the equipment and running day-to-day analysis. 2. High sample throughput. 3. Easy to use. 4. High precision. Flame atomization best for reproducibility (less than 1%)

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DISADVANTAGES 1. Only solutions can be analyzed. 2. Relatively large sample quantities required (1 – 2 mL). 3. Less sensitivity compared to graphite furnace. Flame AAS detection limit is >1ppm while Graphite furnace is <1ppm. 4. Interference due to refractory compound formation. 5. Only metal element can be analysed using AAS. 65

Detection Limits (ppb)

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Applications of AAS   

 



Water analysis. (Ca, Mg, Fe, Si, Al, Ba content) Food analysis. Analysis of animal feedstuffs (Mn, Fe, Cu, Cr, Se, Zn) Analysis of additives in lubricating oils and greases. (Ba, Ca, Na, Li, Zn, Mg) Analysis of soils. Clinical analysis (Ca, Mg, Li, Na, K, Fe)

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