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Chapter 4 Atomic Spectroscopy EST 3203 Instrumental AnalysisRezaul KarimEnvironmental Science and Technology Jessore Science and Technology University

Chapter content Overview of AS 1,2

Advantage and disadvantage of AAS 1 Theory of AS 2

Instrumentation 1 Atomization (flames, furnace and plasmas) 1

How temperature affects on AS1

Background correction1

Detection limits 1

Interference1

Virtues of the ICP 1

Analytical Applications3

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Reference1. Daniel C. Harris , 2010, Quantitative

Chemical Analysis, 8th edition, W. H. Freeman and Company , Madison Avenue New York, NY 10010

2. S. Ahuja and N. Jespersen (Eds), 2006, Comprehensive Analytical Chemistry, Volume 47, Elsevier B.V.

3. Robinson, 1995. Undergraduate instrumental analysis, Marcel Dekker, Inc. NY, USA.

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AA spectrometry

the absorption of discrete wavelengths of light by ground state, gas phase free atoms• .

Free atoms in the gas phase are formed from the sample by an “atomizer” at high temperature• .

AAS was developed in the 1950s by Alan Walshand rapidly became a widely used analytical tool

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Advantages

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1• an elemental analysis technique capable of

providing quantitative information on 70 elements

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• practically independent of the chemical formof the element in the sample

• .e.g. A determination of cadmium in a water

3• used routinely to determine ppb and ppm

concentrations of most metal elements

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its high sensitivity.

its ability to distinguish one element from another in a complex sample.

its ability to perform simultaneous multi-element analyses.

the ease with which many samples can be automatically analyzed.

Disadvantage

no information is obtained on the chemical form of the analyte (no

“speciation”)

often only one element can

be determined at

a time

limited use for

qualitative analysis.

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• An AA spectrometer, Model AA 280 equipped with a graphite furnace and Zeeman device.

• A rotating turret holds height HCL

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Types of atomic spectroscopy:

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• absorption of sharp lines from hollow cathode lamp1

• emission from a thermally populated excited state2

• fluorescence following absorption of laser radiation3

Basis of analytical measurement

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Theory: Atomic absorption spectroscopy

Beer Lambert equationI (λ)= Io(λ)10-K

(λ)b,

where Io(λ) is the radiant power of the incident radiation of wavelength λ,

I(λ) the radiant power of the transmitted radiation, K(λ) the absorption coefficient of the ground state atom, b the path length.

This equation can be expressed in terms of absorbance :

A(λ) = log (I(λ)/Io(λ))= K(λ)b

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Atomic fluorescence spectroscopy

AFS quantifies the discrete radiation emitted by excited state atoms that have been excited by radiation from a spectral source.

If a line source is used for excitation and if the atomic vapor is dilute, then the radiant power of the atomic fluorescence signal (If) can be related to the concentration of ground state atoms by the following equation:

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◦ where ῼf/4π and ῼ A/4π are the solid angles of fluorescence and excitation respectively;

◦ L the length of the atom reservoir in the analytical direction;

◦ F the atomic fluorescence quantum efficiency;

◦ IL the integrated radiant power for the incident beam per unit area;

◦ ∂ a correction factor that accounts for the relative line widths of the source and absorption profiles; and

◦ ΔλD the Doppler half width of the fluorescence profile

Atomic emission spectroscopy AES quantifies discrete radiation that is emitted by

an excited atom when it deactivates to the ground state.

The Boltzmann distribution law gives the concentrations of atoms in the excited and ground states:◦ Nj/N0 = (gj / go)e-Ej/kT

◦ where Nj and No are the number densities of atoms in the excited (jth state) and ground states, ◦ gj and go the statistical weights of these states, ◦ Ej the energy difference between the jth and ground

states, ◦ K the Boltzmann constant; (1.381*10-23 J/K )and◦ T the temperature (K) of the atom reservoir.

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Instrumentation

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Light Source Two radiation sources:◦ the Hollow Cathode Lamp (HCL) and ◦ the Electrodeless Discharge Lamp (EDL). Both types of lamps are operated to provide as

much intensity as possible while avoiding line-broadening problems caused by the collision processes.

Monochromators generally cannot isolate lines narrower than 10-3 to 10-2 nm.

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Hollow Cathode Lamp (HCL) To produce narrow lines of the correct frequency, we use a hollow-

cathode lamp containing a vapor of the same element as that being analyzed.

The hollow-cathode lamp is filled with Ne or Ar at a pressure of 130–700 Pa.

The cathode is made of the element whose emission lines we want. When 500 V is applied between the anode and the cathode, gas is ionized

and positive ions are accelerated toward the cathode. After ionization occurs, the lamp is maintained at a constant current of 2–30

mA by a lower voltage.

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The HCL process, where Ar+ is a positively charged argon ion, M0 is a sputtered ground state metal atom, M* is an excited state metal atom, andλ is emitted radiation at a wavelength characteristic for the sputtered metal

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The HCL emits narrow, intense lines from the element that forms the cathode.

Applying a high voltage across the anode and cathode creates this emission spectrum.

Atoms of the filler gas become ionized at the anode and are attracted and accelerated toward the cathode.

The fast-moving ions strike the surface of the cathode and physically dislodge some of the surface metal atoms (a process called “sputtering”).

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The displaced atoms are excited by collision with electrons and emit the characteristic atomic emission spectrum of the metal used to make the cathode.

The emitted atomic lines are extremely narrow.

Unlike continuum radiation, the narrow emission lines from the HCL can be absorbed almost completely by unexcited atoms.

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Relative bandwidths of hollow-cathode emission, atomic absorption, and a monochromator. Linewidths are measured at half the signal height.

The linewidth from the hollow cathode is relatively narrow because gas temperature in the lamp is lower than flame temperature and pressure in the lamp is lower than pressure in a flame.

Instrumentation

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Atomization: Flames, Furnaces, and Plasmas

In atomic spectroscopy, analyte is atomized in ◦ a flame, ◦ an electrically heated furnace, (graphite) or◦ a plasma (inductively coupled plasma)

The path length of the flame is typically 10 cm.

A detector measures the amount of light that passes through the flame.

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Atomization Process

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In a flame atomizer, a solution of the sample is nebulized by a flow of gaseous oxidant, mixed with a gaseous fuel, and carried into a flame where atomization occurs.

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The processes occurring in a flame atomizer. M+ is a metal cation; A2 is theassociated anion. Mo and Ao are the ground state free atoms of the respective elements

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A complex set of interconnected processes then occur in the flame. ◦ The first is desalvation, in which the solvent evaporates

to produce a finely divided solid molecular aerosol. ◦ The aerosol is then volatilized to form gaseous

molecules.◦ Dissociation of most of these molecules produces an

atomic gas.◦ Some of the atoms in the gas ionize to form cations

and electrons. Other molecules and atoms are produced in the

flame as a result of interactions of the fuel with the oxidant and with the various species in the sample.

A fraction of the molecules, atoms, and ions are also excited by the heat of the flame to yield atomic, ionic, and molecular emission spectra.

Flame Most flame spectrometers use a premix burner,

in which fuel, oxidant, and sample are mixed before introduction into the flame

In atomic absorption, a liquid sample is aspirated (sucked) into a flame at 2000 –3000 K.

Sample solution is drawn into the pneumatic nebulizer by the rapid flow of oxidant (usually air) past the tip of the sample capillary.

Liquid evaporates and the remaining solid is atomized (broken into atoms) in the flame.

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Common fuel-oxidizer

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The most common fuel-oxidizer combination is acetyleneand air, which produces a flame temperature of 2400–2700 K.If a hotter flame is required to atomize high boilingelements (called refractory elements), acetylene and nitrousoxide are usually used

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Graphite Furnaces An electrically heated graphite furnace is more

sensitive than a flame and requires less sample.

From 1 to 100 L of sample are injected into the furnace through the hole at the center.

Light from a hollow-cathode lamp travels through windows at each end of the graphite tube.

To prevent oxidation of the graphite, Ar gas is passed over the furnace and the maximum recommended temperature is 2550°C for not more than 7s.

In flame spectroscopy, the residence time of analyte in the optical path is < 1s as it rises through the flame.

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A graphite furnace confines the atomized sample in the optical path for several seconds, thereby affording higher sensitivity.

1–2 mL is the minimum volume of solution necessary for flame analysis, as little as 1 L is adequate for a furnace.

Precision is rarely better than 5–10% with manual sample injection, but automated injection improves reproducibility to 1%.

A 38-mm-long, electricallyheated graphite furnace for atomic spectroscopy.

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(a) Premix burner. (b) End view of flame. The slot in the burner head is about 0.5 mm wide. (c) Distribution of droplet sizes produced by a particular nebulizer.

Matrix Modifiers for Furnaces Everything in a sample other

than analyte is called the matrix, decomposes and vaporizes during the charring step.

A matrix modifier is a substance added to the sample to reduce the loss of analyte during charring.

E.g. The matrix modifier ammonium nitrate can be added to seawater to increase the volatility of the matrix NaCl.

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a. A graphite furnace heating profile used to analyze Mn in seawater.

b. When 0.5 M NaCl solution is subjected to this profile, signals are observed at the analytical wavelength of Mn

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Inductively Coupled Plasmas ICP is twice as hot as a

combustion flame. The high temperature,

stability, and relatively inert Ar environment eliminate much of the interference encountered with flames.

The plasma instrument costs more to purchase and operate.

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The cross-sectional view of an inductively coupled plasma burner shows two turns of a 27- or 41-MHz induction coil wrapped around the upper opening of the quartz apparatus.

High-purity Ar gas is fed through the plasma gas inlet.

After a spark from a Tesla coil ionizes Ar, free electrons are accelerated by the radio-frequency field.

Electrons collide with atoms and transfer energy to the entire gas, maintaining a temperature of 6000 to 10000 K.

The quartz torch is protected from overheating by Ar coolant gas.

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The concentration of analyte needed for adequate signal is reduced by an order of magnitude with an ultrasonic nebulizer, in which sample solution is directed onto a piezoelectric crystal oscillating at 1 MHz.

The vibrating crystal creates a fine aerosol that is carried by an Ar stream through a heated tube where solvent evaporates.

In the next refrigerated zone, solvent condenses and is removed.

Then the stream enters a desolvator containing a microporous polytetrafluoroethylenemembrane in a chamber maintained at 16°C.

Remaining solvent vapor diffuses through the membrane and is swept away by flowing Ar.

Sensitivity with an inductively coupled plasma is further enhanced by a factor of 3 to 10 by observing emission along the length of the plasma (axial view) instead of across the diameter of the plasma.

Additional sensitivity is obtained by detecting ions with a mass spectrometer instead of by optical emission.

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How Temperature Affects Atomic Spectroscopy?

Temperature determines ◦ the degree to which a sample breaks down

into atoms and ◦ the extent to which a given atom is found in its

ground, excited, or ionized states. Each of these effects influences the strength of

the signal we observe. Affects: ◦ The Boltzmann Distribution◦ Effect of Temperature on Excited-State

Population◦ The Effect of Temperature on Absorption and

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The Boltzmann Distribution It describes the relative

populations of different states at thermal equilibrium.

If equilibrium exists, the relative population (N*/N0) of any two states is

Boltzmann distribution: N*/N0 = (g* / go)e-ΔE/kT where ◦ T, temperature (K) ◦ K, Boltzmann’s constant (1.38*

10-23 J/K)◦ the degeneracies g0 and g*.

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Two energy levels with different degeneracies.

The number of states at each energy is called the degeneracy, denoted as g

Effect of Temperature on Excited-State Population

The lowest excited state of a sodium atom lies 3.371x10-19

J/atom above the ground state. The degeneracy of the excited state is 2, the ground state is 1. The fraction of Na in the excited state in an acetylene-air flame

at 2600 K is, ◦ N*/N0 = (2/1)e- (3.371*10-19J)/[(1.381*10-23 J/K)(2 600 K)]

= 1.67*10-4

That is, less than 0.02% of the atoms are in the excited state. If the temperature were 2610 K, ◦ the fraction of atoms in the excited state would be N*/N0 =

1.74*10-4.

The fraction of atoms in the excited state is still less than 0.02%, but that fraction has increased by 100(1.74 - 1.67)/1.67 = 4%.

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The Effect of Temperature on Absorption and Emission

We see that more than 99.98% of the sodium atoms are in their ground state at 2600 K.

Varying the temperature by 10 K hardly affects the ground-state population and would not noticeably affect the signal in atomic absorption.

How would emission intensity be affected by a 10 K rise in temperature?

Emission intensity is proportional to the population of the excited state.

Because the excited state population changes by 4% when the temperature rises10 K, emission intensity rises by 4%.

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Atomic Linewidths Beer’s law requires that the linewidth of the radiation source should

be substantially narrower than the linewidth of the sample. Otherwise, the measured absorbance not proportional to the

sample concentration. Atomic absorption lines are very sharp, with an intrinsic width of

only ~104 nm. Linewidth is governed by the Heisenberg uncertainty principle,

which says that the shorter the lifetime of the excited state, the more uncertain is its energy:

∂E∂t ≈ h/4π◦ where ∂E is the uncertainty in the energy difference between ground and

excited states, ◦ ∂ t is the lifetime of the excited state before it decays to the ground state

the uncertainty in the energy difference between two states multiplied by the lifetime of the excited state is at least h/4.

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If ∂t decreases, then ∂E increases. The lifetime of an excited state of an isolated gaseous atom is ~10-9 s. Therefore, the uncertainty in its energy is ∂E≤ h/4π∂t = 6.6*1034 J .s/ 4π(10-9 s) ≈ 10-25 J

Suppose that the energy difference (ΔE) between the ground and the excited state of an atom corresponds to visible light with a wavelength of λ = 500 nm. ◦ This energy difference is ΔE= hc/λ = 4.0*10-19J.

The relative uncertainty in the energy difference is ∂E/ΔE ≈ (10-25J) / (4.0*10-19 J) ≈ 2*10-7.

The relative uncertainty in wavelength (δλ/λ) is the same as the relative uncertainty in energy:◦ δλ/λ = ∂E/ΔE ≥ 2*10-7 ¬ δλ = 2*10-7 * 500 nm = 10-4

nm The inherent linewidth of an atomic absorption or emission

signal is ~ 10-4 nm because of the short lifetime of the excited state.

Broadening lines: Two mechanisms broaden the lines to 10-

3 to 10-2 nm in atomic spectroscopy. One is the

Doppler effect pressure broadening

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Doppler effect An atom moving toward the

radiation source experiences more oscillations of the electromagnetic wave in a given time period than one moving away from the source.

That is, an atom moving toward the source “sees” higher frequency light than that encountered by one moving away.

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The Doppler effect. A molecule moving (a) toward the radiation source “feels” the electromagnetic field oscillate more often than one moving (b) away from the source.

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In the laboratory frame of reference, the atom moving toward the source absorbs lower frequency light than that absorbed by the one moving away.

The linewidth, ,δλ, due to the Doppler effect, is Doppler linewidth: δλ ≈ λ(7*10-7) √(T/M)◦ T is temperature (K) and ◦ M is the mass of the atom in atomic mass units.

For an emission line near λ=300 nm from Fe (M = 56 atomic mass units) at 2500 K, the Doppler linewidth is , ◦ 300 nm (7*10-7) √(2500/56)

◦ =0.0014 nm which is an order of magnitude greater than the natural linewidth.

Pressure broadening Linewidth is also affected by pressure broadening

from collisions between atoms. Collisions shorten the lifetime of the excited

state. Uncertainty in the frequency of atomic absorption

and emission lines is roughly numerically equal to the collision frequency between atoms and is proportional to pressure.

The Doppler effect and pressure broadening are similar in magnitude and yield linewidths of 10-3

to 10-2 nm in atomic spectroscopy.

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Multi-element Detection with the Inductively Coupled Plasma An inductively coupled plasma emission

spectrometer does not require any lamps and can measure as many as 70 elements simultaneously.

One photomultiplier detector is required at the correct position for each element.

Dispersed radiation lands on a charge injection device (CID) detector, which is related to the charge coupled device (CCD)

Capabilities of CID detector:◦ pixels are individually addressed◦ rapidly filling pixel can be read, re-zeroed, and read again◦ filled pixel does not bloom into neighbors

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Types of interference spectral: unwanted signals overlapping

analyte signal chemical: chemical reactions decreasing the

concentration of analyte atoms ionization: ionization of analyte atoms

decreasing the concentration of neutral atoms

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Spectral interference Spectral interference refers to the

overlap of analyte signal with signals due to other elements or molecules in the sample or with signals due to the flame or furnace.

Interference from the flame can be subtracted by using D2 or Zeeman background correction.

The best means of dealing with overlap between lines of different elements in the sample is to choose another wavelength for analysis.

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A Cd line at 228.802 nm causes spectral interference with the As line at 228.812 nm in most spectrometers. With sufficiently high resolution, peaks are separated and there is no interference.

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Chemical interference Chemical interference is caused by any

component of the sample that decreases the extent of atomization of analyte. ◦ For example, SO4

2- and PO43- hinder the atomization of Ca2+,

perhaps by forming nonvolatile salts. ◦ Releasing agents are chemicals added to a sample to

decrease chemical interference. ◦ EDTA and 8-hydroxyquinoline protect Ca2+ from interference

by SO4-2 and PO4

3-. ◦ La3+ is a releasing agent, apparently because it preferentially

reacts with PO43 and frees the Ca2+.

◦ A fuel-rich flame reduces certain oxidized analyte species that would otherwise hinder atomization.

Higher flame temperatures eliminate many kinds of chemical interference.

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Ionization interference Ionization interference can be a problem in the

analysis of alkali metals at relatively low temperature and in the analyses of other elements at higher temperature.

For any element, we can write a gas-phase ionization reaction:◦ M(g) ↔ M+

(g) + e-(g) ; K=?

Because alkali metals have low ionization potentials, they are most extensively ionized.

At 2 450 K and a pressure of 0.1 Pa, sodium is 5% ionized. With its lower ionization potential, potassium is 33% ionized. Ions have energy levels different from those of neutral atoms, so the desired signal is decreased.

If there is a strong signal from the ion, you could use the ion signal rather than the atomic signal.

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Background Correction

Atomic spectroscopy must provide background correction to distinguish analyte signal from absorption, emission, and optical scattering of the sample matrix, the flame, plasma, or red-hot graphite furnace.

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the spectrum of a sample analyzed in a graphite furnace.

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Sharp atomic signals with a maximum absorbance near 1.0 are superimposed on abroad background with an absorbance of 0.3.

If we did not subtract the background absorbance, significanterrors would result.

Adjacent pixels of CID display Figure 20-20 shows how background is subtracted in an

emission spectrum collected with a charge injection device detector.

The figure shows 15 pixels from one row of the detector centered on an analytical peak.

Pixels 7 and 8 were selected to represent the peak. Pixels 1and 2 represent the baseline at the left and

pixels 14 and 15 represent the baseline at the right. The mean baseline is the average of pixels 1, 2, 14,

and 15. The mean peak amplitude is the average of pixels 7

and 8. The corrected peak height is the mean peak

amplitude minus the mean baseline amplitude.

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Beam chopping For atomic absorption, beam chopping or

electrical modulation of the hollow-cathode lamp (pulsing it on and off) can distinguish the signal of the flame from the atomic line at the same wavelength.

Figure 20-21 shows light from the lamp being periodically blocked by a rotating chopper.

Signal reaching the detector while the beam is blocked must be from flame emission.

Signal reaching the detector when the beam is not blocked is from the lamp and the flame.

The difference between the two signals is the desired analytical signal.

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D2 lamp background correction Deuterium lamp background correction , broad

emission from a D2 lamp is passed through the flame in alternation with that from the hollow cathode.

The mono-chromator bandwidth is so wide that a negligible fraction of D2 radiation is absorbed by the analyte atomic absorption line.

Light from the hollow-cathode lamp is absorbed by analyte and absorbed and scattered by background.

Light from the D2 lamp is absorbed and scattered only by background.

The difference between absorbance measured with the hollow-cathode lamp and absorbance measured with the D2 lamp is the absorbance of analyte.

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Zeeman effect (pronounced ZAY-mon)

An excellent, but expensive, background correction technique

When a magnetic field is applied parallel to the light path through a furnace, the absorption (or emission) line of analyte atoms is split into three components.

Two are shifted to slightly lower and higher wavelengths (Figure 20-22), and one component is unshifted.

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Zeeman effect on Cofluorescence in a graphite furnace with excitation at 301 nm and detection at 341 nm. The magnetic field strength for the lowerspectrum is 1.2 tesla

The unshifted component does not have the correct electromagnetic polarization to absorb light traveling parallel to the magnetic field and is therefore “invisible.”

To use the Zeeman effect for background correction, a strong magnetic field is pulsed on and off.

Sample and background are observed when the field is off.

Background alone is observed when the field is on. The difference is the corrected signal. The advantage of Zeeman background correction is that it

operates at the analytical wavelength. In contrast, D2 background correction is made over a broad

band.

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Detection Limits` One definition of detection limit is the

concentration of an element that gives a signal equal to two times the peak-to-peak noise level of the baseline.

The baseline noise level should be measured for a blank sample.

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Measurement of peak-to peak noise level and signal level.

The signal is measured from its base at the midpoint of the noise along the slightly slanted baseline.

This sample exhibits a signal-to-noise ratio of 2.4.

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The detection limit for furnaces is typically two orders of magnitude lower than that observed with a flame and that for the inductively coupled plasma are intermediate between the flame and the furnace.

Comparison detection limits for flame, furnace, and ICM

Comparison of atomic analysis methods

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Analytical applications of AAS AAS is used for the determination of all metal and

metalloid elements. Nonmetals cannot be determined directly

because their most sensitive resonance lines are located in the vacuum UV region of the spectrum.

It is possible to determine some nonmetalsindirectly by taking advantage of the insolubility of some compounds. ◦ For example, chloride ion can be precipitated as

insoluble silver chloride by adding a known excess of silver ion in solution (as silver nitrate). ◦ The silver ion remaining in solution can be determined

by AAS and the chloride ion concentration calculated from the change in the silver ion concentration.

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Qualitative Analysis The radiation source used in AAS is an HCL or an

EDL, and a different lamp is needed for each element to be determined.

Because it is essentially a single-element technique, AAS is not well suited for qualitative analysis of unknowns.

To look for more than one element requires a significant amount of sample and is a time-consuming process.

For a sample of unknown composition, multielementtechniques such as XRF, ICP-MS and other atomic emission techniques are much more useful and efficient.

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Quantitative Analysis Quantitative measurement is one of the

ultimate objectives of analytical chemistry.

AAS is an excellent quantitative method.

It is deceptively easy to use, particularly when flame atomizers are utilized.

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