Chapter 8Chapter 8

An IntroductionAn Introduction

toto Optical Atomic Optical Atomic SpectroscopySpectroscopy

► Three major types of spectrometric methods are usedto identify the elements present in samples of matterand determine their concentrations: (1) optical

spectrometry,(2) mass spectrometry, and (3) X-ray spectrometry.In optical spectrometry. discussed in thischapter. the elements present in a sample are convertedto gaseous atoms or elementary ions by a processcalled atomization. The ultraviolet-visible absorption,emission, or fluorescence of the atomic speciesin the vapor is then measured.

Atomic Spectroscopy

Atomic spectroscopy is based on absorption, emission, or fluorescence by atoms or elementary ions.

Atomic spectra are obtained by measuring gaseous atoms or elementary ions that are converted from the components of a sample by a suitable heat treatment.

Sources of atomic spectra:

The emission, absorption and fluorescence spectra of gaseous atomic particles (atoms or ions) consist of well-defined narrow lines arising from electron transitions of the outmost electrons.

Optical Optical AtomicAtomic Spectroscopy Spectroscopy

►Optical SpectrometryOptical Spectrometry AbsorptionAbsorption EmissionEmission FluorescenceFluorescence

►Mass SpectrometryMass Spectrometry►X-Ray SpectrometryX-Ray Spectrometry

8A8A OPTICAL ATOMIC OPTICAL ATOMIC SPECTRASPECTRA

►8A-1 Energy Level DiagramsThe energy level diagram for the outer electrons of anelement is a convenient method to describe the

processesbehind the various methods of atomic spectroscopy.The diagram for sodium shown in Figure 8-1ais typical. Notice that the energy scale is linear in unitsof electron volts (e V). with the 35 orbital assigned avalue of zero. The scale extends to about 5.l4eV. the

energyrequired to remove the single 35 electron to producea sodium ion, the ionization energy.

FIGURE 8-2 Energy level diagram for atomic magnesium. The width of the lines between statesindicates the relative line intensities. Note that a singlet-to-triplet transition is considerably lessprobable than a singlet-to-singlet transition. Wavelengths are presented in angstroms.

Singlet ground state Triplet excited stateSinglet excited state

Spins are pairedNo split

Spins are unpairedEnergy splitting

3p

3s3s

3p

3s

FIGURE 8-3 Spin orientations in singlet ground and excited states and triplet excited state.

Atomic spectroscopyAtomic spectroscopy

► EmissionEmission

► AbsorptionAbsorption

► FluorescenceFluorescence

►Atomic Emission Spectra

at room temperature, all atoms of a sample are in the ground state; electrons can be excited by heat or electric arc or spark (nonradiative); the lifetime of the excited state is short; its return to the ground state is accompanied by the emission of a photon of radiation.

FIGURE 8-4 A portion of the flame emission spectrum for sodium.

Atomic Absorption Spectra:

absorption spectrum typically consists predominantly of resonance lines, which are the result of transitions from the ground state to upper levels.

FIGURE 8-5 Energy level diagram for thallium showing the source of two fluorescence lines.

Atomic Fluorescence Spectra:

atoms in a flame can be made to fluoresce by irradiation with an intense source containing wavelengths that are absorbed by the element. The observed radiation is most commonly the result of resonance fluorescence; first radiation less transition to a lower energy state, thenfluoresce from there to a lower energy state. Thus a longerwavelength than the resonance line will be observed.

FIGURE 8-6 Profile of an atomic line showing definition of the effective line width 1/2.

Line Broadening from the Uncertainty Effect

spectral lines always have finite widths because the lifetimes of one or both transition states are finite, which leads to uncertainties in the transition times and to line broadening as a consequence of the uncertainty principle;

when the lifetime of the two states approaches infinity, then the breadth of an atomic line resulting from a transition between the two states would approach zero.

Doppler Broadening

The l of radiation emitted or absorbed by a fast moving atom decreases if the motion is toward a detector andincreases if the atom is receding from the detector.

magnitude: increases with the velocity at which the emitting or absorbing species approaches or leaves the detector;

FIGURE 8-7 Cause of Doppler broadening. (a) Atom moving toward incoming radiation sees wave crests more frequently and thus absorbs radiation that is actually higher in frequency. (b) Atom moving with the direction of radiation encounters wave crests less often and thus absorbs radiation that is actually of lower frequency.

►Pressure Broadening

Pressure, or collisional, broadening is caused by collisions of the emitting or absorbing species with other atoms or ions in the healed medium.

These collisions produce small changes in energy levels

and hence a range of absorbed or emitted wavelengths.

►8A-3 The Effect of Temperature on Atomic Spectra

Temperature has a profound effect on the ratio betweenthe number of excited and unexcited atomicparticles in an atomizer. We calculate the magnitude ofthis effect from the Boltzmann e4uation, which takesthe form

)exp(00 kT

E

g

g

N

N jjj

Here. Nj and No are the number of atoms in an excited

state and the ground state. respectively. k is Boltzmann's

constant (1.38 x 10-23 J/K), T is the absolute

temperature. and Ej is the energy difference between

the excited state and the ground state, The quantities

gj and go are statistical factors called statistical weights

determined by the number of states having equal energy

at each quantum level. Example 8-2 illustrates a

calculation of Nj/No,

Effect of Temperature on Atomic Spectra

Temperature exerts a profound effect upon the ratio between the number of excited and unexcited atomic particles in an atomizer;

Boltzmann equation: Nj/N0 = Pj/P0 exp(Ej/kT)

Pj, P0 are statistical factors that are determined by the number of states having equal energy at each quantum level.

►8A-4 Band and Continuum Spectra Associated with Atomic Spectra

8B ATOMIZATION METHODS

► To obtain both atomic optical and atomic mass spectra,the const it uents of a sample must he converted togaseous atoms or ions. which can then be determinedby emission, absorption, fluorescence, or mass spectralmeasurements. The precision and accuracy of atomicmethods depend critically on the atomization step andthe method of introduction of the sample into the

atomizationregion.

8C SAMPLE-INTRODUCTION METHODS

► Sample introduction has been called the Achilles' heel

of atomic spectroscopy because in many cases this step limits the accuracy.

►8C-1 Introduction of Solution Samples

►Pneumatic Nebulizers►Ultrasonic Nebulizers►Electrothermal Vaporizers►Hydride Generation Techniques

FIGURE 8-9 Types of pneumatic nebulizers: (a) concentric tube, (b) cross-flow, (c) frilled disk, (d) Babington.

Concentric Tube

Cross-flow

Fritted-disk

Line BroadeningLine Broadening ►Uncertainty EffectsUncertainty Effects

Heisenberg uncertainty principle: Heisenberg uncertainty principle: TheThe naturenature of the matter places limits on of the matter places limits on thethe precision with which certain pairs of precision with which certain pairs of physicalphysical measurements can be made.measurements can be made.

One of the important formsOne of the important forms Heisenberg Heisenberg uncertainty principle:uncertainty principle:

tt ≥ 1 ≥ 1 p156p156 To determineTo determine with negligibly small uncertainty, a huge with negligibly small uncertainty, a huge

measurement time is required.measurement time is required.

►Natural line widthNatural line width

Babington

►8C-2 Introduction of Solid Samples

►Direct Sample Insertion►Electrothermal Vaporizers►Arc and Spark Ablation►Laser Ablation►The Glow-Discharge Technique

FIGURE 8-10 A glow-discharge atomizer. (From D. S. Gough, P. Hanna ford, and R. M. Lowe 61 , Anal. Chem., 1989,1652.