to Optical Atomic Spectroscopy Chapter 8 An Introduction to Optical Atomic Spectroscopy

Three major types of spectrometric methods are used to identify the elements present in samples of matter and determine their concentrations: (1) optical spectrometry, (2) mass spectrometry, and (3) X-ray spectrometry. In optical spectrometry. discussed in this chapter. the elements present in a sample are converted to gaseous atoms or elementary ions by a process called atomization. The ultraviolet-visible absorption, emission, or fluorescence of the atomic species in the vapor is then measured.

Sources of atomic spectra: 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 Atomic Spectroscopy Optical Spectrometry Absorption Emission Fluorescence Mass Spectrometry X-Ray Spectrometry

8A OPTICAL ATOMIC SPECTRA 8A-1 Energy Level Diagrams The energy level diagram for the outer electrons of an element is a convenient method to describe the processes behind the various methods of atomic spectroscopy. The diagram for sodium shown in Figure 8-1a is typical. Notice that the energy scale is linear in units of electron volts (e V). with the 35 orbital assigned a value of zero. The scale extends to about 5.l4eV. the energy required to remove the single 35 electron to produce a sodium ion, the ionization energy.

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

Spins are paired No split Spins are unpaired Energy splitting 3p 3p 3s 3s 3s Singlet ground state Singlet excited state Triplet excited state FIGURE 8-3 Spin orientations in singlet ground and excited states and triplet excited state.

Atomic spectroscopy Emission Absorption Fluorescence

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, then fluoresce from there to a lower energy state. Thus a longer wavelength 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 and increases 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 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 between the number of excited and unexcited atomic particles in an atomizer. We calculate the magnitude of this effect from the Boltzmann e4uation, which takes the form

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 to gaseous atoms or ions. which can then be determined by emission, absorption, fluorescence, or mass spectral measurements. The precision and accuracy of atomic methods depend critically on the atomization step and the method of introduction of the sample into the atomization region.

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 Broadening Uncertainty Effects Natural line width Heisenberg uncertainty principle: The nature of the matter places limits on the precision with which certain pairs of physical measurements can be made. One of the important forms Heisenberg uncertainty principle: t ≥ 1 p156 To determine  with negligibly small uncertainty, a huge measurement time is required. Natural 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 FIGURE 8-10 A glow-discharge atomizer. (From D. S. Gough, P. Hanna ford, and R. M. Lowe 61 , Anal. Chem., 1989,1652.