Chapter 7

Introduction to Atomic

Spectroscopy

Overview

• In atomic spectroscopic methods, the analyte must be converted into the appropriate chemical form to emit or absorb radiation.

• Almost always this involves converting the analyte into free atoms, although occasionally spectroscopic transitions of ions are used.

• Samples for atomic spectrochemical analysis may be in the form of liquids, solids, or gases. Most commonly, sample preparation steps produce an analytical sample that is a solution.

• Thus the sample presentation system has a complex task to perform in order to convert analyte species in solution into vapor phase free atoms.

• This usually entails the application of heat to break up molecules into their component atoms.

• The general routes for introducing solution samples into flame and plasma atomization devices are summarized in the Figure

Plasma and flame sample introduction schemes for solutions

Atomization Devices

• The sample container in which the spectroscopic measurements are made is usually a hot gas or an enclosed furnace.

• Flames, plasmas, electrical discharges (arcs and sparks), and electrically heated furnaces (electrothermal devices) are commonly used. – Flames are formed by combustion of an oxidant

and a fuel, – whereas plasmas are partially ionized gases

maintained either by an electrical discharge or by coupling to a microwave or RF field.

– An arc is a continuous electrical discharge between conducting electrodes, while a spark is an intermittent discharge.

• The process of forming free atoms by applying heat to a sample is known as atomization, and devices that carry out the atomization process are called atomizers.

• These devices can be continuous or pulsed (non-continuous) atomizers.

• With continuous atomizers such as flames or plasmas, the atomization conditions (e.g., temperature) are constant with time.

• With a non­continuous atomizer these conditions vary with time. Usually, electrothermal atomizers such as furnaces are used in a noncontinuous mode.

• The electrical power supplied for heating is varied so that atomization occurs when the temperature reaches a critical value.

• Spark discharges are also noncontinuous atomizers where conditions can change very rapidly (microseconds).

Sample Introduction

• The nature of the atomic population and hence the signals obtained in atomic spectrochemical methods depend on the type of atomizer employed and often the method of sample introduction.

• With continuous sample introduction the sample is constantly introduced in the form of droplets, a dry aerosol, or a vapor.

• A device called a nebulizer is used to convert the solution sample into a fine spray of droplets.

• Usually, continuous sample introduction is used only with continuous atomizers, in which case a steady-state atomic population is produced.

• Samples can also be introduced in fixed or discrete amounts to continuous atomizers, in which case a transient atomic population is produced.

• Discontinuous sampling is almost exclusively used with noncontinuous atomizers to produce a transient atomic vapor cloud.

• Discrete samples can be introduced into atomization devices in numerous ways.

– With electrothermal atomizers, a syringe is often used to transfer an aliquot of sample to the atomizer.

– A transient signal is obtained because the atomization conditions vary with time and because the fixed amount of sample is completely consumed during the measurement period.

– In direct insertion or probe techniques, the sample is placed on a probe (e.g., a carbon rod) and mechanically moved into the atomization region.

– The atomic vapor cloud produced is a transient because of the limited amount of sample.

• In flow injection techniques, a plug of the analytical sample solution in a carrier stream is introduced into the atomizer as a mist with the aid of a nebulizer.

• Dry aerosols and vapors can also be introduced as plugs of material.

• With vapor introduction (e.g., hydride techniques), the volatile analyte species is often stripped from the analytical solution and carried by a gas to the atomizer.

– This stripping step can be preceded by a specific chemical reaction that converts the analyte into a volatile form.

• It is now common to use gas and liquid chromatography to introduce samples into atomizers.

– Nebulizers are often used for liquid chromatographic eluates.

– Chromatographic introduction is a type of discontinuous sample introduction since the atomizer receives a time varying concentration of analyte as components elute from the column.

• With arcs and sparks, the sample introduction and atomization processes are more difficult to separate.

• In many arc and spark determinations, solid samples are employed.

• These are often shaped into the form of an electrode, and the discharge struck between the sample electrode and a second electrode.

• Alternatively, the sample electrode can be in the form of a cup into which powdered samples are packed, a porous cup, or a rotating disk for solutions.

• In any case the atomic signals produced are transient because the discharge conditions vary as the discrete amount of sample is atomized.

• In some cases one atomizer can be used as a sample introduction system for a second atomizer.

• This can be particularly advantageous in atomic emission, where the atomization device is usually called upon to excite the analyte as well as atomize it.

• Separating the Sampling/atomization step from the excitation step can allow optimization of the energy input for each step.

• As an example of this approach, samples can be introduced into flame or plasma excitation sources by means of electrothermal atomizers.

– Here the first atomizer converts the analyte into an atomic vapor, while the flame or plasma must now only supply the energy needed to excite the sample.

• In laser microprobe techniques, a laser beam is

directed onto a small portion of a solid sample.

• The sample is vaporized and atomized by radiative

heating.

• Either the plume of sample formed can be directly

probed by the encoding system or the vapor

produced can be swept into a second atomization

cell for observation of emission, absorption, or

fluorescence.

Processes Occurring During Atomization

• Let us consider the processes that must take place in order to transform a solution sample into an atomic vapor with continuous sample introduction into a continuous atomizer.

• As shown in the Figure, the sample introduction system disperses the sample into the high-temperature environment of a flame or plasma, usually as a fine spray or mist.

• This process is called nebulization.

• Heat from the flame or plasma evaporates the solvent and volatilizes the dry aerosol that remains

• Once free atoms are formed, they can be excited by collisions to produce characteristic spectral lines or an external radiation source can be used to obtain atomic absorption or fluorescence information.

• The atomizer heats the sample to the boiling point of the solvent

and a solid particle remains.

• The solid particle is then heated to a much higher tem­perature

to produce vapor phase species.

• With electrothermal atomizers the heating stages are separated

in time.

• The volatilization step is often carried out in two stages, one of

moderate temperature to drive off organic material (asking or

charing), and one of high temperature to vaporize the analyte

material (atomization).

Formation of atomic vapor with discrete sample introduc­tion

• With continuous sample introduction, a spray is created, while with discrete sample introduction, a fixed amount of sample is transported to the atomizer.

• With electrothermal atomizers,the various heating stages (desolvation and vaporization) are often separated in time by a temperature program, while with continuous atomizers these steps occur sequentially under the influence of heat from the atomization device.

• With arc and spark discharges and solution samples, these same processes occur, although it is difficult to separate the stages.

Nebulizers

• The formation of free atoms in flames and plasmas depends critically on the properties of the sample transport-nebulizer-atomizer system.

• The nebulization step and the remaining steps are all interactive, and the details of the processes that occur are very complex.

• The type of nebulizer used influences directly the efficiency of the nebulization, desolvation, and volatilization steps.

• We consider here the most common types of nebulizers employed in atomic specreometry

Pneumatic Nebulizers • The most commonly used nebulization devices

• They comprise a jet of compressed gas (the nebulization gas) aspirates and nebulizes the solution.

• Two common types :

– Concentric tube nebulizer

– Angular or cross flow nebulizer

• The transport of solution to the nebulizer tip is known as aspiration

• With the concentric nebulizer the nebulization gas flows through an opening that concentrically surrounds the capillary tube, causing a reduced pressure at the tip and thus suction of the sample solution from the container (Bernoulli effect).

• In the angular or cross-flow nebulizer, a flow of compressed gas over the sample capillary at right angles produces the same Bernoulli effect and aspirates the sample.

• In most cases the flow of solution is laminar, and the aspiration rate is proportional to the pressure drop along the capillary and to the fourth power of the capillary diameter; it is inversely proportional to the viscosity of the solution.

• The nebulizing gas flows

through an orifice that

surrounds the

sample-containing capillary

concentrically.

(b) In the angular or

crossed-flow nebulizer the

nebulizing gas flows over

the sample capillary at right

angles and causes

aspiration and nebulization

of the sample solution.

• Both pneumatic nebulizers

have a jet of compressed

gas aspirates and nebulizes

the solution.

• A spray chamber separates

large droplets from small

droplets.

• The latter are carried into

the plasma or flame, while

the former are drained away.

The nebulizer can be made

of glass or metal.

Pneumatic nebulizers

Frit Nebulizers

• A major disadvantage of conventional pneumatic nebulizers is the wide range of droplet diameters they produce.

• When only the small droplets are delivered to the atomization cell, the overall transport efficiency is reduced because of discrimination against the larger droplets.

• Some interference effects may also be related to the process of droplet size discrimination within the spray chamber.

• The glass frit nebulizer produces a much finer aerosol than the conventional pneumatic nebulizer.

• It is not as susceptible to clogging as a conventional nebulizer and has thus found some use with samples of a high salt or particulate content.

• This nebulizer may be advantageous where sample volumes are limited and where long nebulization times must be used (e.g., for some multielement determinations).

• The sample solution flows

over the surface of a fritted

glass disk, while nebulizing

gas is passed through the

many small holes in the disk.

• To reduce memory effects, a

wash solution is applied to the

frit between samples.

• The drain removes the excess

wash solution.

Glass frit nebulizer

Ultrasonic Nebulizers

• With these devices a piezoelectric crystal (a ceramic crystal transducer) is vibrated at ultrasonic frequencies (20 kHz to 5 MHz).

• Ultrasonic vibrations are coupled to the sample solution by a coupling liquid, by a velocity transformer, or by directly flowing the solution onto the vibrating surface.

• The vibrations cause the solution to break up into small droplets which are transported by the carrier gas through the nebulization chamber to the flame or plasma.

• In the vertical, direct-coupled design the sample is fed onto the surface of the vibrating crystal, where nebulization occurs.

• Such nebulizers can produce dense aerosols that are more homogeneous in droplet size than those produced by pneumatic nebulizers.

• Ultrasonic nebulizers have the advantage that the nebulizer parameters (frequency of vibration, power applied to the transducer) are independent of any flame or plasma gas flow rates so that separate optimizations can be made.

• The major limitation of ultrasonic nebulizers is their poor efficiency with viscous solutions and with solutions that have high particulate content.

• Solution is fed onto the surface of the piezoelectric crystal by

gravity flow or by a pump.

• Vibrations of the crystal cause the solution to break into small

droplets, which are transported by the carrier gas to the flame or

plasma.

Ultrasonic nebulizer

High solid nebulizers

• in pneumatic nebulizers have typical inside diameters of 0.1 mm and are thus subject to clogging when samples with a high particulate content are aspirated.

• The nebulizer designs has been found to be quite tolerant to such solutions.

• Both nebulizers are variations on the Babington design, which was originally developed for spraying paint.

• In both designs the solution is delivered through a tube of much larger inside diameter than those used with pneumatic nebulizers.

• Thus these nebulizers are essentially free from clogging, and they are excellent nebulizers when high particulates cannot be avoided.

• The Babington-type devices are, however, less efficient than pneumatic nebulizers in producing small droplets with normal samples.

• Because of this the processes of desolvation, volatilization, and atomization may be less efficient than with conventional nebulizers and some interference effects may be enhanced.

• In (a), the sample is fed by gravity or a pump and forms a film over

the outside surface of a sphere or rounded tip with a small orifice

for the nebulizing gas.

• The gas blows a hole through the film to produce the aerosol.

•In (b), the V-groove nebulizer the sample solution passes down a

V-groove with a small hole in the center for the nebulizing gas.

• Both nebulizers can be made of glass, metal or plastic materials such as Teflon.

Babington-type nebulizers

Free-Atom Formation after Nebulization

• Once the sample solution has been

nebulized and transported to the

atomization device, a variety of

complex processes begins:

– desolvation of the droplets,

– vaporization of the solid or molten

particles,

– dissociation of molecular species,

– ionization of analyte atoms,

Desolvation

• Desolvation leaves a dry aerosol of molten or solid particles and often begins in the nebulization chamber.

• The efficiency of the desolvation step is determined by several experimental variables, such as – the atomizer temperature,

– the trajectories,

– diameters and residence times of the droplets in the atomization cell,

– the nature of the solvent,

– and the design of the nebulizer.

• High-temperature atomizers increase desolvation efficiency, as do relatively low gas flow rates, which increase residence times.

• Organic solvents evaporate more rapidly than does water, and with flame atomizers, the desolvation can be accelerated by the heat of combustion of the vapor.

• Desolvation efficiencies can be quite high for pneumatic nebulizers with spray chambers that reject larger droplets.

• With the total consumption nebulizers, once popular for flame emission, all of the droplets may not reside in the high-temperature environment long enough for compete solvent evaporation, particularly with water as the solvent.

• When flames are used, water also tends to lower the flame temperature.

Volatilization

• The solid or molten particle remaining after desolvation must be vaporized to obtain free atoms.

• Incomplete volatilization leads to – a loss of analyte free atoms and thus a reduction in the

analytical signal.

– nonlinearity in analytical curves,

– continuous background emission from incandescent particles, and light scattering in the case of fluorescence measurements.

• The volatilization efficiency depends on a number of factors, including – the atomizer temperature,

– the composition of the analytical sample (nature and concentration of the analyte, solvent, and concomitants),

– the size distribution of the dry aerosol,

– the trajectories and residence times of the particles,

– and the type of nebulizer. For many metals, oxides are less volatile than the metal itself or salts of the metal.

• The bond dissociation energies of the

compounds that are formed with the analyte

are important in determining volatilization

efficiencies.

• With flames, high temperatures and a

reducing environment tend to increase the

volatilization efficiency and reduce the

formation of refractory oxides.

• The volatilization rate increases as the size

of the droplets introduced into the atomizer

decreases.

Dissociation and Ionization

• In the vapor phase the analyte can exist as free atoms, molecules, or ions.

• The formation of molecular species and ions reduces the concentration of free atoms and thus degrades the detection limit.

• Molecular species, such as CaO, CaOH, KCI, and LiOH, can be formed by reactions of analyte atoms with gaseous flame constituents or with volatilized species from the analytical sample.

• Ionic species, such as K+, Na+, CaOH+, and Ca+, are formed by the loss of an electron by an atom or molecule.In many cases we consider molecules to be in equilibrium with free atoms, at least in localized regions of the atomizer (flame or plasma).

• If chemical equilibrium exists, the law of mass action describes

the degree of dissociation of molecular species, and we can

write a dissociation constant for a molecular species (MX) into

its components

MX M + X

• where n is the number density (number per cm3) and the

subscript indicates the species.

• The dissociation constant for a diatomic molecule is a

function of temperature and the type of reactants according

to

Thus for BaOH at 2200 K is 2.5 x 1012 cm-3, while that for CaOH is

2.5 x 1013 cm-3.

• The ionization of metal atoms can also be

considered an equilibrium process in many flames

and plasmas at least in localized regions

M M++ e

• The ionization constant can be expressed :

where ne is the number density of free electrons.

• where the Zi’s are partition functions for the ion and metal and Eion is the ionization energy in eV.

• Small values of Eion and high temperatures favor the formation of ions.

• For example, if all electrons come from the ionization reaction, at moderate concentrations, K (Eion = 4.34 eV) is about 50% ionized at 2500 K, whereas Na (Eion = 5.14 eV) is only 7% ionized at the same temperature.

• Usually, only the first ionization needs to be considered in most flames and in the observation region of most plasmas

• Ionization constant, Ki is expressed as follows:

• The fraction atomized, also called the free atom fraction,a is a measurable quantity that describes

the loss of analyte atoms due to compound

formation and ionization. 1t can be expressed as

where nM is the number density of free metal

atoms and nT is the total number density of

all metal-containing species (e.g., M, MO,

MOH, MX, M+).

Ground-State Atom Density

• How do we express the analyte ground-state, free atom density (atoms per cm3) in terms of the analyte solution concentration c and the efficiencies of the var­ious processes that must occur.

• The number of analyte atoms that are aspirated per second is given by 10-3NFc, where N is Avogadro's number (atoms per mol), F is the solution flow rate (cm3 s-1), c is the analyte concentration (mol L-1), and 10-3 is a conversion factor (L cm-3).

• Only a fraction of the aspirated atoms pass through the observation zone per second as free atoms.

• This fraction is known as the overall atomization efficiency, a.

• It is a product of the nebulization efficiency en, the local desolvation efficiency n, the local desolvation efficiency, s

• The overall atomization efficiency, a is a product of the

– nebulization efficiency n,

– the local desolation efficiency efficiency, s

– The local volatilization efficiency

– The local free atom fraction a

Thus,

a = n s a

• The final equation for the number density of ground-state analyte atoms in the observation zone no is

go = Statistical weight of the ground

Electronic state

Q = Total gas flow rate-flames

ef = Expansion factor of gas

Z(T) = the internal partition function

golZ(T) = The fraction of analyte free atoms existing as ground-state atoms

Z(T) = gie-E

ilkT

Calibration Curves

• Based on the above we desire that no be linearly related to c.

• This linear relationship can occur only if factors such as F, Z(T), Ea, Q, and ef are constant and independent of analyte concentration.

• This implies indirectly that atomizer temperature, solution viscosity, surface tension, solvent composition, droplet trajectories, residence times, observation heights, and many other variables remain constant and independent of concentration.

• The variation of these factors with analyte concentration can be one cause of nonlinearity.

• At high analyte concentrations, nebulization efficiency may decrease because of solution viscosity and surface tension changes;

– this causes reduced atomization efficiency (decrease in n and thus a with increasing c) and downward curvature of the analytical curve.

• At higher analyte concentrations, negative deviations can also occur because s and decrease as the solids content of the droplets or the size of the dry aerosol particles increases.

• The free atom concentration can also vary with the

analyte concentration when the analyte is involved in

equilibria with, other species.

• For example, consider the atomization of an easily

ionized element; ions and atoms of same element for

example. This causes nonlinearity

• ionization suppressor may be added to avoid this

ionization equilibrium.

• Dissociation equilibria may also cause nonlinearity

Free-Atom Formation with Discrete Sample Introduction • With discrete sample introduction into a furnace, a

droplet of solution is deposited directly in the atomizer (usually a furnace).

• Hence there is no nebulization term in the expression for the overall atomization efficiency (a = s a)

• These factors are usually time dependent because of the discrete sample and the fact that the furnace is usually operated in a noncontinuous mode.

• In most cases the furnace is heated to the boiling point of the solvent for a time long enough that complete desolvation occurs

(s = 1).

• The expression for the instantaneous ground-state free atom density no(t) is similar to that given before except that F is replaced by V, the volume of solution delivered to the furnace.

• Note that most of variables are time dependent because the atomizer temperature varies with time and the amount of sample is limited.

• In many cases we expect no(t) to be linearly related to the analyte concentration [no(t) = kc].

• For this to be true, a , Q, ef, and Z(T) must be constant and independent of analyte concentration.

• This implies that measurements must be made at constant – temperature,

– time,

– flow rate of support gas,

– observation height,

– atomization efficiency, and so on.

Interferences in atomic spectroscopy

Blank interferences

• A blank or additive interference produces an uncompensated signal independent of the analyte concentration.

• In atomic spectroscopic methods these are usually spectral in nature, although nonspectral blank interferences are possible.

Atomic Emission Interferences

• Spectral in­terferences are most troublesome in emission spectroscopy because the selectivity is normally determined by a wavelength selection device (filter, monochromator, polychromator, etc.).

• Whether a spectral interference is observed or not depends on the composition of the sample and the technique used to excite emission.

• Because arcs, sparks, and plasmas have higher excitation energies than flames, spectral interferences are potentially more troublesome with these excitation sources.

• Atomic line interferences in emission methods can, in principle, be reduced by improving the spectral resolution of the wavelength selection device since true spectral line overlaps are rare.

• Thus high excitation energy emission sources normally use (and need) much higher resolution wavelength selection devices than flame

• Molecular band emission can occur with flame excitation when the sample contains combustible materials or constituents that give rise to molecular species.

• For example, high concentrations of Ca in the sample produce CaOH band emission, which can be a blank interference if it appears at the analyte wavelength.

• In contrast to atomic line interferences, the narrow atomic analyte line is superimposed on the broad emission band from the molecular species, and thus band emission interferences cannot be eliminated by improving the spectral resolution of the spectrometer.

• In flame emission the normal background emission from the flame itself is not usually an interference since the blank measurement can compensate for unchanging background.

• However, if a concomitant alters the background emission, it can cause a nonspectral blank interference.

Atomic Absorption Interferences

• Atomic line interferences are rarely observed in atomic absorption spectroscopy with narrow line sources since the source width effectively determines the spectral resolution.

• Hollow cathode lamps produce atomic lines with FWHM values on the order of 0.001 nm.

• In flame AA, for example, there is no interference of Na (285.28 nm) on the Mg 285.21-nm line because of this high selectivity.

• Line sources in AA, however, emit more than just the desired analyte line; lines from metallic impurities and filler gas

• Background emission is normally not a problem in AA because modulation and ac detection are used to discriminate against it.

• However, absorption by molecular species and scattering of the source radiation by nonvolatile salt particles or oxides can occur and can give rise to a blank interference.

• Absorption by molecular species is particularly a problem with electrothermal atomizers and in relatively cool flames.

• Scattering is also said to cause an interference in cool flames.

• At wavelengths shorter than 220 nm, the flame itself becomes absorbing.

Atomic Fluorescence Interferences

• Flame atomic fluorescence (AF) with line source excitation is potentially as free from spectral interferences as line source AA.

• Again continuum sources enhance the risk that concomitants will produce fluorescence within the bandpass of the wavelength selection device.

• In AF, normal background emission from the atomization system is usually compensated by modulation techniques.

• Resonance AF is particularly susceptible to interference from scattering of the source radiation by droplets and nonvolatilized salt particles.

Analyte Interferences

• Analyte or multiplicative interferences alter the magnitude of the analytical signal itself.

• In atomic spectroscopy they are invariably nonspectral interferences because of the narrowness of the line involved.

Nonspecific Interferences • Nonspecific interferences, often called physical

interferences, are fairly independent of the analyte type

• With nebulizers, nonspecific interferences can affect the aspiration, nebulization and desolvation processes.

• Concomitants that affect the sample solution viscosity, surface tension, and density can alter the solution flow rate and the nebulization efficiency.

• Sample constituents that change the evaporation rate of the solvent can affect the desolvation efficiency.

• With electrothermal atomizers, concomitants can affect desolvation rates, and, unless the atomizer temperature program is adjusted to ensure complete desolvation, this can be an interference.

• With arc excitation of solid samples, the matrix can exhibit a large influence on the rate of volatilization of the analyte

Specific Interferences • Specific interferences, often called chemical

interferences, are more analyte specific in that the magnitude of the effect depends strongly on the analyte.

• These interferences can occur in the conversion of the solid or molten particle that remains after desolvation into free, neutral, ground­state analyte atoms.

• Such interferences occur in emission, absorption, or fluorescence measurements.

• Concomitants in the sample can influence the volatilization of the particles containing the analyte and these solute volatilization interferences can be analyte specific.

• For example, in some flames the presence of phosphate in the sample can alter the atomic concentration of calcium in the flame.

• The interference is attributed to the formation of relatively involatile complexes between the metal and phosphate.

• Concomitants in the sample can influence the degree of dissociation of the analyte and cause analyte-specific dissociation interferences.

– For example, the presence of HCl in the sample can affect the dissociation of NaCl and KCl to a different extent because the dissociation energies of the two compounds are different

• Another gas-phase interference is an ionization interference which occurs when a concomitant alters the degree of ionization of the analyte.

• The presence of an easily ionized element, such as K, can affect the degree of ionization of a less easily ionized element such as Ca by increasing the electron concentration.

Selection Rules and Atomic Spectra

• Only a fraction of the total possible number of transitions

between states are observed in the spectrum in practice.

• Allowed transitions are those that occur with high probability

and give reasonably intense lines.

• For­bidden transitions, on the other hand, are those that occur

with low probability and are thus weak in intensity.

• In general,

Energy level diagram for Na and Ca

Allowed transitions

Additional Splitting Effects

• An additional splitting of terms can occur because of magnetic coupling of the spin and orbital motion of the electrons in atoms with the nuclear spin.

• Such splitting results in hyperfine structure. • For Na the 2S1/2 ground level is split into two close

levels, which leads to additional splitting of each D line.

• For Na, the hyperfine splitting is on the order of 0.02 A and not readily detected.

• Other atoms, such as Cu and In, exhibit larger hyperfine splittings that are important in analytical applications

• Other splitting include: – Zeeman effect – Stark effect

Spectral Line Profile

• When atomic emission, absorption, or, fluorescence spectra are recorded, narrow spectral lines are obtained.

• With ordinary spectrometers, the widths of the lines obtained are determined not by the atomic system, but by the properties of the spectrometer employed (slit function and spectral bandpass).

• The finite widths obtained are the result of a variety of line-broadening phenomena.

• These processes give rise to a spectral distribution or spectral profile of photons.

Spectral Line width

• Narrow line desirable for absorption and emission work to

reduce possibility of interference due to overlapping spectra.

• Theoretically atomic lines should have a zero line width but

this does not exist

• The natural line should have a width of 10-5 nm

• Experimentally: spectral lines have definite width and characteristic form

• Actual line width is ~ 10-3 nm. That is the energy emitted in a spectral line is spread over a narrow wavelength range reaching a maximum at o, for

example:

Element o , nm

Ca 422.7 0.0032

Ag 328.1 0.0016

Mn 403.1 0.0026

Cs 455.5 0.0030

• Why do we study the line profile?

Resolution is limited by the finite width of the lines

Line Broadening

1) Uncertainty Effect

– due to finite lifetime of transition states. (10-4 A)

2) Doppler Broadening

– atoms moving toward radiation absorb at higher

frequencies; atoms moving away from radiation

absorb at lower frequencies.

3) Pressure Effects – due to collisions between analyte

atoms with foreign atoms (like from fuel).

4) Electric and Magnetic Field Effects.

The largest two problems:

1. Doppler broadening

2. Pressure broadening

Lifetime Broadening

• Consider a two-level atomic system undergoing the various radiational and nonradiational processes shown

• Because of emission and absorption from the radiation field and collisional processes, states j and i have finite lifetimes, and this gives rise to uncertainties in the energies of both states according to the Heisen­berg uncertainty principle.

• Because photons emitted or absorbed by the atomic system have frequencies determined by = Ej - Ei/h, uncertainties in the energies of the states give rise to a frequency distribution of photons.

• The intensity line width or just half width I may expressed as follows:

Components of lifetime broadening

Natural Broadening: • It is represented by the A/2 term in the previous

equation

• It results from the natural or radiative lifetime r,

given by

Collisional Broadening

• The second term in the previous equation (kjl2) results from collisional deactivation of the excited state.

• Collisions which leave the atom in a different energy level are called diabatic collisions.

• In addition to such state-changing collisions, there can also be collisions which leave the atom in the same energy level. Such collisions are termed adiabatic collisions.

• The amount of broadening caused by collisions increases with the concentration of collision partners (perturbers).

• Consequently, collision broadening is sometimes called pressure broadening.

• Two types of collision partners can be distinguished:

1. When collisions are between atom and partners that are a different species (atoms, molecules, or ions), the broadening is called foreign gas broadening or Lorentz broadening.

2. When collisions are between like atoms, the broadening is called Holtzmark broadening or resonance broadening.

•

Doppler Broadening

• It results because of a statistical distribution

of velocities of the emitting or absorbing

atoms along the observation path.

• Because atoms are in motion with respect to

the observation line, the Doppler effect

causes a statistical distribution in the

frequencies observed that is directly related

to the velocity distribution.

Other Causes of Line Broadening Stark broadening

• results from perturbations of the atomic system by ions, electrons, or molecules with a permanent dipole moment.

• It is generally negligible in flames, but can be a significant broadening source in sparks and plasmas with a high amount of ionization.

Radiation or power broadening

• It can occur when strong radiation fields (i.e., high-powered lasers) are applied to an atomic system.

• High rates of induced absorption and emission lead to a reduction in the radiative lifetime which is a function of the energy density of the field

Saturation broadening

• An apparent broadening occurs when a high-intensity laser is scanned across an atomic absorption line.

• This is not the result of change in the profile itself, but the result of a decrease in the population difference between the two levels under strong fields when the laser is tuned to the line center.

• Because the fluorescence or absorption increases less near the line center than in the wings with increasing laser power, an apparent broadening is seen under conditions of saturation.

Self Absorption and Self Reversal

of Spectral lines

• When sample concentration increases, there would be an increase in the possibility that the photons emitted from the hot central region of the flam collide with the atoms in the cooler outer region of the flame and thus be absorbed

• Curvature of the calibration curves at high concentrations would be observed

• The effect is minimal with ICP until very high atom concentrations are reached

• Intensity of emission

line is reduced

• half-width is increased

Self reversal

• It occurs when emission line is broader

than the corresponding absorption line

• Resonance line suffers the greatest self

absorption

• The center of the resonance line will be

affected more than the edges

• The extreme case of self absorption is:

self reversal

Effect of self absorption and self reversal on

measurements

• When self absorption occurs

– Line intensity interpretation becomes

difficult and inaccurate

– Thus non-resonance line is selected for

analysis or concentration is reduced

Choice Of Absorption Line

• Resonance line is the best

• Resonance line is always more intense than other lines i.e., more sensitive for analysis

• Resonance line is used always for small concentrations

• Most elements require from 6-9 electron volts for ionization to occur. 1ev = 1.6X10-19 J. Thus using appropriate excitation, the spectra of all metals can be obtained simultaneously

1/T5

Distribution of atomic population:

Effect of Temperature on Atomic Spectra

Nj Pj ---- = ---- exp -(Ej/kT)

No Po

where Nj => # atoms excited state

No =># atoms ground state

k => Boltzmann constant

Pj & Po => statistical factors determined by

# of states having equal energy at

each quantum level

Ej =>energy difference between energy

levels

for Ca atoms: Pj/Po = 3

Ej = 2.93 ev for 4227 D line

(a) 2000 K

Nj (2.93 ev)(1 erg/6.24 X 1011 ev) --- = 3 exp - -------------------------------------- No (1.38 X 10-16 erg/K)(2000 K)

= 1.23 X 10-7

The ratio of Ca atoms in the excited state to ground state at

(a) 2000 K and (b) 3000 K.

(b) 3000 K Nj (2.93 ev)(1 erg/6.24 X 1011 ev) --- = 3 exp - ---------------------------------- No (1.38 X 10-16 erg/K)(3000 K)

= 3.56 X 10-5

% Increase in the excited atoms =

(3.56X10-5 – 1.23X10-7)/1.23X10-7 = 288 time

• Atomic population of Na atoms for the

transition 3s 3p

Nexcited / Nground = 1X 10-5 = 0.001%

• That is 0.001% of Na atoms are thermally excited

• Thus 99.999% of Na atoms are in the ground state

• Atomic emission uses Excited atoms

• Atomic absorption uses Ground state atoms

Effect of temperature on the

atomic population

•The energy source must have a stable temperature since

this factor affects the number of free atoms drastically

Conclusions about atomic population

• Number of excited atoms is very temperature dependent. Temp. should be carefully controlled

• Number of ground state atoms is insensitive to temp.; but subject to flame chemistry which is dependent on temp., as well as type of the flame

• Most atoms are in ground state (resonance state). Resonance absorption lines are the most sensitive

• The fraction of excited atoms is very dependent on the nature of element and temperature

• Which of the two techniques (AA or AE) is more sensitive? Why?

• Are all elements, including

nonmetals, accessible on the AA or

FE instruments?

• Yes for metals; no for nonmetals

• Excitation of nonmetals e.g., Noble

gases, Hydrogen, Halogens, C, N, O,

P, S necessitates the application of

special techniques

Ionic spectra versus atomic spectra

• Spectra of excited atoms differ from those of excited ions of the same atoms

• Spectrum of singly ionized atom is similar to the atomic spectrum of the element having an atomic number of one less e.g.: – spectrum of Mg + is similar to that of Na atom

– spectrum of Al+ is similar to that of Mg atom

• Ionic spectra contain more lines than atomic spectra; however the intensity of ionic spectra is much less than that of atomic spectra