Chapter 8: Introduction to Atomic SpectrometryChapter 8: Introduction to Atomic SpectrometryRead: pp. 215 – 228 Problems: 2,4,5,6,9

• Why choose atomic spectrometry?

• Three major types of spectrometric methods for identifying elements present in matter: – optical spectrometry (Chap. 8, 9, 10)– mass spectrometry (Chap. 11)– x-ray spectrometry (Chap. 12)

• In optical spectrometry, the elements present in a sample are converted to gaseous atoms or elementary ions and then analyzed by optical methods.

Excellent series of methods for determining elemental composition in environmental samples, foods and drinks, potable water, biological fluids, and materials.

Notice high resolution!Notice high resolution!

Wavelength (nm)

403.31

403.08

403.45

Mn

Atomic SpectraAtomic Spectra

• Outer shell or valence electrons are promoted to unoccupied atomic orbitals by incident radiation.

E = hν = hc/λ

• Small energy differences between the different transitions – so high resolution instruments are needed.

• Transitions are observed only between certain energy states.

Na

Figure 8-1a

Mg+Na

Figure 8-1

Mg

Figures 8-2 and 8-3

Chemical ProblemChemical Problem

The first excited state of Mg is reached by absorption of 457.1 nm light. Calculate the energy difference (kJ/mol) between the ground and excited states.

E = hν = hc/λ

(6.62 x 10-34 J s)(3.00 x 108 m/s)(457.1 nm)(1.00 x 10-9 m/nm)

E = = 4.34 x 10-19 J/photon

(4.34 x 10-19 J/photon)(6.02 x 1023 photons/mol) = 2.62 x 105 J/mol

(2.62 x 105 J/mol) (1 kJ/1000 J) = 262 kJ/mol

Atomic Line WidthsAtomic Line Widths

Sources of Line Broadening

1. Uncertainty effect

2. Doppler effect

3. Pressure or collisionaleffects

4. Electric and magnetic field effects

Figure 8-6

Uncertainty EffectUncertainty Effect

• Spectral lines have finite widths because lifetimes of one or both transition states are finite, which leads to uncertainties in transition times.

∆ν • ∆t ≥ 1

• To know ∆ν with high accuracy, then time of measurement, ∆t, must be very long!

• Lifetime of ground state is long, but lifetime of excited state is short, 10-8 s.

• Line widths due to uncertainty broadening, sometimes called natural line widths, are on the order of 10-4 Å.

Doppler BroadeningDoppler Broadening

∆λ/λo = v/c v = velocity of moving atom

Encounter wave crests more frequently Encounter wave crests less frequently

None MixedMaximum

Wavelength of radiation emitted or absorbed by rapidly moving atom decreases if motion is toward the detector and increases if motion is away from the detector. Broadening on the order of 10-2 to 10-1 Å.

Pressure BroadeningPressure Broadening

• Broadening arises from collisions of the absorbing or emitting species with other atoms or ions in the heated medium.

• Collisions cause small changes in the ground state energy levels and, thus, a range of absorbed or emitted wavelengths.

• Broadening on the order of 10-1 Å.

Temperature EffectsTemperature Effects

• Temperature influences velocity and, thus, the extent of Doppler broadening and pressure broadening.

• Temperature also influences the number of atoms in the ground (N0) and excited (Nj) states

where g0 and gj are statistical factors and Ej is the energy difference between states.

• So temperature should be maintained constant, as much as possible.

⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

TkE

expgg

NN j

0

j

0

j

Least effect on atomic absorbance and fluorescence measurements, because fraction of atoms in ground state is large.

Greatest effect on atomic emission measurements, because fraction in excited state is small.

Sample Introduction MethodsSample Introduction Methods

• Goal is to produce neutral atoms or simple ions in the gas phase

• Sample may be gas, liquid/solution, slurry, or solid

• Sample introduction may be continuous or discrete

• Great challenge – generally limits accuracy, precision, and detection limits of atomic spectrometric methods

Steps in Sample Introduction ProcessSteps in Sample Introduction Process

Several methods for introduction of liquid samples, main one is pneumatic nebulization:

Figure 8-10

Pneumatic Pneumatic NebulizerNebulizer DesignsDesigns

Cross-FlowConcentric Tube

Figure 8-11

Graphite Tube FurnaceGraphite Tube Furnace

• Liquid or solid samples

• Smaller sample volumes

• More efficient atomization

• Lower detection limits

• Poorer precision

• Slow – requires heat cyclesto dry, ash, atomize

Figure 9-6a

hν

Sample