OPTICAL ATOMIC SPECTROMETRYOPTICAL ATOMIC SPECTROMETRY

Chap 8Chap 8

Three major typesThree major types

Optical spectrometryOptical spectrometry

Mass spectrometryMass spectrometry

(X-ray spectrometry)(X-ray spectrometry)

In all three, atoms or ions are In all three, atoms or ions are atomizedatomized

ENERGY LEVEL DIAGRAMS

Sodium “D-lines” at Sodium “D-lines” at 589.6 and 590.0 nm589.6 and 590.0 nm

SHC, 6e, Fig. 8-1 (a)SHC, 6e, Fig. 8-1 (a)

Electronic States

SHC, 6e, Fig. 8-3

Ground state

Singletof Mg atom

3s For a moleculeFor a molecule

Absorption, emission, and fluorescence by atoms in a flame.

Profile of an Atomic LinewidthSHC, 6e, Fig. 8-6

Signal Δλ1/2 = FWHM

λ0

FWHM FWHM ≡ ≡ Full Width at Half MaximumFull Width at Half Maximum

SOURCES OF LINE BROADENINGSOURCES OF LINE BROADENING(in order of increasing effect)(in order of increasing effect)

(1)(1) Uncertainty Effect (Natural Linewidths)Uncertainty Effect (Natural Linewidths)

ΔΔνν · · ΔΔt > 1t > 1

Because excited state lifetimes (t) are Because excited state lifetimes (t) are brief (brief (∼ns – ∼ns – μμs), the uncertainty (s), the uncertainty (ΔΔt) is t) is

small and small and ∴ ∴ ΔΔνν is relatively large is relatively large

(2)(2) Doppler BroadeningDoppler Broadening

Wavelength shift caused by motion of Wavelength shift caused by motion of atoms relative to detectoratoms relative to detector

The Doppler Effect

Analogous to SHC, 6e, Fig. 8-7

Blue shift

Red shift

(3)(3) Pressure Broadening (Collisional Broadening)Pressure Broadening (Collisional Broadening)

Collisions cause small changes in ground Collisions cause small changes in ground state energy levels (i.e., smearing)state energy levels (i.e., smearing)

ΔλΔλ1/21/2 >> >> ΔλΔλ1/21/2 of isolated atom of isolated atom

At high pressures At high pressures continuum radiation continuum radiation

e.g., high-pressure Hg and Xe lampse.g., high-pressure Hg and Xe lamps

TEMPERATURE EFFECT ON ATOMIC SPECTRATEMPERATURE EFFECT ON ATOMIC SPECTRA

Effect described by Boltzmann distributionEffect described by Boltzmann distribution

kT

E

P

P

N

N jjj exp00

j

0 ooooooooooooooooooo

oooo o o

P ≡ degeneracy of level

TEMPERATURE EFFECT ON ATOMIC SPECTRATEMPERATURE EFFECT ON ATOMIC SPECTRA

At low TAt low Tj

0 ooooooooooooooooooo

ooo

At high TAt high T

0

j

ooooooooooooo

ooooooooo

SAMPLE INTRODUCTION METHODSSAMPLE INTRODUCTION METHODS

Common Types of AtomizersCommon Types of Atomizers (from SHC, 6e, Table 8-1) (from SHC, 6e, Table 8-1)

FlameFlame 1700 – 3100 °C1700 – 3100 °C

Electrothermal (“furnace”)Electrothermal (“furnace”) 1200 – 3000 1200 – 3000 °C°C

Inductively coupled plasma (ICP)Inductively coupled plasma (ICP) 4000 – 6000 4000 – 6000 °C°C

Electric arc Electric arc 4000 – 5000 °C4000 – 5000 °C

(e.g., Vreeland spectroscope)(e.g., Vreeland spectroscope)

SHC, 6e, Fig. 8-6SHC, 6e, Fig. 8-6

Processes leading to sample atomizationProcesses leading to sample atomization

INTRODUCTION OF SOLUTION SAMPLESINTRODUCTION OF SOLUTION SAMPLES

Nebulization: sample is aspirated (“sucked in”) and Nebulization: sample is aspirated (“sucked in”) and converted to fine mist or aerosolconverted to fine mist or aerosol

PneumaticPneumatic

Ultrasonic Ultrasonic

Hydride generation (for species of low volatility, e.g., Hydride generation (for species of low volatility, e.g., Hg, Pb, Se, Sb, etc.)Hg, Pb, Se, Sb, etc.)

INTRODUCTION OF SOLID SAMPLESINTRODUCTION OF SOLID SAMPLES

Electrothermal (introduces Electrothermal (introduces andand vaporizes sample) vaporizes sample)

Arc AblationArc Ablation

SHC, 6e, Fig. 8-11 Pneumatic nebulizersSHC, 6e, Fig. 8-11 Pneumatic nebulizers