Atomic spectroscopy
The excitation process is similar as for the molecular spectroscopy, but in case of atoms there
are no vibrational, rotational transitions, only electrons can be excited. Therefore the atomic
spectrum includes lines (not bands):
The electron transition corresponds to the line of Na
589.0 nm: 31s1/2 → 32p1/2
Methods of atomic spectroscopy:
resonant line: the lower state of the transition is the
• atomic absorption spectroscopy ground state (intensive)
• atomic emission spectroscopy
• atomic fluorescence spectroscopy
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Atomic spectrum
Fraunhofer (1815):
atomic spectral lines in the spectrum
of the sun
self-absorption: emission from the central region is
absorbed in the outer region. The free atoms in the light
source absorb the the light emitted by the source.
Bunsen, Kirchhoff (1860) the birth of spectrochemistry
spectral lines of hydrogen
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Atomic spectrum
continuous spectrum
glowing solid body spectrum of emission lines
or high density gas
glowing low density gas (high T) spectrum of absorption lines
low density gas (smaller T)
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Atomic spectrum
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Spectral lines
the atomic absorption spectral lines are very narrow: ~10-4 nm
Linewidth is governed by the Heisenberg uncertainty principle (the shorter the
lifetime (t) of the excited state, the more uncertain is its energy (dE/dt ≥ h/4p).
The lifetime of an excited state of an isolated gaseous atom is ~10-9 s.
Two major effects boaden the lines to 10-3-10-2 nm:
Doppler effects: a molecule moving toward the radiation source "feels" the
elctromagnetic field oscillate more often than one moving away from the source
T
D 7.16 10 7 0
M
Lorentz (pressure) broadening: due to collision between atoms.
Collisions shorten the lifetime of the excited state 5
Atomization, ionization
n= nm + na + ni (nm: atoms bonded in the molecule, na: free atoms, ni: ions in gas phase)
Degree of atomization: na/n
Effect of the temperature
N * g * E / kT
Bolzmann distribution: e
N0 g0
N: population (*excited, 0 ground states)
atomization degree
g: number of stets at tha given energy
(degeneracy)
k: Boltzmann's constant
Atomic absorption is not as sensitive to
temperature (2000-3000 K) as atomic
emission, which is exponentially sensitive
to temperature. temperature
ionization buffer
Atomization curves of Na and Si
reducing (fuel-rich) flame
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Schematic arrangement of the atomic spectrometers
Atomic emission
monochromator display
atomizer detector
Atomic absorption
light source
monochromator
atomizer detector display
Atomic fluorescence
atomizer
monochromator display
detector
light source
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Methods of atomic emission spectrometry
• Spark and arc atomic emission spectroscopy
(spectrography)
used for the analysis of metallic elements in solid
samples, now it is rarely applied.
• Flame atomic emission spectrometry (FAES)
flame can be used for the most easily excitable elements (alkali, alkaline earth metals)
simple, cheap but not very sensitive
I kC m
Scheibe-Lomakin
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flame of a Bunsen-burner
Sample introduction with nebulization
goal:
forming monodisperse aerosol with small particles
pneumatic or ultrasonic nebulizers (h ~1-10 %)
the glass bead, baffles, chamber cut off or break the
larger droplets (modify the droplet size distribution)
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Emission spectrum of flames
Emission spectrum of a salty solution nebulized into acetylene/air flame
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Methods of atomic emission spectrometry
• Inductively coupled plasma (ICP)
high temperature source
RF field frequency: 5-75 MHz, U=2-15 KV, P= 2 kW
plasma temperature: 6000-1000K
large amount of Ar gas is needed (carrier, cooling, plasma gas)
ICP-AES: complete atomization at high temperature
simultaneous detection of 70 elements
atom/ion spectra
small background radiation
ICP-MS:
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Atomic absorption spectrometry (AAS)
Hollow-cathode lamp
The linewidth of the source must be narrower
than the linewidth of the atomic vapor.
Monochromators generally cannot isolate lines
narrower than 10-3 nm.
~500 V are applied between the anode
and cathode, gas is ionized and positive
ions are accelerated toward the cathode.
Ne/He Cations strike the cathode to sputter
metal atoms from the cathode into the
gas phase. gaseous atoms excited by
collisions with high-energy electrons emit
photons.
includes the element 12
to be determined
Atomic absorption spectrometry (AAS)
• Flame AAS
the monochromator and detector the same as in spectrophotometry
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Atomic absorption spectrometry (AAS)
• Graphit furnace AAS
electrically heated, inert atmosphera (Ar)
steps of the programmed heating:
drying, charring/pyrolysis, atomization, cleaning
advantages: char
sensitive dry
small sample solution is required (10-50 µL) atomize
direct analysis of solid samples
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Detection limits of atomic spectrometric methods
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Interferences in atomic spectometry
• Spectral (additive)
• Physical (proportional)
• Chemical (exponential): in condensed or gaseous phase
(eg. sulphate forms nonvolatile compoud with Ca2+)
releasing agents (eg. EDTA protect the Ca2+ )
Background correction:
• signals on either side of a peak is substracted
• chopping of the analytical beam/modulation
• deuterium lamp
• utilising Zeeman effect
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Quantitative analysis
• external calibration
• standard addition
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Speciation analysis: identification and determination of element species
Species (Specific form of an element defined as to isotopic composition, electronic or
oxidation state, and/or complex or molecular structure):
• isotopes (eg. 204Pb, 206Pb, 207Pb, 208Pb)
• different oxidation forms of an element (eg. Cr(III)-Cr(VI))
• different compounds, complexes of an element (eg. Ni3S2, NiO, Ni, NiSO4, NiCl2)
• metal-organic compounds (eg. CH3Hg+, (CH3)2Hg)
• complexes of metals with macromolecules (eg. with humic acids, serum proteins)
Application of hyphenated techniques:
separation technique (GC, HPLC, CE, IC) + atomic spectrometer (ICP/AES, ICP/MS)
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Hg speciation:
• elements (metal) Hg
• inorganic (HgCl2, Hg2Cl2)
• organic (CH3HgCl, (CH3)2Hg)
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