Atomic Spectroscopy Atomic Spectroscopic Methods Covered in Ch 313: Optical Atomic Spectrometry (Ch...
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Transcript of Atomic Spectroscopy Atomic Spectroscopic Methods Covered in Ch 313: Optical Atomic Spectrometry (Ch...
Atomic SpectroscopyAtomic Spectroscopic Methods Covered in Ch 313:
• Optical Atomic Spectrometry (Ch 8-10)
• Atomic X-ray Spectrometry (Ch 12)
Atomic Mass Spectrometry (Ch 11) is combined later on with Molecular Mass Spectrometry (Ch 20)
•the elements in a sample are converted to gas-phase atoms or ions
•detected based on UV-Vis absorption, emission, or fluorescence
•detection limits in the part-per-billion (ppb) range
Energy Level Diagrams - valence electron transitions
Na
1s22s22p63s1
Mg+
1s22s22p63s1
Atomic Emission Spectra - valence electrons excited by a flame, plasma, or electric spark to higher energy levels; emission to ground state produces the emission spectra.
Na excitation Na emission
Atomic Absorption Spectra - the gas phase atoms and ions can also absorb radiation directly from an outside source
monitor missing
wavelength
LightSource
Po Po
absorption
Atomic Fluorescence Spectra - atoms in a flame made to fluoresce by irradiation with an intense light source, e.g. laser
MolecularAtomic
→ →
1. Doppler Broadening - frequency shift of light due to source motion
2. Pressure Broadening - increased pressure increases the number of atomic collisions
Na Na
Atomic
collisions can activate or deactivate an excited state
collisions shift ground state energy because of electron cloud interactions
kT
Eexp
g
g
N
N j
o
j
o
j
Boltzmann Distribution
Nj = excited state populationNo = ground state populationPj and Po = degeneracy termsEj = excited state energyK = Boltzmann’s constant = 1.38 x 10-23 J/KT = Kelvin temperature
For the 3p level of Na –
2500 K
2510 K
4
o
j 10 x 1.72N
N
4
o
j 10 x 1.79N
N
0.5 % change
4 % change
1. Flame Atomization
Flame Backgrounds
O2 – H2
O2 – C2H2
N2O – C2H2
Increasing T
Processes leading to flame atomization -
1. aspiration
2. Nebulization
3. Desolvation
4. Volatilization
5. Free atoms
Further reactions with O2 and N2 in the atmosphere = BACKGROUND
Non-equilibriumunstable combustion productsNOT USED
Homogenous T and compositionequilibriumfree atomsOBSERVE HERE
Temperatures in the primary combustion zone are the hottest
oxides
Must choose the height within the flame to do the analysis without creating oxides
Laminar Flow Burner
•Oxidant (air or O2) nebulizes sample
•Aerosol mixed with fuel and past baffles that remove larger drops
•Mixture ignited in slotted burner head
•Longer path length
•Danger - “flashback” explosion
Graphite Furnace Atomizer - all parts made from graphite
1. Drying or desolvation step – 110 oC, evaporates solvent
2. Ash step – 350-1200 oC, organics converted to CO2 + H2O
3. Atomization step – 200 Amps, 2000-3000 oC, vaporization
L’vov Platform 1 x 5 cm
Liquid sample injected using a syringe
Graphite Furnace Atomizer – cont’d
Constant inert gas flow (Ar) protects the graphite from oxidation and removes analyte from chamber walls.
1. Radiation Sources – require a very narrow linewidth because of negative deviations in calibration curve due to polychromatic radiation effect .
To solve this problem, the lamp is constructed out of the same metal element being analyzed.
As long as the temperature of the lamp is less than the flame temperature, Doppler and collisional broadening will be greater in the flame, and the source wavelength will be narrower in the sample.
Hollow Cathode Lamps
500 Volts
1. Cathode consists of metal to be analyzed2. 500 Volts across electrodes ionizes inert gas3. Cations migrate towards the negative hollow cathode4. Collisions with cathode “sputters” metal from surface5. Metal atoms in excited states emit characteristic wavelengths6. Metal redeposited on cathode or glass
2. Instrumentation – single beam; dual beam also available
3. Interferences
A. Spectral – unresolveable peaks
The 308.211 nm line of V interferes with the 308.215 nm line of Al. To resolve them –
1ωD308.211308.2152
1
If = 1.0 µm then D-1 = 2.0 nm/mm
but the slit widthis so narrow that diffraction will cause loss of signal.
-100
1020
-10
0
10
20
0.2
0.4
0.6
0.8
long axis of aperture
Fraunhofer Diffraction from a Rectangular Aperture
short axis of aperture
Inte
nsity
of
Diff
ract
ed L
ight
3. Interferences
B. Chemical
(i) Releasing Agents – a cation that preferentially reacts with the interferent and prevents interaction with the sample.
e.g. Ca (analyte) in the presence of PO43-
3 Ca2+ + 2 PO43- Ca3(PO4)2
insoluble product that passes through flame without atomizing the Ca. Add La3+ or Sr2+ (both of which form even more insoulbe compounds with PO4
3-)
(ii) Protecting Agents – prevent interference by forming a stable but volatile species with the analyte
e.g. EDTA combines with Ca while leaving interferents behind like Al, PO4
3-, and SO42-
3. Interferences
B. Chemical
(iii) Ionization in Flames
M(g) = M+(g) + e
1. less interelemental interference (many emission lines to choose from)
2. simultaneous detection of dozens of elements
3. good for compounds with high Hvap (difficult to vaporize)
4. can detect elements that form "refractory compounds" (difficult to thermally decompose) such as the oxides of B, P, W, U, Zr, and Nb
5. can detect nonmentals such as Cl, Br, I, and S
6. wide dynamic range
7. no extensive sample pretreatment
Plasma, arc and spark emission spectrometry have advantages over flame and electrothermal atomization techniques:
Echelle Gratings
n = d (sin i sin r ) same side
i r =
n = 2d sin
nF
2dcosβ
dβ
dλ
F
1D echelle 1
nF
dD echellette 1
No matrix (background) effect from –
•Natural substances like dissolved organic materials (e.g. humics) and microorganisms
•No spectral interference from other ions