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