Chapter 8-9 Atomic Spectroscopy Based on Flame and Electrothermal by daa16358


									Chem 311                                                                      Feb. 2, 1999
Chapter 8-9 Atomic Spectroscopy Based on Flame and Electrothermal Atomization
Used for the determination of more than 70 elements; sensitivity: ppm to ppb
Highly selective procedure

1. Sample atomization: The process of converting sample into an atomic vapor
1) Continuous atomizers: A solution of sample is converted to a mist of finely divided droplets by a jet
of a compressed gas (nebulization). The flow of gas then carries the sample into a heated region where
atomization takes place.
2) Discrete atomization: A measured volume of a solution is introduced into the device (electrothermal
analyzers). Desolvation is then carried out by raising the temperature to a level at which solvent
evaporation occurs rapidly. The temperature of the device is then increased drastically so that the other
atomization steps occur over a brief period of time.

2. Types and sources of atomic spectra
1) types: atomic absorption and atomic emission
2) sources: see Figure 8-1 and Figure 8-2, page 193-194.
3) Atomic Line Widths
Narrow lines are highly desirable for absorption and emission work because they reduce the possibility
of interference due to overlapping spectra. Furthermore, Line widths are of prime importance in the
design of instruments for atomic absorption spectroscopy.
Sources of line broadening:
i) The uncertainty effect (page 197), the natural line widths are around 10-4Å.
ii) Doppler Broadening
The wavelength of radiation emitted or absorbed by a fast-moving atom
--decreases if the motion is toward a detector.
--increases if the atom is receding from the detector.
iii) Pressure broadening:
Pressure, or collisional, broadening arises from collisions of the emitting or absorbing species with other
atoms or ions in the heated medium. These collisions cause small changes in ground-state energy levels
and hence a spread of absorbed or emitted wavelengths.
4) Effect of temperature on atomic spectra
                                         N     P         E
The very famous Boltzmann equation: j = j exp (- j )
                                        N0 P0            kT
where Nj and N0 is the number of atoms in an excited state and the ground state respectively, k is the
Boltzmann constant (1.38 x 10-23 J/K), T is the temperature in Kelvins, and Ej is the energy difference in
Joules between the excited state and the ground state.

Hollow Cathode Lamps (Fig. 9-11, p215)
Consist of a tungsten anode and a cylindrical cathode sealed in a glass tube that is filled with neon or
argon at a pressure of 1 to 5 torr. The cathode is constructed of the metal whose spectrum is desired or
serves to support a layer of that metal.
The cylindrical configuration of the cathode tends to concentrate the radiation in a limited region of the
tube, and enhances the probability that redeposition will occur at the cathode.

The operation of HCL:
1. Apply 300V across the electrodes, ionization of inert gas occurs which results in a 5 to 20 mA
2. The gaseous cations dislodge some of the metal atoms from the cathode surface and produce an
atomic cloud (sputtering).
Chem 311                                                                                      Feb. 2, 1999
3. A portion of the sputtered metal atoms is in excited states and emits their characteristic radiation as
they return to the ground state.
4. Eventually, the metal atoms diffuse back to the cathode surface or to the glass walls of the tube and
are redeposited.

3 Atomization
1. Flame atomization, see Figure 9-2 and Figure 9-5, page 208 - 209.
The flame: primary combustion zone, interconal region, and outer cone
The process:
1) Transport - sample solution to nebulizer
2) Nebulization - solution to spry in the flame
3) Desolvation - spray in the flame to salt particles
4) Vaporization - salt particle to vapor in the flame
5) Equilibration of Vaporized Species - neutral atoms, ionic species, and molecules
6) Can be used for Atomic Emission, Absorption, or Fluorescence Measurement

2. Electrothermal atomizer designs
-- graphite tube, 1 cm dia., 1 cm long
-- aqueous sample (10 µl)
-- heating the tube by passage of a current
1) Drying cycle - low T (< 110 ˚C), solvent is vaporized
2) Ash cycle, 200 - 500 ˚C, burn off any organic component or salt in the sample, 30 - 90 seconds
3) Atomization cycle - 2000 - 3000 ˚C, 3 - 5 seconds.
-- heated in an inert atmosphere (e.g. Ar)
See Figure 9-6, p211

4. Interferences in atomic absorption spectroscopy
1. Spectral interferences
i) overlapping emission or absorption lines -- use another line
ii) broad band absorption or scatter from the combustion products
a) use different temperature or fuel-to-oxidant ratio.
b) Broad band absorption correction: continuous source correction (by using a D2 lamp), Smith-Hieftje
background correction (current programming for HCL)
c) Zeeman effect background correction

2. Chemical interference
1) releasing agents: Strontium or lanthanum ions
Used when there are phosphate or sulfate ions when determining a cation (e.g. Ca2+).
2) protective agents: EDTA, 8-hydroquinoline, or APDC
3) fuel rich combustion to prevent the formation of metal oxides
4) Ionization suppressers (alkali metals)

5. Flame emission spectroscopy

6. Atomic fluorescence spectroscopy

To top