分析化学英文课件12原子吸收分光光度计 AAS

The relationship of number of atoms at ground state and the total number of atoms should be considered.
At thermodynamic equilibrium：
Nj
Pj
E0 E j
e kT
Pj
(2) In fact, the shape is also peak with certain width. (3) It = Io e-Kvb
Kv: absorptivity diagram: Kv~ ,
Parameters of absorption peak:
Central frequency: o (peak frequency) frequency of maximum absorptivity
excited state ground state
emit radiant energy of certain frequency.
resonance emission line
resonance line
K energy level diagram
2. Characteristic line of element
§12.1.1 Resonance Line
1. Atomic energy level and transition
ground state first excited state,
absorb radiant energy of certain frequency.
resonance absorption line resonance line
E
e kT
Pj
h
e kT
N0 P0
P0
P0
Pj and PO in the above formula represent respectively the statistical weighting of the excited state and the ground state
. The ratio of atom numbers of the excited state Nj and the
one of ground state No is small (<1%). So we can use the
atom number of the ground state to represent the atom
number of the measured element. Excluding the temperature
Otherwise, it will be higher.
VD 7.162 107 V0
T M
Lorentz broadening
(collision broadening) ΔVL The collision between atoms makes the energy change slightly. ΔVD is mean factor for broadening.
§12.2.3 Light Source
1. Function
Supply characteristic spectrum of element determined. Obtain high sensitivity and accuracy. Requirements for light source： (1) Can emit resonance line of element determined (2) Can emit sharp line (3) High strength, good stability
If use normal source, change of absorptive strength: 0.5%. Sensitivity is poor.
Integrated absorption: use peak area for quantitative analysis, can not be realized.
T, there are all constants on the right side of the equation . If
the T is determined, the ratio is also determined.
§12.1.3 Basis of Quantitative Analysis
§12.2 Atomic Absorption Spectrometer
§12.2.1 Atomic Absorption Spectrometer §12.2.2 Process §12.2.3 Light Source §12.2.4 Atomization System §12.2.5 Monochromator §12.2.6 Detector
4. Integrated absorption and peak absorption
Tungsten halogen lamp or deuterium lamp, after monochromatization: width of band is 0.2 nm. Atomic adsorption line: half width: 10-3 nm.
Analytical Chemistry 分析化学
Chapter 12 Atomic Absorption Spectrophotometry
§12.1 Principles §12.2 Atomic Absorption Spectrometer §12.3 Condition and Quantitative
§12.2.1 Atomic Absorption Spectrometer
§12.2.2 Process
moving
Difference between atomic absorption spectrometer and ultraviolet-visible spectrophotometer: (1) Sharp line light source (2)Monochromator is between flame and detector
(4) Use different elements to be measured to make the hollow cathode lamp. (5) Intensity of radiation is related to the working current of the lamp.
π e2
K v dv mc N 0 f
5. Sharp line source
(1) Frequency of emission line of source is the same as that of absorption line (Vo). ( 2 ) ΔV1/2 of emission line is less than ΔV1/2 of absorption line
(1) Ground statefirst excited state radiant energy absorbed is different for different
atom—— characteristic (2) Ground statefirst excited state
most easy to produce，most strong absorption，most sensitive line—characteristic line (3) Can be applied to quantitative analysis
Central wavelength: λ (nm)
Half width: Δo
Factors for broadening of peak
Incident line is wide
Doppler broadening
(temperature broadening) ΔVD Doppler effect: The frequency of light which is
2. Hollow cathode lamp structure
3. Principles of hollow cathode lamp
(1) Certain pressure is exerted, the electrons will flow from the hollow inner wall of cathode to the anode. (2) The collision of electrons with the inert gas makes the latter ionize and generates ions with positive charges, which under the effect of electric field bombard the inner wall of cathode violently. (3) The metallic atoms spray out of the surface of the cathode. Then the spraying atoms collide with the electrons, inert gas atoms and ions and are excited. The spectrum of the cathode material and the inert gas are generated in the cathode light.
emitted by the moving atom, if the atom is moving from
the observer (the receptor), will be er than the one which is sent out by the static atom.
(3) Hollow cathode lamp
§12.1.2 Number of Atom at Ground State and T for Converting to Atomic Vapor
The relationship between number of atoms at ground state and absorption of resonance line is used for determination
Analysis
§12.4 Interference
§12.1 Principles
§12.1.1 Resonance Line §12.1.2 Number of Atom at Ground State and
T for Converting to Atomic Vapor §12.1.3 Basis of Quantitative Analysis
3. Shape of absorption peak
(1) There are fewer possible energy-level transitions in atoms than in molecules . In theory, the shape of atomic absorption peak is of line.
It I0e K b
When sharp line source is used，
K0 can substitute Kv，then:
A
lg
I0 I
0.434 K 0
b
2 π ln 2 e2
0.434 vD
mc N0 f b
Absorptivity at peak：
K0
0.434
2
π ln vD
2
e2 mc
N0
f
A = k N0 b ;
N0 ∝ N∝ c
N0 : number of atoms at excited state; N : number of atoms at ground state; c : concentration to be determined. Then A = lg(I0 / I ) = K' c