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1. Atomic absorption spectroscopy

2. Introduction:
• The concentration of the gaseous atoms is measured by
absorption spectroscopy using Beer’s law.
• From a sample in solution (used because solutions help to
reduce matrix effects), generate the elemental gaseous
atoms in their ground state at a concentration proportional to
the concentration of the element in the sample.
• There are two commonly used methods for achieving this
goal:
a. flame AAS
b.electrothermal method.

3. A. Flame Atomic Absorption Spectroscopy (Flame
AAS).
Youtube
• The most common flame method is based on a
nebulizer and a slot burner.
• Oxidant gas causes the nebulizer to draw up the liquid
sample and produce a spray of tiny droplets.
• The oxidant and fuel are pre-mixed before entering the
slot burner.
• The slot burner produces a linear flame 5-10 cm long.

4. Desirable attributes of the nebulizer and burner include:
(a) Production of small & reproducible droplets.
(b) No carryover effects (good drainage).
(c) Production of a quiet and stable flame.
(d) Prevention of flashback.

6. The choice of fuel and oxidant is limited.
The most common combinations are:
1. acetylene - air (T = 2400 K)
2. acetylene - nitrous oxide (T = max 2800 K).
 Fuel/oxidant ratios can be adjusted to be
1. stoichiometric (hottest flame),
2. reducing or rich (excess fuel helps to reduce metal oxides),
3. or oxidizing or lean (excess oxidant helps to break down
organics).
The flame must:
(a) evaporate the droplets,
(b) decompose the organics and salts to atoms
(c) reduce ions to atoms.

7. https://www.youtube.com/watch?v=g3rSwl2hfBw&t=127s
https://www.youtube.com/watch?v=L3OFSJ8ZHog
https://www.youtube.com/watch?v=d7EEfDuPEZk
graphite furnace: https://www.youtube.com/watch?v=qA3qb4pgqP4

9. B. Electro thermal Atomization (Graphite Furnace Atomic Absorption).
Use a graphite furnace - a graphite tube heated by electrical current.
1. Inject the sample (5-50 µL) into the sample cup.
2. Dry the sample (100 C).
3. Ash the sample (500-1000 C) to break down the organics.
4. Atomize the sample by ramping the temperature rapidly to 2000-
2500 C.
For a fraction of a second, a plume of atoms fills the tube;
the absorbance measurement must be made during this time.
5. Clean the sample cup by heating to 3000 C and then cool to room
temperature.
* The total time per sample is typically about 3 minutes.

12. Comparison of the flame and graphite furnace atomization
methods:
Flame: better precision, better accuracy, more rapid analyses of
samples, less expensive equipment.
Furnace: greater sensitivity, much lower LOD, uses much less
sample.

13. Atomic Absorption Spectrometers.
Assumptions:
(i) Beer's law holds for the atoms in the flame or graphite furnace,
(ii) the concentration of atoms in the flame or furnace is proportional
to the concentration of analyte in the sample.
The block diagram:

15. 1. Line Source: hollow cathode lamp (HCL).
• The HCL consists of a hollow cup cathode lined with the desired
element and an anode in a low pressure argon atmosphere.
• Application of several hundred volts causes electrons to pass
through the argon, forming argon cations (Ar + ).
• The Ar + ions blast atoms of the element into the gas phase and
excite them by collisions. A(g) ↔ A(g)*
• Consequently, the HCL emits lines of the element coating the
cathode.
• To change the element analyzed, one must change the HCL
(although some HCL’s are coated with several elements).
• Beer's law is generally valid for Abs. up to 0.8 (the upper limit of
linearity in calibration curves).

17. 2. Chopper:
 Light emitted by the flame or graphite furnace (including emission by
the analyte atoms) is stray light.
 So the HCL light is modulated by the chopper and the Amp/Processor
synchronously detects the signal due to the HCL light.
 The chopper sends the light in a path around the flame/furnace to
provide Po .

18. 3. Monochromator:
(a) isolates one spectral line from the HCL lines (a band pass of 0.2
nm is usually sufficient to isolate 1 line);
(b) prevents most of the flame or graphite furnace emission from
reaching the detector.
* Because the dynamic range of calibration curves is so limited (ca. 1
order-of-magnitude), it is convenient to switch to another line of the
element to change the sensitivity of the method.

19. 4. Detector: Photomultiplier (PMT).
To obtain an absorbance measurement:
(a) Run a blank (e.g. pure water). The instrument records Poor
adjusts PMT sensitivity to get a preset Po
(b) Run a sample. The instrument calculates and displays A.
Most modern instruments are automated with computer control and
data acquisition.

20. Interferences
* Chemical interferences: affect the concentration of ground state
atoms in the flame/furnace.
1. Formation of stable compounds i.e. refractory oxides (WO,
CaAlO2) or refractory salts (CaPO4).
Possible cures:
 use hotter temperatures to break apart the compounds;
 use a rich flame to reduce metal oxides;
 avoid certain anions with certain analyte cations;
 chelate the metal analyte.

21. 2. Ionization (M º → M+ + e- ).
Especially a problem for Group I metals which have low
ionization energies.
Cure:
 raise the free electron concentration in the gas phase by
adding a large excess of Cs salt (ionization suppressant) to
the sample;
 the excess electrons from the Cs atoms suppress ionization
of other metal atoms.