SOIL AND PLANT ANALYSIS (1+1)

Instrumentation

1. Hollow-cathode lamp (light source or radiation emitting source),
2. Atomizer unit (Flame burner and electrically heated furnace-graphite furnace),
3. Monochromator (to select the analysis wavelength of the target element)
4. Detector (photomultiplier) and Recorder.

The hollow cathode lamp for the light source consists of a hollow cathode and an anode enclosed in a glass (quartz) tube and neon or argon gas is filled at around 10 Torr in pressure in it. The cathode is made of the element to be measured or its alloy, so it emits the light its wavelength is equal to that absorbed by the atoms of the sample.

Resonance lines of common elements for AAS

S.NO.	Elements	WL of absorption
1.	Zinc (Zn)	213.90
2.	Copper (Cu)	324.80
3.	Iron (Fe)	248.30
4.	Manganese (Mn)	279.50
5.	Calcium (Ca)	357.90
6.	Magnesium (Mg)	285.20
7.	Potassium (K)	766.50
8.	Sodium (Na)	589.00
9.	Cadmium (Cd)	228.80
10.	Nickel (Ni)	232.00
11.	Chromium (Cr)	357.90
12.	Lead (Pb)	283.30
13.	Barium (Ba)	553.60
14.	Gold (Ag) check	242.80

When a solution containing metallic species is introduced into a flame, the vapour of metallic species will be obtained. Some of the metal atoms may be raised to an energy level sufficiently high to emit the characteristic radiation of the metal-a phenomenon that is utilized in the familiar technique of emission flame photometry. But a large percentage of the metal atoms will remain in the non-emitting ground state. These ground state atoms of a particular element are receptive of light radiation of their own specific resonance wave-length (in general, the same wavelength as they would emit if excited). Thus, when a light of this wavelength is allowed to pass through a flame having atoms of the metallic species, part of that light will be absorbed and the absorption will be proportional to the density of the atoms in the flame. Thus, in atomic absorption spectroscopy, one determines the amount of light absorbed. Once this value of absorption is known, the concentration of the metallic element can be known because the absorption is proportional to the density of the atoms in the flame. Mathematically, the total amount of light absorbed may be given by the expression as follows:

At v total amount of light absorbed=πe2

__
mcNf …(1.1)

where e is the change on the electron of mass m, c the speed of light, N the total number of atoms that can absorb at frequency v in the light path and f the oscillator strength or ability for each atom to absorb at frequency, v. As π, e, m and c are constants, equation (1.1) can be simplified to the following expression

Total amount of height absorbed=constantxNxf …(1.2)

From expression (1.2), it follows that

Firstly, there is no term involving the wavelength (or frequency) of absorption other than the indication of the actual absorption wavelength.
Secondly, there is no term involving the temperature.

From above, it follows that absorption by atom is independent of the wavelength of absorption and the temperature of the atoms. These two features provide atomic absorption spectroscopy distinct advantages over flame emission spectroscopy.

Applications

• This is the most widely used technique for the quantitative determination of metals at trace levels (0.1 to 100ppm), which present in various materials.
• It utilizes Beer - Lambert Law for the analysis and a standard curve is obtained by plotting absorbance vs concentration of the samples taken.
• The usual procedure is to prepare a series of standard solutions over a concentration range suitable for the sample to be analyzed.
• Then, the standards and the samples are separately aspirated into the flame, and the absorbances are read from the instrument. The plot will give the useful linear range and the concentrations of the samples can be found out from the plot.

Disadvantages

• Sample must be in solution or at least volatile.
• Individual source lamp.
• Though multielement lamp sources are availabe nowdays, only limited combinations are there due to metallurgical properties or spetrual limitations.
• Use of multielement lamp results in poorer signal/noise ratio which can influence the precision of analyses and detection limit.