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Atomic Absorption Instrument Spectrophotometer

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Atomic absorption spectroscopy (AAS)

Atomic absorption is the determination of the presence and concentrations of metals in liquid samples. Metals include Fe, Cu, Al, Pb, Ca, Zn, Cd and many more. Typical concentrations range in the low mg/L (ppm) range.

In atomic absorption (AA) spectrometry, light of a specific wavelength is passed through the atomic vapor of an element of interest, and measurement is made of the attenuation of the intensity of the light as a result of absorption.

Quantitative analysis by AA depends on:

(1) Accurate measurement of the light intensity.
(2) The radiation absorbed must be proportional to the atomic concentration.

Metals will absorb ultraviolet light in their elemental form when they are excited by heat, either by flame or graphite furnace. Each metal has a characteristic wavelength that will be absorbed. The AAS instrument looks for a particular metal by focusing a beam of uv light at a specific wavelength through a flame and into a detector.

The sample of interest is aspirated into the flame. If that metal is present in the sample, it will absorb some of the light, thus reducing its intensity. The instrument measures the change in intensity. A computer data system converts the change in intensity into an absorbance.

The atomic absorption spectrophotometer.
The following components make up the AA spectrometer :

Hollow cathode lamp
This is the Source of the analytical light line for the element which is to be measured.
It gives a constant and intense beam of that specific analytical line

The sample of interest which is a liquid is aspirated at a controlled rate
This creates a fine aerosol spray for introduction into the flame
It mixes the fuel and oxidant thoroughly for introduction into the flame

The Flame is the Atomizer in which the sample undergoes desolvation and vaporization at high temperature. It destroys any analyte ions and break up complexes
It creates the atoms of the element of interest, eg. Feo, Cuo, Zno, etc.

The main purpose of the monochromator is to isolate the absorption line from background light due to interferences.
It thus isolates the analytical line photons and removes scattered light of other wavelengths from the flame.
With a selected wavelength and slitwidth, the monochromater isolates the Hollow Cathode lamp's analytical line.

Photomultiplier tube (PMT)
This is the detector. The PMT determines the intensity of photons of the analytical line exiting the monochromator. Before an analyte is aspirated, a signal is generated by the PMT as a measurement of light from the HCL.

When the sample is aspirated in the flame, some of the light is absorbed by atoms now present in the flame and causes a decrease in PMT signal that is proportional to the amount of analyte.

The PMT is the most commonly used detector for atomic absorption spectroscopy.

However, solid state detectors are now replacing conventional vacuum-type photomultipliers.

High tech electronics amplify, filter, and process the electrical signal, using a series of chips and microprocessors, transmitting the result to an internal or external computer which manage all data-handling and display.

The samples and standards are often prepared with duplicate acid concentrations to replicate the analyte's chemical matrix as closely as possible. Acid contents of 1% to 10% are common. High acid concentrations help keep all dissolved ions in solution.

Liquid sample not flowing into the flame collects on the bottom of the nebulizer chamber and flows by gravity through a waste tube to a glass waste container (highly acidic).

Atomic absorption instruments always use a nebulizer and a slot burner to increase the path length for sample absorption.

Temperature of some flames 0C

Fuel oxidant Temperature
Natural gas Air 1700-1900
H2 Air 2000-2100
C2H2 Air 2100-2400
C2H2 N2O 2600-2800
C2H2 O2 3050-3150

For some elements that form refractory oxides (molecules hard to break down in the flame) nitrous oxide (N2O) needs to be used instead of air (78% N2 + 21% O2) for the oxidant. In that case, a slightly different burner head with a shorter burner slot length is used.

Most modern instruments control the ignition and shutdown procedures automatically.

Signature: Dhanlal De Lloyd, Chem. Dept, The University of The West Indies, St. Augustine campus
The Republic of Trinidad and Tobago.
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