Atomic absorption spectrophotometer
Atomic absorption spectroscopy 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 intensity of the light and (2) the assumption that the radiation absorbed is proportional to 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.
Hollow cathode lamp
The following components make up the AA instrument.
Source of the analytical light line for the element of interest
Give a constant and intense beam of that analytical line
Suck up liquid sample at a controlled rate
Create a fine aerosol spray for introduction into the flame
Mix the aerosol and fuel and oxidant thoroughly for introduction into the flame
Destroy any analyte ions and breakdown complexes
Create atoms of the element of interest, Feo, Cuo, Zno, etc.
Isolate the analytical line photons passing through the flame
Remove scattered light of other wavelengths from the flame
In doing this, only a narrow spectral line impinges on the PMT.
Photomultiplier tube (PMT)
This is the detector. The PMT determines the intensity of photons of the analytical line exiting the monochromator.
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 handle all data-handling and display.
Graphite Fufnace AA
The graphite furnace is an electrothermal atomiser system that can produce temperatures as high as 3,000°C.
The heated graphite furnace provides the thermal energy to break chemical bonds within the sample held in a graphite tube, and produce free ground state atoms. Ground-state atoms then are capable of absorbing energy, in the form of light, and are elevated to an excited state. The amount of light energy absorbed increases as the concentration of the selected element increases.
Flame AA can only analyse solutions, but graphite furnace can accept very small absolute quantities of solution, slurry or solid samples.
Fully automatic atomic absorption spectrometers are now available. All operations are controlled through the PC, with the AA-Win software based on a WINDOWS platform. The design with the flame burner and graphite furnace in one compact unit enables automatic interchanging of two analytical methods : Flame atomic absorption and Graphite Furnace.
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|
|Fuel ||oxidant ||Temperature (K)|
|H2 ||Air ||2000-2100|
|C2H2 ||Air ||2100-2400|
|H2 ||O2 ||2600-2700|
|C2H2 ||N2O ||2600-2800|
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.
Copyright: delloyd2000© All rights reserved.