atomic absorption spectrophotometer hossein piri hossein piri department of biochemistry and...
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![Page 1: Atomic Absorption Spectrophotometer Hossein Piri Hossein Piri Department of Biochemistry and Genetics, School of Medicine, Qazvin University of Medical](https://reader035.vdocuments.us/reader035/viewer/2022062322/56649e205503460f94b0c692/html5/thumbnails/1.jpg)
Atomic Absorption Atomic Absorption SpectrophotometerSpectrophotometer
Hossein PiriHossein PiriDepartment of Biochemistry and Genetics, School of Medicine, Qazvin University of Medical Sciences, Qazvin, Iran
September 2012September 2012
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Atomic absorption spectrophotometer
• Is used to measure concentration by:– detecting absorption of electromagnetic radiation
by atoms rather than by molecules.
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Usual light source
• Hollow-cathode lamp, consists of:– Anode– Cylindrical cathode– Inert gas (helium or argon)
• When voltage is applied:– gas is ionized– Ions collide with metal, and cause atoms to be excited
• When they return to ground state, light energy is emitted that is characteristic of metal in cathode
• Separate lamp is required for each metal
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Electrodeless discharge lamps
• Are a relatively new light source
• A bulb is filled with argon and element to be tested
• A radiofrequency generator around the bulb supplies the energy to excite the element
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SampleSample
• Sample must contain reduced metal in atomic vaporized state
• This is done by using the heat of a flame to break chemical bonds and form free, unexcited atoms
• The flame is the sample cell in this instrument, rather than a cuvet.
• There are various designs; however, the most common burner is the premix long-path burner.
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• The sample is aspirated as a spray into a chamber, where it is mixed with air and fuel.
• This mixture passes through baffles, where large drops fall and are drained off.
• Only fine droplets reach the flame.
• The burner is a long, narrow slit, to permit a longer path length for absorption of incident radiation.
• Light from the hollow-cathode lamp passes through the sample of ground-state atoms in the flame.
SampleSample
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• The amount of light absorbed is proportional to the concentration.
• When a ground-state atom absorbs light energy, an excited atom is produced.
• The excited atom then returns to the ground state, emitting light of the same energy as it absorbed.
• The flame sample thus contains a dynamic population of ground-state and excited atoms, both absorbing and emitting radiant energy.
SampleSample
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Mechanical rotating chopper
• The emitted energy from the flame will go in all directions, and it will be a steady emission.
• Because the purpose of the instrument is to measure the amount of light absorbed, the light detector must be able to distinguish between the light beam emitted by the hollow-cathode lamp and that emitted by excited atoms in the flame.
• To do this, the hollow-cathode light beam is modulated by inserting a mechanical rotating chopper between the light and the flame or by pulsing the electric supply to the lamp.
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Detecting the pulsed signal• Because the light beam being absorbed enters the
sample in pulses, the transmitted light also will be in pulses
• There are, therefore, two light signals from the flame—– Alternating signal from hollow-cathode lamp – Direct signal from flame emission
• Interference from the constant flame emission is electronically eliminated by accepting only the pulsed signal from the hollow cathode.
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MonochromatorMonochromator
• Is used to isolate the desired emission line from other lamp emission lines
• It serves to protect the photodetector from excessive light emanating from flame emissions
• A PM tube is the usual light detector
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Flameless atomic absorption Flameless atomic absorption
• Uses an electric furnace to break chemical bonds (electrothermal atomization)
• An electric current, evaporates the solvent, ashes the sample and, finally, heats the unit to incandescence to atomize the sample.
• This instrument, like the spectrophotometer, is used to determine the amount of light absorbed.
• Again, Beer’s law is used for calculating concentration.
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• For example, phosphate may interfere with calcium analysis by formation of calcium phosphate.
• This may be overcome by adding cations that compete with calcium for phosphate.
• Routinely, lanthanumlanthanum or strontiumstrontium is added to samples to form stable complexes with phosphate.
ProblemsProblems
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• Another possible problem, • Ionization of atoms, which can be decreased by
reducing the flame temperature.
• Matrix interference, can be another source of error.
• This interference may be overcome by pretreatment of the sample by extraction.
ProblemsProblems