FLAME SPECTROSCOPY The concentration of an element in a solution is determined by measuring the absorption, emission or fluorescence of electromagnetic by its monatomic particles in gaseous state in the flame.
Atomization - It is the conversion of molecules to their component atoms in gaseous state ; and it is carried out by introduction of the molecules solution in the flame in very fine droplet In Flame Emission -Atoms in gaseous state in the flame absorb thermal energy from the flame itself,some of the atoms get excited & as they return back to the ground state they emit radiation having energy equal to that absorbed. -The emission is proportional to the number of excited atoms, which is proportional to the total number of atoms in the flame i.e. the sample concentration
Flame Spectra The spectra of gaseous, atomic particles consist of well defined narrow discrete lines arising from electronic transition of outermost electrons. Since there is no bonds, atoms undergo electronic transition only, no vibrational or rotational transitions. The energy to which the atoms are subjected must be less than the ionization potential. The resonance wave - length (at which the most intense absorption and emission occur) is : 671 nm for lithium, 589 nm for sodium and 767 nm for potassium.
Energy Level Diagram:
The number of atoms of an element excited by the flame depends on : 1- Flame temperature 2- The energy difference between the excited and ground states. Accordingly -The number of excited atoms in the flame is considerably small, even in the case of alkaline metals which are easily excited. Sodium at 2500 k 0, 0.017% of the atoms are excited. Other metals the number of excited atoms is extremely small e.g. in case of zinc only 10-9 are excited. -Any increase of the flame temperature is accompanied by great increase in the number of excited atoms
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Limitation of Flame Emission Photometry 1-The number of excited atoms in flame is very small. It is the alkaline and alkaline earth metals that can be practically determined. 2-It needs perfect control of flame temperature. 3- Interference by other elements is not easy to be eliminated. 4-Heavy and transition metals, the number of absorption and emission lines is enormous and the spectra are complex.
INSTRUMENT FOR FLAME EMISSION 1-Flame atomizer. 2- Monochromator 3- Detector. 4- Readout meter.
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1-Flame Atomizer a-atomization of the sample. b- Source of thermal energy to excite the atoms. The atomizer is composed of : Nebulizer: Nebulizer, and burner is a device by which sample solution is divided into very fine droplets which are aspirated into fine spray or aerosol. As the oxidant flows it withdraws the sample from the capillary in very fine droplets Then mixed in the premixing chamber with the fuel gas. The fuel-oxidant-sample aerosol mixture passes to the burner producing the necessary heat for atomization and excitation r
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In the burner The combustion of fuel occurs producing the necessary heat for atomization and excitation but not ionization. The temperature of the flame produced depends on fuel-oxidant ratio and kind.
In case of potassium, sodium, lithium and calcium, they are atomized and excited below 20000 k above 25000 k ionization occurs. The flame temperature must be : Regular :an increase by 10 0 c is accompanied by increase of 4% in the excited atoms. It must be sufficient to cause atomization only and not ionization. 2- Monochromator Either grating or interference filters which allow the resonance wavelength to pass to the detector.
ATOMIC ABSORPTION SPECTROSCOPY atoms in the vapor state are subjected to external source of radiation which produces one line or beam of monochromatic light with single wavelength. This wavelength is a resonance one for the atoms and that will be absorbed by them. All elements can be determined by this technique. INSTRUMENT FOR ATOMIC ABSORPTION 1- Source of radiation 2- Chopper 3- Atomizer 4-Monochromator 5- Detector 6- Read out meter
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1- Source of radiation ; Hollow cathode lamp: is a tube with a front quartz window contains an anode of tungsten and a cylindrical cathode, the material of which is the same element of the sample to be determined. The glass tube is filled with neon or argon at a pressure of 1 to 5 tore. When a low potential is applied between the two electrodes ionization of the gas occurs ions move rapidly to the cathode
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2-The chopper its function is to fluctuate the source output. It is a circular disc divided into four quarters two are mirrored and two are opened. The disc rotates at high constant speed, when the mirrored quarter in front of the lamp, it reflects the radiation the second moment the open in front of the lamp and the radiation passes to the sample being absorbed by it and reaches the detector in pulses. The detector converts the radiation to alternating current signal and amplified it. The radiation coming from the flame itself and from atoms excited by the flame will reach the detector continuously and converted to direct current signal which can be suppressed and eliminated. This process is known by modulation of the source output.
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3- Atomizer: a- Flame atomizer b- Flameless or non flame atomizer It is a graphite furnace heated electrically up to 6000 o C and contains a ribbon or boat in which one can inject the sample. Upon heating the furnace the sample is ashen, then atomized by action of heat. Advantages of non flame atomizer -The sample volume is small. - Unusual high sensitivity -Solid sample can be used directly. - No need for fuel -oxidant mixture. - No flame noise. - Heat distribution is uniform and temperature is steady.
4- Monochromator: Grating to eliminate other resonance lines from the source or other radiation from the flame or sample Schematic of an atomic-absorption experiment