Presentation on theme: "FLAME SPECTROSCOPY The concentration of an element in a solution is determined by measuring the absorption, emission or fluorescence of electromagnetic."— Presentation transcript:
1FLAME SPECTROSCOPYThe 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.
2Atomization In Flame Emission - It is the conversion of molecules to their component atomsin gaseous state ; and it is carried out by introductionof the molecules solution in the flame in very fine dropletIn Flame Emission-Atoms in gaseous state in the flame absorb thermal energyfrom the flame itself ,some of the atoms get excited & as theyreturn back to the ground state they emit radiation having energyequal to that absorbed.-The emission is proportional to the number of excited atoms,which is proportional to the total number of atoms in the flamei.e. the sample concentration
3Flame SpectraThe spectra of gaseous, atomic particles consist of well defined narrowdiscrete 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 beless than the ionization potential.The resonance wave - length (at which the most intense absorption and emission occur) is : nm for lithium, nm for sodiumand nm for potassium.
5The number of atoms of an element excited by the flame depends on : 1- Flame temperature2- 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 k0, 0.017% of the atoms are excited.Other metals the number of excited atoms is extremely smalle.g. in case of zinc only 10-9 are excited.-Any increase of the flame temperature is accompanied by greatincrease in the number of excited atoms
7Limitation of Flame Emission Photometry 1-The number of excited atoms in flame is very small. It is the alkalineand 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 absorptionand emission lines is enormous and the spectra are complex.
8INSTRUMENT FOR FLAME EMISSION 1-Flame atomizer.2- Monochromator3- Detector.4- Readout meter.
101-Flame Atomizer a-Atomization of the sample. b- Source of thermal energy to excite the atoms.The atomizer is composed of :Nebulizer, and burnerNebulizer:is a device by which sample solution is divided into very finedroplets which are aspirated into fine spray or aerosol.As the oxidant flows it withdraws the sample from the capillary invery fine dropletsThen mixed in the premixing chamber with the fuel gas .The fuel-oxidant-sample aerosol mixture passes to the burner producingthe necessary heat for atomization and excitation r
13In the burnerThe 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.
14In case of potassium, sodium, lithium and calcium, they are atomized and excited below k above k ionization occurs.The flame temperature must be :Regular :an increase by 100 c is accompanied by increase of 4% in the excited atoms.It must be sufficient to cause atomization only and not ionization.2- MonochromatorEither grating or interference filters which allow the resonance wavelengthto pass to the detector.
15ATOMIC 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 ABSORPTION3- Atomizer1- Source of radiation2- Chopper6- Read out meter4-Monochromator5- Detector
181- Source of radiation ; Hollow cathode lamp: is a tube with a front quartz window contains an anode of tungstenand a cylindrical cathode , the material of which is the sameelement 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 electrodesionization of the gas occursions move rapidly to the cathode
222-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 ofthe lamp, it reflects the radiationthe second moment the open in front of the lamp and the radiation passesto 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 theflame will reach the detector continuously and converted to direct currentsignal which can be suppressed and eliminated.This process is known by modulation of the source output.
243- Atomizer: a- Flame atomizer b- Flameless or non flame atomizer It is a graphite furnace heated electrically up to 6000oC andcontains a ribbon or boat in which one can inject the sample.Upon heating the furnacethe sample is ashen, then atomized by action of heat.Advantages of non flame atomizer-The sample volume is small .- No need for fuel -oxidant mixture.- Unusual high sensitivity- No flame noise.-Solid sample can be used directly.- Heat distribution is uniform and temperature is steady.
254- Monochromator:Grating to eliminate other resonance lines from the source or other radiation from the flame or sampleSchematic of an atomic-absorption experiment