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M Asif Shaheen
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Introduction Flame photometry more accurately called Flame
Atomic Emission Spectrometry
A flame photometer is an instrument used to
determine the concentration of certain metal ions
among them sodium, potassium, calcium and
lithium.
Flame Photometry is based on measurement of
intensity of the light emitted when a metal is
introduced into flame.
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Principle When a solution of metallic salt is sprayed as fine
droplets into a flame. Due to heat of the flame, the
droplets dry leaving a fine residue of salt. This fine
residue converts into neutral atoms.
Due to the thermal energy of the flame, the atoms get
excited and there after return to ground state. In this
process of return to ground state, exited atoms emit
radiation of specific wavelength. This wavelength of
radiation emitted is specific for every element
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– The wavelength of colour tells what the element is
(qualitative) –
The colour's intensity tells us how much of theelement present (quantitative)
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Nebulizer
Flame
Burners
Mirrors
Slits 11
This is the component of sample delivery system. which breaks up the bigger liquid droplet to smaller liquid droplets.
The process of conversion of sample to a fine mist of finely divided droplets using a jet of compressed gas is known as Nebulization.
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Pneumatic nebulizers
Electro thermal vaporizer
Ultrasound nebulizer
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PNEUMATIC NEBULIZERS
CONCENTRIC TUBES
CROSS FLOW
BAGINGTON
FRITTED DISK
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The liquid sample is sucked through a capillary tube by a high pressure jet of gas flowing around the tip of the capillary.
The high velocity breaks the sample into a mist and carries it to the atomization region.
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The jet stream flows right angles to the capillary tip.
It uses a high speed stream of gas perpendicular to the tip of the sample capillary
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The jet is pumped through a small orifice in a sphere on which a thin film of sample flows
In this type of nebulizer the sample solution flows freely over small aperture, rather than passing through a fine capillary
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The sample is pumped into a fritted disk through which the gas jet is flowing and this gives fine aerosol than others
High efficiencies can be obtained by introducing the sample at predetermined location of the fritted surface
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It is an electro thermal vaporizer contains an evaporator in a closed chamber through which an inert gas carries the vaporized sample into the atomizer
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The sample is pumped onto the surface of a vibrating piezoelectric crystal.
The resulting mist is denser and more homogeneous than pneumatic nebulizers
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It should have proper temperature
Temperature should remain constant throughout the operation
There should not be any fluctuation during burning
To convert the analyte of the liquid sample into vapour state To decompose the analyte into atoms and simple moleculesTo excite the formed atoms/free atoms/simple molecules to emit radiant energy
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Mecker burner
Total consumption
burner
Premix of laminar flow
burner
Lundergraph burner
Shielded Burner
Nitrous Oxide-
Acetylene Flames
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This burner was used earlier and employed natural gas and oxygen. Produces relatively low temp. and low excitation energies. This are best used for ALKALI metals only. Now-a-days it is not used.
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In this burner fuel and oxidant are hydrogen and oxygen gases. Sample solution is aspirated through a capillary by high pressure of fuel and Oxidant and burnt at the tip of burner. Entire sample is consumed.
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In this type of the burner, aspirated sample, fuel and oxidant are thoroughly mixed before reaching the burner opening and then entering the flame. There is high loss of sample(95%) as large droplets are drained out.
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In this sample and air is mixed in a chamber, this mixed composition is send to fuel nozzle where it is atomized. Here the sample reaches the flame is only about 5%
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In this flame was shielded from the ambient atmosphere by a stream of inert gas. Shielding is done to get better analytical sensitivity and quieter flame
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These flames were superior to other flames for effectively producing free atoms. The drawback of it is the high temperature reduces its usefulness for the determination of alkali metals as they are easily ionized and Intense background emission, which makes the measurement of metal emission very difficult
NITROUS OXIDE ACETYLENE FLAME
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Fuel Oxidant Temperature 0C
Natural gas Air 1700-1900
Natural gas Oxygen 2700-2800
Hydrogen Air 2000-2100
Hydrogen Oxygen 2550-2700
Acetylene Air 2100-2400
Acetylene Oxygen 3050-3150
Acetylene Nitrous oxide 2600-2800
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Flame Photometry
Non Flame Atomizers
For example: Heated Gravite Furnace
Sample evaporation→ time and temp. controlled drying and ashing
Advantages
1. small samples are analysed
2. 1000-fold more sensitive than flame
3. Oven is adaptable to determination of solid samples
Disadvantages
1. Low accuracy 2. Low precision
2. More ionic interferences due to very high temp.
The radiation from the flame is emitted in all the directions in space. Much of the radiation is lost and loss of signal results. A mirror is located behind the burner to reflect the radiation back to the entrance slit of the monochromator. The reflecting surface of the mirror is front-faced.
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The entrance and exit slits are used before and after the dispersion elements.
The entrance slit cuts off most if radiation from the surroundings and allows only the radiation from the flame and the mirror reflection of flame to enter the optical system.
The exit slit is placed after the monochromator and allows only the selected wavelength range to pass through the detector
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Photocell
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Measures the amount of light passing through the sample. Usually works by converting light signal into electrical signal The least expensive of the devices is known as a barrier-layer cell,
or photocell. The photocell is composed of a film of light-sensitive material,
frequently selenium, on a plate of iron. Over the light-sensitivematerial is a thin, transparent layer of silver. When exposed tolight, electrons in the light-sensitive material are excited andreleased to flow to the highly conductive silver in comparisonwith the silver, a moderate resistance opposes the electron flowtoward the iron, forming a hypothetical barrier to flow in thatdirection. Consequently, this cell generates its own electromotiveforce, which can be measured. The produced current isproportional to incident radiation.
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Phototube
The third major type of light detector is the photomultiplier (PM)tube, which detects and amplifies radiant energy.
incident light strikes the coated cathode, emitting electrons. Theelectrons are attracted to a series of anodes, known as dynodes,each having a successively higher positive voltage These dynodes areof a material that gives off many secondary electrons when hit bysingle electrons. Initial electron emission at the cathode triggers amultiple cascade of electrons within the PM tube itself. Because ofthis amplification, the PM tube is 200 times more sensitive than thephototube
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PM tubes are used in instruments designed to be extremely sensitive to very low lightlevels and light flashes of very short duration.
The accumulation of electrons striking the anode produces a current signal,measured in amperes, that is proportional to the initial intensity of the light. Theanalog signal is converted first to a voltage and then to a digital signal through theuse of an analog to- digital (A/D) converter. Digital signals are processedelectronically to produce absorbance readings
Readout device.
In the past nearly all spectrophotometer used ammeters or galvanometers. Newer digital devices and printers have now replaced these, and many instruments relay their electrical output directly to computer circuits where calculations are performed, allowing direct reporting of sample concentration.
Microprocessor and recorders
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Limitations Limited number of elements that can be analyzed.
The sample requires to be introduced as solution intofine droplets. Many metallic salts, soil, plant and othercompounds are insoluble in common solvents. Hence,they can’t be analyzed by this method.
Since sample is volatilized, if small amount of sampleis present, it is tough to analyze by this method. Assome of it gets wasted by vaporization.
Further during solubilisation with solvents, otherimpurities might mix up with sample and may lead toerrors in the spectra observed.
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Chatwal & Anand; Instrumental Methods of Chemical Analysis, 5/e 2013, page no- 2.370 to 2.375, Himalaya Publishing House.
B.K Sharma; Instrumental Methods of Chemical Analysis, 26/e 2007, page no- 430 to 437, GOEL Publishing House.
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