Flame Photometry
1) Principal of Flame Photometry: 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 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 When a metallic salt solution is aspirated into path of flame, following events takes place; 1) Vaporization: The solvent is vaporized leaving particles of solid salt. 2) Atomization : The salt is vaporized and converted into free neutral gaseous atoms or radicals 3) Excitation: Some of these atoms are excited by the thermal energy of the flame to higher energy levels.The various events are as follows,

Sample Solution

Fine Mist M+ X

Solid Particles MX

M+ X

Ground State gaseous atoms

Excited Gaseous Metal atoms

Gaseous Atoms M(g)+ X(g)

Gaseous Molecules MX

The excited metal atoms are unstable at higher energy level, and quickly returns to lower energy state, while coming to lower energy they emit light of their own characteristic wavelength. The measurement emitted photons (light) forms the basis of flame photometry. The colour of the flame tells us which element is present (qualitative analysis) and intensity of emitted radiation is directly proportional to concentration of metallic element present in the sample. If E1 and E2 are the energy of lower and higher levels, the radiation emitted during the jump is given by the equation E2-E1 = hV ………….(1) Where h = Planks constant. And V the frequency of radiation. …………….(2)

But V = C/ Lambda E2-E1 = hc/ Lambda

Lambda = = h C / E2-E1 …………(3) The wavelength of emitted radiation is characteristic of the atoms of particular element present in the sample. It can be calculated by using equation no.(3) The fraction of the atoms that are thermally excited is given by Boltzman distribution equation i.e.

N*/No= Ae---

E/

kT

N*= The number of excited atoms. No= The number of atoms remain in ground state. A = Constant of a particular element. E= Difference in the energy level of two states. K= Boltzman constant equal to R/N T= Temperature of flame in Kelvin

2) INSTRUMENTATION OF FLAME EMISSION SPECTROSCOPY:

1-Flame atomizer. 2- Monochromator 3- Detector. 4- Readout meter.

1-Burner:
Atomization of the sample is carried out in the path of flame. Thermal energy is used to excite the atoms. The atomizer is composed of : Nebulizer and burner. Burners are of two types;
Total Consumption Burner and Premix or Laminar flow Burner.

Total Consumption Burner: The fuel gas, oxidant gas and liquid samples are drawn to the opening of the burner through separate inlets. The fuel gas burns in presence of oxygen to produce the flame attached to the base of burner. Nebulizer is attached to the base of burner. ( Nebulizer: 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. All the sample irrespective of their size is evaporated hence the name total consumption burner. Advantage: 1) Entire sample enter into flame and hence it is more sensitive. 2) The flame can be adjusted to produce high temperature.

Disadvantages: 1) The flame produced is noisy and turbulent. 2) The reading obtained are non reproducible since the size of droplets vary.

Total Consumption Burner

Flame

Slit

Aspirated Sample

Fuel Oxidant

Sample Solution

Premix or Laminar Flow Burner: The fuel, oxidant gas and liquid sample are thoroughly mixed before entering the flame. In premix burner only small droplets of the sample are reach the flame and large droplets are trapped by baffles and drained off and sample is wasted. Advantages: 1) The flame produced is stable, no turbulent, and 2) Vaporization, atomization and excitation of atoms takes place 3) Sensitivity and reproducibility is good. noiseless. efficiently.

Disadvantages: 1) only small portion of small reaches to flame. 2) Due to large wastage fewer atoms are excited and consequently the emission intensity is weak. This lowers the sensitivity of the estimation.

Premix Burner
Fuel

Flame Flame

Sample solution

Burner Head

Oxidant

Drain

Baffles for Mixing

atomization and excitation.

2- Monochromator : Either grating or interference filters which allow the resonance wavelength to pass to the detector 3) Detector and Read out Device: The light selected by the monochromator is directed onto a detector that is typically a photomultiplier tube , whose function is to convert the light signal into an electrical signal proportional to the light intensity. The processing of electrical signal is fulfilled by a signal amplifier . The signal could be displayed for readout , or further fed into a data station for printout by the requested format.

Instruments:
The instrument used is called as Flame photometer

Flame
Collimating Mirror

Photometer
Slit P.M.T.Detector

Amplifier Fuel Oxidant Prism Monochromator

Read Out

Sample Solution

Total Consumption Burner

Flame

Slit

Aspirated Sample

Fuel Oxidant

Sample Solution

Premix Burner
Fuel

Flame Flame

Sample solution

Burner Head

Oxidant

Drain

Baffles for Mixing

3) Application of Flame Emission Spectroscopy F.E.S. Used for Qualitative analysis and Quantitative analysis of Alkali and alkaline earth metals. Soil analysis: Na,K,Ca,Mg. Fertilizer : Na,K,Ca,Mg.

Biological Fluids : Na,K,Ca,Mg. Petrol : Lead Cement: Ca and Mg. Glass Analysis: Na,Ca,Mg, K,and Li. Organic Compounds : Boron Physiology and Clinical Studies : Electrolyte Balance. Quantitative Analysis Calibration Curve A calibration curve is used to determine the unknown concentration of an element in a solution. The instrument is calibrated using several solutions of known concentrations. The emission intensity of each known solution is measured and then a calibration curve of emission intensity v/s concentration is plotted. The sample solution is fed into the instrument, and the emission intensity of the element in this solution is measured .The unknown concentration of the element is then calculated from the calibration curve

Emission Intensity is Directly proportional to concentration

•Emission Intensity --

Concentration-----------

1) Internal Standard Method:A series of standard solution containing the same elements as that present in sample solution is prepared. A fixed quantity of suitable internal standard is then added to each of standard solutions, blank and sample solutions alike .Each of above standard solutions and sample solutions are then aspirated into flame one by one .The emission intensity for each of above standard solution (Is) & (Ii) and sample solution (Ix&Ii) are measured at different wavelength one corresponds to element and other corresponds to the internal standard. These measurements are made against blank .The ratio of absorbance of the standard solutions to that of internal standard (Is/Ii) are plotted against the concentration of standard solutions. This

gives a straight line from this curve concentration of sample solution can be read by finding where the ratio (Ix/Ii) falls on concentration scale.

Emission Intensity Standard Standard

__________ Internal

Concentration‐‐‐‐‐

3)) Method of Standard Addition Method:-

In this method the emission intensity of unknown (X) is first found out by aspirating into flame against blank. Then a series of standards having definite amount of unknown (X) plus varying

amount of standard are prepared and diluted to same volume in each case .Their emission intensity are then obtained . A graph of emission intensity (EI ) against concentrations of standard (S) gives a linear curve. The concentration of the unknown can be determined by extrapolation of line which cuts to X axis.

Emission Intensity‐‐‐

Concentration‐‐‐‐‐ Concentration of Unknown.

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