Flame Photometry, by Dr. Umesh Kumar Sharma & Shyma M. S.

FLAME PHOTOMETRY By- Dr. UMESH KUMAR SHARMA AND SHYMA M S

1. FLAME EMISSION 1 SPECTROSCOPY By : Dr . UMESH KUMAR SHARMA & SHYMA M. S. DEPARTMENT OF PHARMACEUTICS MAR DIOSCORUS COLLEGE OF PHARMACY , THIRUVANANTHAPURAM, KERALA, INDI A

2. FLAME EMISSION SPECTROSCOPY Flame photometry is also known as flame emission spectroscopy. Neutral atoms are involved in emission of radiation when introduced into flame. Atoms are simplest & pure form of matter. Atoms exhibit electronic transitions when absorb energy. Such discrete transitions are quantised & line spectra is observed. 2

3. Atoms in the form of atomic vapour are produced in higher energy level. It returns to ground energy state by emitting photons. Generating sharp line emission spectra. Flame photometry is based on the measurement of intensity of light emitted when a metal is introduced into flame Wavelength of light emitted indicates the type of element present Intensity of light indicates the quantity of element. 3

4. When a liquid sample containing a metallic salt is introduced into the flame : Solvent vaporises leaving particles of solid salt. Salt vaporises into gaseous state. Gaseous molecules dissociated to neutral atoms. Neutral atoms are excited by thermal energy of flame. Excited unstable atoms quickly emit photons and returns to ground state. Emitted radiation intensity is measured. Permitted energy levels of all atoms can be represented diagrammatically in Grotrian chart. 4

5. In flame spectroscopy source of excitation energy is a flame. It is a low energy source. Emission spectrum produced is simple. It has few emission lines. Quantitative determinations are made by aspirating sample into flame. Intensity of characteristic radiation emitted by flame for individual elements is correlated with concentration of element in sample. Specific wavelength emitted by elements appear as spectral lines in UV & visible regions. 5

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7. Wave length of light emitted is : λ = hc / E Where: h – Planks constant, c –velocity of light, E-Difference energy levels of ground state & excited state. Intensity of radiation emitted depends upon concentration of element in solution. Higher the concentration the more is flame intensity. Intensity of spectral emission line : I = VATh υ Ng B(T) Where : E – Energy of excited state, T – Absolute temperature, υ – Frequency of radiation, AT -No of transitions each excited atom undergoes, N - No of free metal atoms in ground state per unit volume, g-Statistical weight of excited atomic state, B-Partial function of atom Fraction of free atoms thermally excited: / =A Where : -Number of atoms in excited state, -Number of atoms in ground state, A - Constant for element, -Difference in energy levels of excited & ground state, K - Boltzman constant, T -Flame temperature.   7

8. MEANS OF EXCITATION ELECTRIC ARC ELECTRIC SPARK THERMAL ENERGY OF FLAME Powdered s ample is incorporated in carbon electrode. Series of sparks applied which carries current across gaps. Arc source has high sensitivity of detection than spark. 8

9. STRUCTURE OF FLAME : 9 PREHEATING ZONE In this combustion mixture is heated to ignition temperature by thermal conduction from primary reaction zone.

10. PRIMARY REACTION ZONE : About 0.1mm thickness. No thermodynamic equilibrium. Concentration of ions & free radicals are very high. This region is not used for flame photometry. INTERCONAL ZONE: Can extend up to considerable height. Maximum temperature is attained just above tip of inner zone. This zone is used for flame photometry . SECONDARY REACTION ZONE : Products of combustion process burnt to stable molecular species by surrounding air. 10

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13. Characteristics of suitable flame for producing emission spectra : Should have proper temperature. Temperature should remain constant throughout operation. There should not be any fluctuations during burning Spectrum of flame must not interfere with observations when emissions is being measured. 13

14. When sample in the form of aerosol is uniformly delivered into flame: Water or other solvent is vaporised leaving minute particles of dry salt. At high temperature of flame, dry salt is vaporised. Part or all of gaseous molecules are dissociated to give neutral atoms which are potentially emitting species. Some of free metal atoms may unite with other radical or atoms in flame gases. They are introduced into flame with test element. Vapours of neutral atoms of test elements are excited by thermal energy of flame. Ionisation of neutral atoms occur to some extent. Excited atoms fall back to ground state by collision & by emission of light. 14

15. PROCEDURE : A small volume of sample is dissolved in water or organic solvent placed in a cup of atomiser. Air, oxygen & combustible gas is fed into atomiser under controlled conditions. This allows a thin spray of solution to be introduced in the flame. Solvent evaporates rapidly to give dehydrated salt. It then dissociated into free gaseous atoms in ground state. Some of atoms absorb energy from flame & raised to excited electronic states. They drop back to ground level emitting photon. Characteristic wavelength are detected with a monochromator - detector assembly 15

16. OPTIMISATION OF FLAME Number of free atoms will be an equilibrium value which depend on rate of nebulization & atomization. Rate of fuel flow : Rate of nebulization & residence time of atoms with in flame should be properly controlled. Viscosity of solvent : If sample solution is viscous, rate of nebulization is largely diminished. Chemical nature of solvent: Sometimes a stable solvated species may be formed. Hence a modification in flame process is necessary. 16

17. Other chemical species in solution / flame These may form non volatile stable compounds with sample. Flame temperature Rate of solvent evaporation, molecular dissociation, molecular association, extent of ionization depends on flame temperature. Flame photometry is now mainly used for analysis of sodium, potassium in biological fluids & tissues. It is widely applicable, specific & sensitive. Useful in case of elements whose resonance lines associated with relatively low energy value. Sodium, potassium , lithium calcium shows higher sensitivity to this technique. More than half of elements in periodic table have analysed by flame photometry. Many exist in flame as molecules , especially such as oxide produce molecular band spectra. 17

18. Only those elements which give line spectra are generally determined by flame emission spectroscopy in a clinical laboratory. Cool gaseous atoms in outer region of source cause self absorption. It decrease the intensity of transition. Use of high energy source is not always helpful since it ionizes gas with loss of one or more electrons. Spectrum of singly ionised ion (Mg+) is similar to neutral Na atom with an atomic number less than one unit. Na - 2 2 3 Mg+ -1 2 2 3   18

19. Hottest practical flame temperature (~4575) is produced by burning cyanogen in oxygen. C2N2+O2 Optimum flame temperature depends on: Excitation energy of flame Sensitivity of measurement Presence of other elements Element combination in sample High temperature increase emission intensities & thereby provide higher sensitivity. Sensitivity depends upon response & stability of detector & stability of flame aspiration system.   19

20. Measurement of intensities of spectral lines depends on : Amount of salt impregnated in flame. Amount of salt dissociated. Degree of ionization of compound. Number of atoms excited. Chances of transition from excited to ground state. Self absorption. Fraction of excited atom depend on temperature of flame. Temperature of flame depends on type of fuel & oxidant used. Flames have limited amount of energy compared to excitation sources. 20

21. They are useful to detect alkali & alkali earth metals. Cannot be used for transition metals or other metals which requires significant energy. Fuel rich oxy acetylene flame generates intense radiation bands at shorter wave length ( 300-200nm ). Ion emission lines can be detected. If the flame is oxygen rich they operate at same temperature , but ion emission lines are not observed. Nitrous oxide-Acetylene flame gives higher temperature Useful for oxides of Aluminium , Titanium etc. But high temperature ionises alkali metals. 21

22. LIMITATIONS OF FLAME PHOTOMETRY: As natural gas & air flame is used for excitation, temperature is not high enough to excite transition metals. Method is selective towards detection of alkali & alkaline earth metals. Low temperature makes this method susceptible to interference, stability of flame, aspiration conditions. Identical conditions are necessary for measuring emission of standard & unknown solutions. Relatively low energy available from the flame leads to low intensity of radiation. It helps in determining total metal concentration. It tells us nothing about molecular form of metal in original sample. Only liquid samples can be used. Preparation of liquid samples involves lengthy steps & time consuming. 22

23. REFERENCE: Instrumental methods of analysis, b y Willards , 7 th edition. Pharmaceutical Analysis , by P.C Kamboj.Volume 3 rd . Principle of instrumental analysis, by Doglas A Skoog , James Holler.5 th edition. 23

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