Flame photometry ppt

Principle, Instrumentation, Applications, and Interferences are discussed in this presentation.

1. R. Deepthi Assistant Professor Vignan Institute of Pharmaceutical Technology Flame Photometry

2. Flame Photometry is also called as Flame Emission Spectroscopy , since neutral atoms are involved in the emission of radiation when introduced into the flame. In early experiments, the visible colour of the flame was used to confirm the presence of certain elements in the sample, particularly alkali metals and alkaline-earth metals. Later the whole ultraviolet and visible range was utilized using a spectrophotometer. This instrument permitted us to select the wavelengths of the radiation and measure its intensity with considerable accuracy.

3. Principle When a solution of metallic salt is sprayed onto a flame, fine droplets are formed. Due to the thermal energy of the flame, the solvent in the droplets evaporate, leaving behind fine residue, which are converted to neutral atoms. These neutral atoms are converted to excited state atoms by the thermal energy of the flame. As the excited state is not stable, these excited atoms return to ground state, with the emission of radiation of specific wavelength.

4. The wavelength of the radiation emitted is characteristic of the element and is used to identify the element (Qualitative analysis). The intensity of the radiation emitted depends upon the concentration of the element analysed (Quantitative analysis).

5. Although it is theoretically possible to analyse all the elements by flame photometry, the availability of burner, fuel and oxidant combinations and technical reasons make it practically possible to analyse Group 1 A elements like Li, Na, K and Group II elements like Ca, Mg. The intensity of the radiation emitted depends upon the proportion of thermally exited atoms, which in turn depends upon the proportion of thermally exited atoms, which in turn depends upon the temperature of the flame.

6. Fraction of free atoms thermally exited = N*/N o = Ae -∆E/ kT Where N* = No. of atoms in excited state N = No. of atoms in ground state A = Constant for element ∆E = Difference in energy level of excited and ground state K = Boltzman constant T = Flame temperature

7. The wavelength of the light emitted depends upon the difference in the energy levels of atoms in the excited and ground state. Since atoms of each element has specific exited and ground state energy levels, the wavelength of radiation emitted is different for different elements. For example, sodium emits yellow radiation (589nm), potassium emits orange (767nm), calcium emits brick red colour (422, 554 and 626nm) and lithium at 670nm. Some of the elements emit at a single wavelength (primary emission), whereas other elements emits secondary emissions at different wavelengths.

8. The wavelength of the radiation emitted is given by the following equation: Wavelength of light emitted (λ) = hc /E 2 -E 1 Where, h = Plank’s constant C= Velocity of light E 2 & E 1 = Energy levels of excited and ground state respectively

9. The intensity of the radiation emitted depends upon the concentration of the element present in the solution. Higher the concentration, more is the flame intensity and lower the concentration, less is the flame intensity. The intensity of the spectral emission line is given by the following equation: Intensity of spectral emission line (Iv) = VA T hγ N ga e -E/KT B(T) Where, E = Energy of exited state T = Absolute temperature γ = Frequencyof radiation AT = No. of transitions each exited atom undergoes/sec N = No. of free metal atoms in ground state per unit volume ga = Statistical wt. of excited atomic state B(T) = Partial function of the atom overall states

10. Instrumentation

11. Instrumentation/ Components of a Flame Photometer Nebulizer Burner (with fuel and oxidant) Filter/ Monochromator Detector Readout device

12. Burner The flame used in the flame photometer must have the following characteristics: The flame should possess the ability to evaporate the liquid droplets from the sample solution resulting in the formation of solid residue. The flame should decompose the compounds in the solid residue formed in step 1, resulting in formation of atoms. The flame must have the capability to excite the atoms formed in step 2 and cause them to emit radiant energy. For analytical purposes, it becomes essential that emission intensity should be steady over reasonable periods of time (1-2min).

13. Mecker burner: This burner was used earlier and employed natural gas and oxygen. As this burner produced relatively low temperatures and low excitation energies , this was generally used for the study of alkali metals only. The flame produced by Mercer burner is not homogeneous chemically . It means that there are different regions in the flame, i.e., an “oxidising” region and a reducing region appear in the flame. As the processes leading to atomic excitation in the flame differ in the oxidising and reducing regions in flame, different concentrations of excited atoms are obtained in these regions. These days Meckler burner is not used.

14.   Total consumption burner The construction of a Total consumption burner is simple. In this, the sample solution is aspirated through a capillary by the high pressure of fuel and oxidant and burnt at the tip of the burner. Advantage The design is simple and The entire sample is consumed Disadvantage An uniform and homogenous flame is not obtained, since droplet size vary. This leads to fluctuations in the flame intensity.    

15. Laminar flow (premix) Burner: This burner is most widely used , because of its merits like uniformity in the flame intensity. In this type, the sample solution, fuel and oxidant are mixed before they reach the burner tip. Only few droplets of uniform size reaches the flame and the remaining are drained through an outlet at the bottom.  

16. Fuel and Oxidants If the temperature of the flame is too low, it may not cause excitation of neutral atoms. If the temperature is too high, it may cause ionisation of atoms and thus sufficient population of atoms in exited state may not occur. Hence the temperature of the flame is critical. This makes it necessary to select ideal combination of oxidant and fuel which gives the desired temperature in flame photometry. The following table shows the different combinations of fuel and oxidant to get desired flame temperature. In most cases, a conventional flame photometer uses compressed air as oxidant and liquefied petroleum gas (LPG) as fuel.

17. Fuel Flame temperature Oxidant Air Oxygen Propane 2100 C 2800 C Hydrogen 1900 C 2800 C Acetylene 2200 C 3000 C

18. Filters/ Monochromators : In flame photometry, the wavelength as well as intensity of the radiation emitted by the element has to be monitored, hence a filter or monochromator is to be used. Typically, a simple flame photometer contains a filter wheel (containing several filters for either Ca, Li, Na or K) and when a particular element has to be analysed, the specific filter is selected.

19. Detector: The radiation emitted by the elements is mostly in the visible region. Hence, conventional detectors like Photo voltaic cell or Photo tubes can be used. In a Flame photometer, Photomultiplier tube is used as detector.

20. Read out device The signal from the detector is shown as a response in the Digital Read out device. The readings are displayed in an arbitrary scale (%Flame Intensity).

21. Simple flame Photometer A simple flame photometer contains a burner, where the solution to be analysed is sprayed into fine droplets after mixing with fuel and oxidant. A compressor compresses the air or oxygen and fuel, at a pressure of about 0.5kg/sq.cm. This high pressure is required to spray the sample solution to fine droplets. When the sprayed droplets are ignited, the solvent is evaporated by the thermal energy of the flame, leaving behind fine residue. This fine residue is converted to neutral atoms. The neutral atoms are excited by the thermal energy of the flame to exited atoms. These excited atoms are unable and hence loose energy in the form of radiation and returns to ground state. The wavelength as well as the intensity of the light radiation emitted is detected by using a filter and a Photometric detector.

22. A concave mirror is used to focus the light onto a filter wheel (containing Li, Na, K, and Ca filters), in which the desired position can be selected based upon the element to be measured. This light passes through a slit and falls on the detector. The detector response is shown in the digital display device (%Flame Intensity).  

23. Flame Spectrophotometer A flame spectrophotometer consists of similar components of a flame photometer, in which the filter is replaced by a prism or monochromator and the photovoltaic cell is replaced by a Phototube or Photomultiplier tube. Double Beam Flame Spectrophotometer In this, the radiation from the flame passes through two different filters or monochromators and the resulting beams fall onto two different detectors. The double beam configuration is used to monitor a sample element and an internal standard element (a second substance used in quantitative analysis by internal standard method).

24. For ex, lithium is used as internal standard in the estimation of calcium, sodium or potassium. Double beam flame spectrophotometer is used when internal standard method of quantitative analysis is used, to avoid certain errors like fluctuations in flame intensity, errors in atomising the sample due to high viscosity of solutions etc.

25. Pharmaceutical applications of Flame Photometry Qualitative analysis: Flame photometry is used to identify the elements in a given sample solution. This is done by a technique called peak matching , where at least 3 peaks of emission spectrum should match when sample and standard spectra are recorded. For ex; Calcium emits radiation at 422nm, 554nm and 626nm. If the spectrum of the sample shows emission maximum at these wavelengths, then the sample can be identified by using the standard.

26. Quantitative analysis Flame photometry is mostly used to determine the concentration of the elements belonging to Group 1 A and Group II A elements . Most often, the concentration or the amount of elements like lithium, calcium, sodium or potassium in samples can be determined. The following are some of the quantitative applications: Concentration of calcium in serum Concentration of calcium, sodium, potassium in urine. Amount of sodium, potassium, calcium and magnesium in intravenous fluids, oral rehydration salts. Assay of potassium chloride in syrup. Concentration of lithium in serum for therapeutic drug monitoring.

27. The concentration or the amount of elements can be determined by any one of the following four methods: Direct comparison method Calibration curve method Standard addition method Internal standard method

28. Direct comparison method: In this method, the % flame intensity (%F.I) of a standard solution and sample solution is compared. Merits Less time is required for the estimation and more standards need not be prepared. Demerits It is subjected to more errors, especially when the concentration of the sample and the standard are in the non-linear region of calibration range. Conc. of ion in sample solution = % F.I. of Sample × Conc. of ion in Std. % F.I. of Standard

29. Calibration curve method In this method, a series of standard solutions of the element to be estimated is prepared and a calibration curve of Concentration Vs % Flame intensity is made. From this calibration curve, the concentration of sample solution is determined from its %Flame intensity.

30. The detailed procedure is as follows ( for determination of Na ) : Prepare a stock solution of Sodium chloride solution (1000µg/ mL ). From the stock solution, prepare a series of standard solutions of Sodium ( 10, 20, 30, 40 and 50 µg/ mL ). Select the sodium filter in the instrument. Set the air pressure of 0.4-0.5 kg/sq.cm and set the flame in the burner. Use distilled water (or demineralised water) and set to 0% flame intensity. Set 100% Flame intensity by using the maximum concentration of standard solution in the calibration region (50µg/ mL ). Use the other standard solutions (10-40µg/ mL ) as well as sample solution and determine the %Flame intensity. Construct a calibration curve of concentration of standard Vs % Flame intensity. From the %Flame intensity of sample solution, the concentration of ion in the sample solution can be determined.

31. Standard addition method In this method, the given sample solution is divided into aliquots and to each, increasing concentrations of standard solution of the same element is added. The % Flame intensity of sample solution as well as those of sample solutions to which standard solutions were added were determined. A calibration curve is constructed, with negative x-axis. The line when intrapolated , meets at negative x-axis, which is the concentration of the ion present in the given sample solution. This method is used, when the interfering elements cannot be removed or difficult to be removed from the sample matrix. Standard addition method overcomes Physical or anionic interference , but cannot avoid cationic interference. Eg . Standard addition method is used in the estimation of calcium in Magnesium chloride for dialysis .

32. Let us assume that the concentration of calcium in the given sample is X µg/ mL. Pipette out 5ml of aliquots of sample solution into each of four 10ml standard flasks. To the flasks, add respectively 5ml of 10µg/ mL , 20µg/ mL , 30µg/ mL , and 40µg/ mL of standard solution of calcium ions. Determine the %Flame intensity of sample solution ( Xµg / mL ) as well as those of (X+10µg/ mL ), (X+20µg/ mL ),(X+30µg/ mL ), and (X+40µg/ mL ). Plot a calibration curve. Intrapolate the line to meet negative X axis which is the concentration of the sample ion in the given sample solution.

33. Internal standard method Internal standard method is used to avoid or minimise the errors due to fluctuations in the flame intensity , errors in atomising the sample solution due to high viscosity , etc. In this method, a known concentration of a different element is added to standard solutions as well as to sample solution. The following example will illustrate the procedure adopted in internal standard method. Ex: Estimation of Ca in unknown sample Add known concentrations of strontium to various standard solutions of Ca and to sample solution. Measure the flame intensity of standard and sample at 227nm for Ca and 260 nm for strontium, simultaneously (since double beam flame photometer is used.

34. Plot a graph of conc. of Ca Vs I 227 /I 260 for standard solution. Concentration corresponding to I 227 /I 260 of sample is read from the graph.

35. Background absorption This occurs due to the sample matrix, flame itself, scattering, absorption by similar alkali halides etc. Remedy: Use of Gratings will avoid or minimise these interferences.

36. Spectral line Interferences/ Cation-Cation Interference Atomic line Interference, occurs due to the presence of other cations , which can emit radiation in the same region of emission by that of the element under analysis. Orange band of Ca-543-622nm interfere with Na doublet 589 and 589.6nm and Ba line at 553.6nm. Iron 324.7nm interferes with copper 324.8nm Iron 285.2nm interferes with Mg 285.2nm Aluminium interferes with emission lines of Ca and Mg Na and K mixtures interfere with each other. Remedy : Extraction of interfering material Calibration curve of interfering material Use of grating instead of prisms /Filters.

37. Vapourisation interference Chemical type, which occurs due to presence of some acids, by affecting the dissociation with other metals. Physical type, due to high viscosity eg dextrose, sucrose (interferes with atomisation) Overcome by Choice of Flames, burners, atomisers and additives ex: Use of acetylene/Nitrous oxide flame for thermally stable phosphates, sulphates, silicates, aluminates etc. Additives: Releasing agents: Add few ppm of Lanthalum /Strontium as ionisation suppressant to overcome interference due to PO 4 . Chelation / Masking: Add EDTA to mask Ca in the presence of PO 4 ( cation -anion-interference)

38. Ionisation interference More ionisation depopulates neutral atom both in ground state and excited state. Hence, it decreases the sensitivity of the method. Remedy: Add excess of easily ionisable ions like K, Cs, strontium as suppressants to the sample and standard solution (100-1000µg/ml). These have lower ionisation potential (LT 7.5 ev ), and hence they are preferentially ionised over the elements to be analysed.

39. To watch utube video of Flame Photometer, copy and paste the below link in utube search https ://youtu.be/tzloYQxLAkQ