Lecture 06; atomization by Dr. Salma Amir
- 1. Lecture No. 06 Course title: Atomic Spectroscopy Topic: Atomization Course instructor: Dr. Salma Amir GFCW Peshawar
- 2. Atomization Atomization is the separation of particles into individual molecules and breaking molecules into atoms. This is done by exposing the analyte to high temperature in flame or graphite furnace. Sample atomization techniques Flame atomization Electrothermal atomization Hydride generation atomization Cold vapour atomization
- 5. Steps in Flame Atomization Aspiration: The transport of solution to the nebulizer tip is known as aspiration Nebulization: The sample introduction system disperses the sample into the high-temperature environment of a flame or plasma, convert the solution sample into a fine spray of droplets or mist. At this point, the metals are still in solution in the fine aerosol droplets. Evaporation/Desolvation: The atomizer heats the sample to the boiling point of the solvent and a solid particle remains Voltalization: The solid particle is then heated to a much higher temperature to produce vapor phase species. Atomization: The process of forming free atoms by applying heat to a sample is known as atomization. Ionization/dissociation:
- 7. Steps in Electrothermal Atomization The graphite furnace tube is subjected to a multistep temperature program. The program controls the temperature ramp rate, the final temperature at each step, the length of time the final temperature is held at each step, and the nature and flow rate of the purge gas through the furnace at each step. A typical graphite furnace program consists of three main steps: (1) dry, (2) pyrolyze (ash, char), (3) atomize,
- 8. The process of atomization is extremely fast and must be rigidly controlled. The temperature program is therefore very carefully controlled, both with respect to the times used for each section of the heating program and the temperature range involved in each step. It is vital to avoid loss of sample during the first two steps but it is also extremely important to eliminate as much organic and other volatile matrix material as possible. Sample introduction: A small volume of solution, between 5 and 50 μL, is injected into the graphite tube via a micropipette or an autosampler Dry step: The “dry” step is used to remove the solvent. The solvent must be removed without splattering the sample by rapid boiling, which would result in poor precision and accuracy. A slow temperature ramp from room temperature to about 110°C is used for aqueous solutions. The upper temperature is held for about a minute. The purge gas during this step is the normal inert gas (nitrogen or argon) at its maximum flow of about 300 mL/min to remove the solvent vapor from the furnace.
- 9. The pyrolyze step: The “pyrolyze” step is also called the ashing step or the “char” step. Its purpose is to remove as much of the matrix as possible without volatilizing the analyte. The “matrix” is everything except the analyte; it may consist of organic compounds, inorganic compounds, or a mixture of both. The sample is again subjected to a temperature ramp. The upper temperature is chosen to be as high as possible without losing the analyte and held for a short time (350-1200°C for 45 sec) The gas flow is normally 300 mL/min and is usually the inert purge gas. With some organic sample matrices, switching to air is done in this step to help oxidize the organic materials. The atomization step: The atomization step must produce gas-phase free analyte atoms. The temperature must be high enough to break molecular bonds. In general, the temperature is raised very rapidly for this step. Typically, the temperature of the furnace is adjusted to between 2000- 3000°C for a period of about 5sec. The purge gas flow is stopped to permit the atoms to remain in the light path. Stopping the purge gas flow increases the sensitivity of the analysis. The absoption of the analyte is measured during the automization step.
- 10. Factors affecting Atomization 1. Droplet size 2. Sample feed rate 3. Flame temperature 4. Fuel and oxidant ratio 5. Chemical nature of analyte 6. Solvent affect
- 11. 1. Droplet size To measure an atomic absorption signal, the analyte must be converted from dissolved ions in aqueous solution to reduced gas- phase free atoms. The solution is converted into a fine mist or aerosol, with the analyte still dissolved as ions. When the aerosol droplets enter the flame, the solvent (water) is evaporated, followed by vaporization and atomization. If the droplet size is large, surface area per unit weight is small and evaporation is slow. And there is incomplete evaporation results in incomplete atomization that affects absorption signal. On the other hand, If the droplet size is small, surface area per unit weight is large and there will be rapid evaporation, leaving a solid metal residue which is then thermally decomposed to free atoms and contribute to the absorption signal.
- 12. 2. Sample feed rate If the sample feed rate is too high, much of the energy of the flame is used up in nebulizing and disintegration of the sample. The final atomization step is insufficient. Under these conditions, the flame is swamped with the sample. On the other hand, if the sample feed rate is too low, the production of the neutral atoms is reduced and again the absorption signal is diminished.
- 13. 3. Flame temperature Lower temperature flames cannot convert the substances into atoms and there will be incomplete atomization of the analyte resulting in less absorption. Too much energy of the flame convert the free atoms to ions. Atoms with small ionization energies will ionize readily at high temperatures (and even at moderate temperatures) and decreases the absorption signal.
- 14. 4. Fuel and oxidant ratio Flames are classified as oxidizing (excess oxidant) or reducing (excess fuel). Air–acetylene flames can be used in either an oxidizing mode or a reducing mode; nitrous oxide–acetylene flames are usually run in a reducing mode. In general, excess oxidant helps to destroy organic material in samples. However, excess oxidant can react with elements that exist as stable oxides to form oxide molecules. These oxide molecules cannot undergo atomic absorption. Elements that form stable oxides, such as aluminum, boron, molybdenum, and chromium, are therefore determined using reducing flame conditions, usually with the high-temperature nitrous oxide–acetylene flame, to prevent the formation of oxide molecules.
- 15. 5. Chemical nature of analyte The formation of free atoms depends on the flame temperature and on the chemical form of the sample. Chemical species with small dissociation energies at high temperatures will dissociate to form free atoms. For a given flame temperature, free atom formation depends on the chemical species and its dissociation constant. If the metal exists in the sample in a stable chemical form, it may be difficult to decompose, and atomization efficiency is low. On the other hand, if the metal is in a chemical form that is easily decomposed, the number of atoms formed is high and the atomization efficiency is high.
- 16. 6. Solvent effect Since most organic solvents have lower viscosity, surface tension and specific gravity than water, so they are more easily aspirated, bring about finer nebulization so that considerably more sample reaches the flame per unit time, resulting in higher atomization