Atomic Absorption Spectroscopy, Principles and Applications.pptx

AAS spec principles

1. Atomic Absorption Spectroscopy, Principles and Applications

2. What is atomic absorption spectroscopy? Technique for measuring the concentrations of metallic elements in different materials. Involves absorbance of light by metallic atoms in the gas phase, the light typically being sourced from a lamp specific to the measurement of a single element. Determination of most metals and metalloids with this technique offers sufficient sensitivity for many applications and is relatively interference free. Metals make up around 75% of the earth’s chemical elements. Metal content in a material is desirable, but metals can also be contaminants (poisons). Used in quality control, toxicology and environmental testing.

3. Introduction Atoms absorb light at a definite  wavelength  depending on the nature of chemical elements. Sodium  is absorbed in 589 nm Uranium  is absorbed in 589 nm Potassium  is absorbed in 766.5 nm Light at this wavelength has absorbed energy to excite another electronic state. The electronic transition is specific to a particular element. From the ground state of an atom is excited to a higher energy level by absorption of energy. Atomic spectra are identified by sharp lines which can be distinguished from broadband spectra associated with  molecules . Usually, lines arising from the ground state are almost important in atomic absorption spectroscopy. These are called resonance lines.

4. Simple diagram showing atomic energy levels

5. Schematic diagram of a typical atomic absorption spectrometer.

6. What is the principle of atomic absorption spectroscopy? Firstly, all atoms or ions can absorb light at specific, unique wavelengths. When a sample containing copper (Cu) and nickel (Ni), for example, is exposed to light at the characteristic wavelength of Cu, then only the Cu atoms or ions will absorb this light. The amount of light absorbed at this wavelength is directly proportional to the concentration of the absorbing ions or atoms. The electrons within an atom exist at various energy levels. When the atom is exposed to its own unique wavelength, it can absorb the energy (photons) and electrons move from a ground state to excited states. The radiant energy absorbed by the electrons is directly related to the transition that occurs during this process. Furthermore, since the electronic structure of every element is unique, the radiation absorbed represents a unique property of each individual element and it can be measured.

7. How does an atomic absorption spectrometer work? The AA spectrometer works by: Creating a steady state of freely dissociated ground state atoms using a heat source (flame) Passing light of a specific wavelength through the flame. The wavelength corresponds to the amount of energy required to excite an electron from (typically) the ground to first excited state for a specific element. Measuring the amount of the light absorbed by the atoms as they move to the excited state (the atomic absorption). Using the measured absorbance to calculate the concentration of the element in a solution, based on a calibration graph.

8. The electromagnetic spectrum. Atomic absorption spectroscopy uses light in the ultraviolet-visible range.

9. Beer-Lambert Law describes the relationship between light absorption and concentration of the element The Beer-Lambert Law defines the relationship between the concentration and absorption of an absorbing species A =     ε  * c * l Where: A is the absorbance (Abs). Abs is measured by the AAS. ε is the molar absorption coefficient. This is the absorptivity of the sample at a particular wavelength. c is the determined concentration of the element. l is the path length. For flame AAS, this is typically the path length through the flame (along the burner) and is fixed for all measurements.

10. Instrumentation Simple flame atomic absorption spectrometer includes: A sample introduction system The burner (flame) and its associated gas supplies: air-acetylene or nitrous oxide-acetylene A light source, the hollow cathode lamp (HCL) A monochromator (the optical components inside the box in the diagram) An optical detector (photomultiplier tube or PMT) Computerized instrument control, data collection, and analysis.

11. AA spectrometer sample introduction system The liquid sample is transported via capillary tubing into the nebulizer. The pneumatic nebulizer makes use of the Venturi effect, the principle that fluid flows at a higher velocity through a narrower tube, to accelerate the solution stream. The fluid then impacts a glass bead to create a fine spray of droplets, known as an aerosol. In nebulizer the gaseous sample is broken up into a fine mist which is then carried to the atomizer, such as a flame, by a carrier gas.

12. Atomization Can be carried out either by a flame or furnace. Atomization refers to breaking bonds in some substance to obtain its constituent atoms in gas phase. Means by separating something into fine particles Heat energy is utilized in atomic absorption spectroscopy to convert metallic elements to atomic dissociated vapor. The temperature should be controlled very carefully for the conversion of atomic vapor. At too high temperatures, atoms can be ionized.

13. Atomizing techniques - flame atomic absorption spectroscopy (FAAS) FAAS is mainly used to determine the concentration of metals in solution in parts per million (ppm) or parts per billion (ppb) ranges. The metal ions are nebulized as a fine spray into a high-temperature flame where they are reduced to their atoms and subsequently absorb light from an element-specific hollow cathode lamp. Robust technique for routine metal determinations. But, it has limited sensitivity because of the spectral noise created by the flame. Can only measure one metal at a time and as different lamps are required for each element, the lamp must be changed each time you want to analyze for something different. Also, a large part of the sample is lost in the flame (up to 90%) in FAAS, further influencing the sensitivity.

14. The atomization process in FAAS.

15. Fuels and Oxidants Used for Flame Combustion fuel oxidant temperature range ( o C) natural gas air 1700–1900 hydrogen air 2000–2100 acetylene air 2100–2400 acetylene nitrous oxide 2600–2800 acetylene oxygen 3050–3150

16. Advantages and Disadvantages of Flame Atomization Advantage ; reproducibility with which the sample is introduced into the spectrophotometer; Disadvantage ; efficiency of atomization is quite poor. There are two reasons for poor atomization efficiency. First, the majority of the aerosol droplets produced during nebulization are too large to be carried to the flame by the combustion gases. Consequently, as much as 95% of the sample never reaches the flame, which is the reason for the waste line shown at the bottom of the spray chamber. Second reason for poor atomization efficiency is that the large volume of combustion gases significantly dilutes the sample.

17. Atomizing techniques - graphite furnace atomic absorption spectroscopy (GFAAS) In GFAAS, a type of electrothermal atomization, a sample is placed in a hollow graphite tube which is heated until the sample is completely vaporized. GFAAS is much more sensitive than FAAS and can detect very low concentrations of metals (less than 1 ppb) in smaller samples. Using electricity to heat the narrow graphite tube ensures that all of the sample is atomized in a period of a few milliseconds to seconds. The absorption of the atomic vapor is then measured in the region immediately above the heated surface. Naturally, the detection unit does not have to contend with spectral noise, leading to improved sensitivity.

18. Typical graphite tube atomization process.

19. Atomization Method

20. Comparison of standard burner and high temperature burner Standard burner High temperature burner Gas system Air-acetylene: 2000°C Air-hydrogen: 2000°C Argon hydrogen: 1600°C Dinitrogen oxide-acetylene: 2900°C Tip size 0.5 mm x 100 mm 0.4 mm x 50 mm Measured elements Pb Cd Fe Cu Mn Cr Au K Ag Zn Na Ca Mg etc. Al B Ba Be Ge Si Ti V W etc.

21. LIGHT SOURCE Hollow cathode lamp (HCL) is used as the light source. It consists of a cylindrical hollow cathode made up of the same element under consideration. The anode is made up of tungsten. Both anode and cathode are enclosed in a glass tube with a quartz window. The metal which is used in the cathode is the same as that metal that we analyzed . The lamp is filled with noble gas at low pressure. The lamp forms a glow of emission from the hollow cathode.

22. Monochromator An optical device that transmits a narrow band of wavelengths of light or other radiation from a wider range of wavelengths. The atoms in the AAS instrument accept the energy of excitation and emit radiation. A desired band of lines can be isolated with a monochromator by passing a narrow band. The spectra through a monochromator can be shown by a curve. Types of monochromators ; Prism Diffraction gratings Dispersion from the grating is more uniform than dispersion from the prism. So, the grating can maintain a higher resolution over a longer range of wavelengths.

23. AAS Monochromator

24. Detector Detector can convert light coming from a monochromator to a simplified electrical signal. Photomultiplier tubes are the most commonly used detectors in atomic absorption spectroscopy. These are used to convert electromagnetic waves into electric currents.  They are made up of photoemissive cathodes and dynodes. These dynodes provide electron multiplications. Photodiode array detectors These detectors convert the transmitted light signals into an electric pulse.

25. Photomultiplier tube (PMT)

26. Computerized instrument control/Recorder Recorder can receive electrical signals from the detector to convert them into a readable response. In atomic absorption spectroscopy instrumentation, today we used a computer system with suitable software for recoding signals coming from the detector. Modern AA spectrometers contain a network of sensors and use sophisticated algorithms to monitor and control their operating conditions. They notify the user when there is a problem with the system and ensure safety of operation.

27. Atomic Absorption Spectrometer Model: AAnalyst 100 and PinAAcle 900F Manufacturer: Perkin Elmer

28. Calibration The instrument must be calibrated before analysis for better results.  There must be five standard solutions containing a known concentration of metals to be analyzed for calibration of the instrument and of the same concentration as the sample solutions.  Calibrating solution is generally prepared as a sample of salt dissolved in water or dilute acid. 

29. Strengths and limitations of atomic absorption spectroscopy (AAS) Advantages Limitations Low cost per analysis Cannot detect non-metals Easy to operate New equipment is quite expensive High sensitivity (up to ppb detection) More geared towards analysis of liquids High accuracy Sample is destroyed Mostly free from inter-element interference   Wide applications across many industries  

30. What are the benefits of Atomic Absorption Spectroscopy? Despite being one of the first elemental analytical techniques on the market, Atomic Absorption Spectroscopy (AAS) is still in widespread use across many industries. This is largely because the benefits of AAS are simplicity, reliability, and low cost while still delivering precise, accurate results.

31. The benefits of flame AAS Flame atomic absorption spectrometry (FAAS) is an entry-level spectroscopy technique that is ideally suited to labs in which a small number of elements are routinely measured. Compared with other atomic spectroscopic techniques, flame AAS: Has the lowest capital cost, making it a more affordable option Is a good technique for those new to spectroscopic analysis, due to the simplicity of operation Can deliver excellent sample throughput for a single element analysis Can measure samples over a wide concentration range, from low ppm to percent levels, without the need to dilute the sample Delivers accurate results Can easily be operated manually or can be automated with an additional autosampler

32. The benefits of graphite furnace AAS    In contrast to FAAS,  Graphite Furnace Atomic Absorption Spectroscopy (GFAAS)  is a highly sensitive technique that allows labs to achieve ultra-trace analysis down to low ppb levels. Other benefits of GFAAS include: Robust technique capable of handling samples with high total dissolved solids (TDS) Cost-effective trace element analysis option Can be operated without constant supervision, as no flammable gases are required Ideal option for labs with trace detection limit requirements and small numbers of samples Ideal when a very small volume of sample is available Suitable for a limited budget where ICP-MS is out of reach or unnecessary

33. What are the applications of atomic absorption spectroscopy? Today, the atomic absorption spectroscopy technique is the most powerful tool in  analytical chemistry ,  forensic science , environmental analysis, and food industries. It is popular for analysts due to several advantages ; Speed of analysis. It can analyze various samples within a day. Possible to determine all elements at trace  concentration . Not always essential to separate the element before analysis because AAS can be used to determine one element in presence of another. The atomic absorption spectroscopy principle or instrumentation can be used to analyze sixty-seven metals and several nonmetals such as  phosphorus  and  boron .

34. APPLICATIONS OF ATOMIC ABSORPTION SPECTROSCOPY Atomic absorption is the most widely used technique for the determination of metals at trace and ultra-trace levels in a solution. It is used to analyze metals in biological fluids such as blood, hair, and urine. Atomic absorption is used to find out the number of various metals in the environment. It is used in pharmaceutical industries for quantitative analysis of metals in samples for example, in multivitamins tablets. Atomic absorption is used to find the concentration of metals (Ca, Mg, Fe, Si, Al, )  analysis of water. Atomic absorption is used to determine the amount of catalyst.

35. Application of atomic absorption spectroscopy… It is used for the analysis of crude oil, and petroleum. Atomic absorption is used for the analysis of soil samples. For example, minerals inland before cultivation are tested to get maximum yields. It is used for trace element analysis of cosmetics. Atomic absorption is used to find the amount of metals such as gold in rocks. It is used to determine the amount of various metals (Mn, Fe, Cu, Zn) in foodstuff. It is used for the analysis of additives (Ba, Ca, Na, Li, Mg) in lubricating oils and grease.

36. References Atomic Absorption Spectroscopy, Principles and Applications https://www.technologynetworks.com/analysis/articles/atomic-absorption-spectroscopy-principles-and-applications-356829 Atomic Absorption Spectroscopy https://psiberg.com/atomic-absorption-spectroscopy/ Atomic Absorption Spectroscopy https://www.priyamstudycentre.com/2021/11/atomic-absorption-spectroscopy.html