Atomic Spectroscopy AS (Part 3) (Chapter 20) Summary (1) PDF

Quick Review on previous Instruments Figure 23-1a shows the arrangement for absorption measurements. Figure 23-1b illustrates the configuration for fluorescence measurements.  Atomic Spectroscopy ❑ Atomic spectroscopy, AS, is the study of the interaction of electromagn...

Quick Review on previous Instruments Figure 23-1a shows the arrangement for absorption measurements. Figure 23-1b illustrates the configuration for fluorescence measurements.  Atomic Spectroscopy ❑ Atomic spectroscopy, AS, is the study of the interaction of electromagnetic radiation (EMR) with atoms. ❑ AS is based upon the absorption, emission, or fluorescence of EMR by atoms AAS, AES, AFS. ❑ Samples to be analyzed needs to be in atomic state through a process of atomization. ❑ AS can be used for the qualitative and quantitative determination of elements in solution or directly in solid samples. ❑ AS deals with atoms (elements). i.e. will not differentiate between ions of the same element such as Fe2+ and Fe3+. Schematic of AAS 3 Energy Level Diagrams The energy level diagram for the outer electrons of an element provides a convenient method for describing the processes upon which the various methods of atomic spectroscopy are based Na AAS, and AES Absorption lines for sodium vapor ❑ In a hot gaseous medium, Na atoms are capable of absorbing radiation of λ characteristics of electronic transitions from the 3s to higher excited states. ❑ For example, sharp absorption peaks at 5890. 5896, 3302 and 3303 oA correspond to transitions from 3s to 3p and 4p orbitals. ❑ Typically, an atomic absorption consists predominately of resonance lines. (non resonance transitions like 3p to 5s are weak and undetected). Fig. Emission spectra of several elements along with the spectrum of white light 6  Atomic absorption Spectrum It is made up of a limited number of narrow peaks or lines due to electron transitions within atoms Resonance line is the longest, most intense (strongest) line with higher wavelength and lower energy. λem= λex. The most probable transitions (strongest) used in quantitative analysis. Resonance line is used in qualitative analysis too. Bands N.B. only electronic transition is expected in atom (since there are no bonds. So; There are no vibrational and rotational states. E, V, and R transitions could be available in UV-Vis absorption of Molecules. 9 Atomization: The process by which the sample is converted into an atomic vapor” Gaseous state MS F-AES F-AAS F-AFS  Main types of AS Atomic Emission spectroscopy, AES (thermal Energy/ excitation) Sample is heated to excitation of the sample atoms. Excited atoms decay to a lower energy state through emission (no light source). Atomic Absorption spectroscopy, AAS (optical excitation) Light of a wavelength characteristic of the element of interest radiates through the atoms. The atoms absorb some of the light. The amount absorbed is measured (Light source; HCL). Atomic Florescence spectroscopy, AFS Atoms are excited to higher energy levels by absorption of radiation. A radiation characteristic of the element is emitted and measured (Light source; HCL). 10  The Copper Flame The Calcium Flame The Potassium Flame The Manganese Flame 12 Atomic Absorption Spectrometer 13 3 Main types of AS based on Different Types of Atomization (a)Flame atomization e.g. flame atomic absorption spectroscopy (FAAS) (Flame Photometry), there is also F-AES (b) Electro-thermal atomization: e.g. flameless graphite furnace atomic absorption spectroscopy (GFAAS), there is also GF-AES (c) Plasma atomiztion: e.g. inductively coupled plasma-optical emission spectroscopy (ICP-OES) Advantages of AS 1. High selectivity: each element has a characteristic spectrum. 2. Wide applicability: perhaps 70 elements can be determined 3. Excellent sensitivity: Ability to make ppm or ppb determinations. 4. High speed, and convenience; An atomic spectral analysis can be completed in a few minutes, sample preparation is simple, instrument easy to operate 14  Differences between AS & Molecular Spectroscopy Molecular AS spectroscopy Spectra Sharp lines Bands Light Source HCL D, T lamp Selectivity More selective, Less selective because each element has a characteristic spectrum. Interference Less, because there is a little More due to overlap overlap between sharp lines. between spectral bands. Q: Which is more selective & less interference? Give Reasons……. 15  Atomic Absorption Spectroscopy, AAS Definition & principle: Analytical technique for the quantitative determination of metallic elements and metalloids based on excitation of atom in gaseous state by absorption of radiation. This absorbed radiation is related to the sample concentration. Beer's law can be successfully applied for AAS A = ԑbC - Atomic absorption spectroscopy can be used to analyze the concentration of over 62 different metals. Atomic Absorption Spectrometer-FAAS 16  Instrumentation (Atomic Absorption Spectrometer) Principal components: 1. An atom cell (atomizer) (flame or electrothermal) 2. A light source (usually a hollow cathode lamp) 3. Monochromator (discussed before) 4. Detector and readout device (discussed before) 17  1- Atomizer: Atomizers are devices that carry out atomization The most commonly used atomizers are flame and electrothermal (graphite furnace) atomizers. Atomization : A process of forming free atoms by high temperatures 18  a) Flame atomizer The aerosol, formed by the flow of oxidant, is mixed with fuel b then burned in a slotted burner that provides a flame that is 5- 10 cm length. Laminar flow burners are designed to provide a quiet, stable flame with a long path length. This is due to the premixed fuel and oxidant gases, which burn smoothly and steadily(this enhances the sensitivity and reproducibility). It is important to have a close control on the flow rates of both the oxidant and fuel. Flame atomizers are the best in reproducibility. But in terms of sampling efficiency and sensitivity, other atomizers are better. Atomizer  a) Flame atomizer Three regions are involved in flame atomizer a. Nebulizer : Used to spray the sample in flame as aerosol (fine solid particles are dispersed in a gas). b. Premix Burner (Spray Chamber): The place in which fuel & oxidant are mixed with sample evenly. c. Burner Head (Flame) : The main function of the flame is atomization. Nebulizer 27  Flame Outerzone ▪ Several centimeters height ▪ Rich with free atoms ▪ Hottest region, Widely used for spectroscopy ▪No thermal equilibration ▪Seldom used for spectroscopy b) Electrothermal atomizers (Non flame atomizer) They are graphite tubes that are heated by passing an electrical current through it, where temperature is increased in a stepwise manner. N.B. *To prevent oxidation of the furnace, it is operated in presence of inert gas (Ar) * There is no nebulization step. The sample is introduced as a drop, slurry or solid particle Atomization process: A few microliters of samples are placed directly in the graphite furnace and the furnace is electrically heated where the following processes take place: 1. Evaporation of the sample solvent at low temperature (110 ºC), 20 Sec. 2. Ashing the sample to burn organic matter associated with the sample at moderate temperatures (1200 ºC), 60 Sec. 3. Atomization of the sample by rapid increase of the current so that a very high temperature is obtained. (2000-3000 ºC), 10 Sec (Measurement). 4. The temperature is raised to an extremely high value to clean the graphite tube. (3.0 minutes). 22  b. Graphite Furnace Atomization 2000-3000 oC Temperature Charring Data collection 500-800oC cooling Drying 50-200oC Time  Graphite furnace Electrothermal atomizer provides an enhanced sensitivity because the entire sample is atomized in a short period, and the average residence time of the atoms in the optical path is a second or more. 24 Features of Graphite furnace: 1. Solutions or solid samples could be determined. 2. Very small quantities of sample is needed (5-20 uL). 3. Temperature is controlled. 4. Atomization of the entire sample in a very short time 4. No explosion (no need for fuel-oxidant mixture). 5. Higher sensitivity (1000 times more than flame). Adv 1. high sensitivity (1000 times > FAAS). 2. Reasonable reproducibility and precision (FAAS>ET-AAS). 25 Comparison between flame and electrothermal atomizers Parameter Flame Electrothermal Sample efficiency low (~1%) high (100%) Residence time ~10-4 s ~ 1s Sensitivity Low ~100 times more sensitive Analysis time short long (several minutes/element) Minimum sample ~ 1 mL ~0.5 L Accuracy and Precision < 1% ~ 5-10% (% Error) Atomization process Less efficient More efficient Compatibility with solids Incompatible compatible (liquid)  2. Light (Radiation) Source: The produced atoms are then irradiated by optical radiation. A light source with a narrow bandwidth for light output is needed The light source is usually a hollow cathode lamp of the element that is being measured Hollow cathode lamp (HCL)  Sputtering process in HCL Application of potential difference (300 V) Ar Ar+ (Ionization) Absorbed M by Atoms M * * Ar+ in atomizer h Mo Mº 28 A hollow cathode lamp for Aluminum (Al) 29 Flame emission versus Flame atomic absorption FES AAS - Source of excitation: Flame (no - Hollow cathode lamp light source) (Light source) - measures: the radiation emitted by the radiation absorbed by the ground state the excited atoms atoms. - depends on: the number of excited - the number of ground state atoms. state atoms. - The temperature in the atomizer - The temperature in the atomizer is is high enough to atomize the adjusted to atomize the analyte only, analyte and excite them to a higher keeping it in the ground state energy level. (atomization) (atomization+ excitation) 30  The Lamps From bottom to top, the lamps are for Mg, Ca, K, and a combination of Fe, Co, Ni, Mn, Cu, and Cr. Each element uses a specific wavelength of light. 31  Hollow cathode lamps: - Consist of tungsten anode and a cylindrical cathode sealed in a glass tube that is filled with neon or argon at 1-5 torr. The cathode is constructed of the metal whose spectrum is desired, (or some cathodes are made of a mixture of several metals). Ionization of the inert gas occurs when a potential on the order of 300 V is applied across the electrodes, which generate a current of about 5 to 15 mA as ions and electrons migrate to the electrode. If the potential is sufficiently large, the gaseous cation acquire enough kinetic energy to dislodge some of the metal atoms from the cathode surface and produce an atomic cloud in a process called sputtering. A portion of the sputtered metal atoms are in excited state and thus emit their characteristic radiation as they return to the ground state. The efficiency of the hollow cathode lamp depends upon its geometry and operating potential. Figure 23-1c illustrates the configuration for emission spectroscopy.