Atomic Absorption Spectroscopy (AAS) PDF

## Atomic Absorption Spectroscopy (AAS) ### Introduction - AAS is a quantitative analysis used to determine the amount or concentration of trace metals in liquid having concentration about 1 ppm. - One of the most widely used modern instrumental technique. - Very versatile being capable of detecti...

## Atomic Absorption Spectroscopy (AAS) ### Introduction - AAS is a quantitative analysis used to determine the amount or concentration of trace metals in liquid having concentration about 1 ppm. - One of the most widely used modern instrumental technique. - Very versatile being capable of detecting 68 elements. - Extremely sensitive, detecting concentration of elements at parts per million (ppm) level - 1 ug = 10^-6 g - 1 ug in 1 g sample = 1 ppb = 10^-9 g ### Principle "If the light of resonance wavelength is passed through a flame containing the atoms than the part of light is absorbed and extent of absorption will be proportional to the no. of ground state atom present in flame." - ΔE = E_i - E_f = hc/λ - ΔE = hν, hν = hc/λ, Bohr's planck equation - no. of atoms in excited state: N_i / N_o = g_i / g_o * e^-ΔE/RT - no. of atoms in G.S: log F_o / F = KLN_o * path length - A = mx c x path - A = molar absorption - A = molar path absorption ### no. of molecules in upper level: - N_upper = g_upper / g_lower * e^-ΔE/K_B.T - energy difference (Per molecule) between upper and lower level - K_B: Temperature - K_B = 1.381 x 10^-23 J/K - no. of molecules in lower level ### Diagram **[Describe the diagram, include the description of the light source, sample, metal, excited state, ground state, GS, solution, mist, solid, MX, gas, gas, absorption of radiation energy, M^*(gas) and Flame: thermal excitation]**. ### Application - Determination of - Na, K, Ca, Mg in blood serum - Pb, Cr, Ag, Cd in food stuffs - Pb in Petrol - Mg in tap water - V in lubricating oils - Concentration of Hg in fish. - Forensic Science: Gun-Shot wound analysis of bullet (As, Pb, Cd). - Geographical Surveying: Analysis of metal soil, sediments & rock - Agriculture - Medical Pathology: Zn in tissue - Analysis of toxic metals in food and drinks: Fe, cu, zn, Mg in hairs & nails. ### Instrumentation - Atomic vaporin frame - Lines - Hollow Cathode lamp - Nebuliser - sol of Sample sprayed into flame - Detector - slit - Readout: AC- amplifier - Resonane line Source (Hallow Cathode lamp): responsible for excitation of ground state metal ions. They dislodge atom from at the cathode surface. They are the main source of current. ### Atominsation unit - ovens, electric arc, Sputtering devices, nebulizers burner system (flame AAS) - Convert the test solution into gaseous atoms. - The nebuliser. burner produces the mist or aerosol of the test solution. ### Mono-Chromatar - Selects a given emission line and isolates it from other lines and occasionally from molecular band emission. ### Detectors - Photomultiplier is the universally accepted detector. - In AAS the radiation received by the detector originates, not only from resonance line which has been selected but also from emission, within the flame. - Read out device:- meters, Chart recorders or digital display, may be used as readout devices. ### AAS for arsenic - "NO" Suitable. ### Hydride generation method - Elements like As, Sn, se etc are difficult to analyze by flame AAS because it is difficult to reduce compound of this elements to gaseous atomic state so hydride generation method is preferred. - Here compounds are converted into volatile hydrides at first: - As (v) + NaBH4 / H+ → AsH3 → As + H2 in flame - Suitably used Continuous flow vapour system can be suitably used ### Graph - Absorbance of standard solution ### AAS & flame Photometry **AAS** 1. Hollow Cathode lamp 2. Absorption of radiation energy. 3. Many metal Cation can be detected 4. Expensive and sophisticated instrument 5. It provides information about moleculer Structure **Flame** 1. Flame is used for thermal excitation 2. Thermal excitation of molecules 3. Limited metal cation can be detected 4. Cheap & simple technique 5. It does not provides ### Interference 1. Spectral interference: - Overlapping of spectra due to: selected wavelength (similar wavelength), Flame emission, Impurities. - Absorption value ↑ increases due to spectral interference, overcome by: seperation of impurities (solvent extraction), improved resolution, use of prism rather than filter, resonance source line 2. Chemical interference: - stable compound formation, By ionisation of sample during atomization process in flame, lowering temperature, by changing the fuels: propane, CH = CH, by changing the oxidant (O2, air, NO) - matrix nitric oxide. 3. Background inferference: - it can be reduced by monochromatic wavelength 4. Other effects: - matrix effect - The disturbance produced by physical factors like viscosity, suface tension , etc.. use to prepare solvent ### Limitation - Back ground absorption effect. - Analyte may be lost - especially for volatile compounds - Incomplete atomization of Sample - matrix effect - It is not useful for volatile metal - It is not useful for non-metal - Costly - Specialized chemist required - Not useful for radioactive metal ### MCQ - **Which of the following lamps is a light source for atomic absorption measurements.** - (A) Hollow cathode lamp - (B) Sodium lamp - (C) Deuterium lamp - (D) Tungesten filament lamp. - **The universal accepted detector in AAS is:** - (A) Photo multiplier - (B) Electrochemical - (C) Uv-visible - (D) Luminescence - **The effect of other components of the sample upon the behaviour of the analyte, in an analytical procedure is termed as:** - - (A) Interference effect - (B) Matrix effect ✓ - (C) Concentration effect - (D) Resonance effect