Atomic Absorption (AA) Spectrometer Atomic absorption spectroscopy is a quantitative method of analysis that is applicable to measure many metals and a few nonmetals. Keep in mind: All instruments work quite similar! What is the similarity? What is the difference? Objectives Principles Major components Light Sources Flame Monochromators / Wavelength selectors Detectors Application Inductively Coupled Plasma – a new phase of metal analysis Principles Atomic Absorption spectroscopy involves the study of the absorption of radiant energy by neutral atoms in the gaseous state. Some facts: A much larger number of the gaseous metal atoms will normally remain in the ground state. These ground state atoms are capable of absorbing radiant energy of their own specific resonance wavelength. Excited State Emits Special Electromagnetic Radiation Ground State Principles Then it will be possible: If light of the resonance wavelength is passed through a flame containing the atoms in question, then part of the light will be absorbed. The extend of absorption will be proportional to the number of ground state atoms present in the flame. What we have in mind? Specific resonance wavelength The gaseous metal atoms Measurement of absorption The relation of absorption vs the number of ground state atoms present in the flame. AA - Major components Light source Flame Monochromator Detector Nebulizer Read-out Schematic diagram of a flame atomic absorption spectrophotometer AA - Major components Flame Detector Light source Monochromator AA - Light Source Remember the facts? A neutral atom in the gaseous state can absorb radiation and transfer an electron to an excited state. But: This change of electron state occurs at discrete λ So: Bandwidth (wavelength range) much narrower! Why: Non-linear behavior observed when λ range of excitation source is greater than λ range of absorber; In conclusion, bandwidth of excitation source must be narrower than bandwidth of absorber. Monochromator cannot be used to select λ range in AA (bandwidth = few tenths of a nm, some even 0.002 nm). AA - Light Sources How to solve the problem? This difficulty was overcome by using a source of sharp emission lines with a much smaller half-width than the absorption line, and the radiation frequency of which is centered on the absorption frequency. Excited State Emits Special Electromagnetic Radiation Solution to line width problem: Ground State Use atomic source of the same material of your interest, e.g. for Na analysis, Na vapor is used. Atoms are excited by electrical discharge; the excited atoms emit a characteristic λ. Resonance line sources Provide the sharp emission lines with a much smaller halfwidth than the absorption line Emit the specific resonance lines of the atoms in question Considerations? - Intensity - Purity - Background - Stability - Life-time Hollow cathode lamp (HCL) Cathode-in the form of a cylinder, made of the element being studied in the flame Motorized Mirror HCL A hollow cathode lamp for Aluminum (Al) AA - Flame Purpose To convert the test solution to gaseous atoms in neutral state. How we can make a flame ? Oxidant and Fuel Air-propane Air-acetylene Air-hydrogen When finish the analysis, which should be turned off first, the fuel or the oxidant? A question in the test! AA - Flame How to do it? Nebulizer (or burner) - to produce a mist or aerosol of the test solution Vaporizing chamber - Fine mist is mixed with the fuel gas and the carrier gas. Larger droplets of liquid fall out from the gas stream and discharged to waste. Burner head -The frame path is about 10 –12 cm Nebulizer-burner A typical premix burner Flame characteristics: Sample enters flame, is vaporized, reduced and eventually oxidized. Exact region of flame in which each of these occurs depends upon: Flow rate Liquid drop size Oxidizability of sample Optimum position of flame for many metals. Flame Profiles in AA FUELS/OXIDANTS • Fuels: natural gas, propane, butane, H2, and acetylene • Oxidants- Air and O2 (low temperature flames), N2O (high temperature flames) • Low temperature flames: easily reduced elements (Cu, Pb, Zn, Cd) • High temperature flames: difficult to be reduced elements (e.g. alkaline earth elements, Na, K) Disadvantages of flame atomization Only 5 –15 % of the nebulized sample reaches the flame A minimum sample volume of 0.5 –1.0 mL is need to give a reliable reading Samples which are viscous require dilution with a solvent Advanced techniques Graphite furnace technique Cold vapor technique AA - monochromators / Wavelength selectors • Needed to choose one of several possible emission lines (λ emitted) associated with HCL. • They are usually reasonably well separated from the line of interest, a common monochromator can effectively eliminate this interference. • Why not put the monochromator ahead of the flame (sample) as in a vis-uv spectrophotometer? AA - Detectors Photo Multiplier tube Photodiode Photodiode & CCD AA - Application Lambert-Beer’s Law • Absorbance = abC a : Absorbance Constant b : Sample Path length - the path length through the flame (cm) C : Sample Concentration Experimental procedures Preparation of sample solutions Optimization of the operating conditions -resonance line -slit width -current of HCL -atomization condition Calibration curve procedure Interferences need to be considered? Spectral interferences -spectral overlap -non-absorption line -molecular absorption -light scatter Chemical interferences -Stable compound formation (increase in flame temperature, use of releasing agents, separation) -Ionization (adding an excess of an ionization suppressant) Physical interferences -viscosity -density -surface tension -volatility AA - Application Advantages: Very sensitive. Fast. Disadvantages: Hollow cathode lamp for each element. Expensive element. Inductively Coupled Plasma Excited State Emits Special Electromagnetic Radiation Ground State
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