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Transcript of Chemistry 311: Instrumentation Analysis Chemistry 311: Instrumentation Analysis Topic 2: Atomic...

  • 9/30/09

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    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 41

    Topic 2: Atomic Spectroscopy Atomic line widths: Narrow line widths reduce the possibility of spectral overlap and thus interferences. The band width at half height is used to indicate width. This is also sometimes called the effective line width Δλ½.

    Line broadening arises from 4 sources; Uncertainty Effect: Broadening due to the uncertainty principal relating to the uncertainty in state lifetimes Doppler effect: Broadening due to the Doppler effect (see below) Pressure effects: Broadening due to collisions of the emitted or absorbing species causing small changes in the energy level of the ground state. Electric and magnetic effects: Broadening due to the Zeeman effect or in other words the splitting of states due to presence of electric (or mag. fields)

    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 42

    Topic 2: Atomic Spectroscopy

    Results from the Doppler shift phenomenon common for sound waves.

    The magnitude of the Doppler shift increases with the velocity that the emitting species is traveling to or from the detector (observer). No shift is observed by species moving perpendicular to the detector.

    Atomic velocity is distributed over a range described by a Maxwell-Boltzman distribution. The average velocity increases as the square root of the absolute temperature. In flames, Doppler effect broadens lines two orders of magnitude greater than natural line widths.

    Introduction to Atomic Absorption Spectrometry: Doppler effect:

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    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Page 43

    Temperature Effects on Atomic Spectra: Temperature has a significant effect on the population of excited vs ground atomic species. The distribution is described by the Boltzman equation;

    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 44

    Topic 2: Atomic Spectroscopy Temperature Effects on Atomic Spectra: Emission spectra are very dependant on the temperature of the atomizer. However, even in very hot flames only a small fraction of atoms are in excited states, Absorption and fluorescence are much less temperature dependant. High temperature increases atomization of the sample, while also enhancing Doppler effect and increasing ionization. Although, adsorption involves more states, it also involves measuring a small difference (A = log P0 - log P), thus trade off

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    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 45

    Complications and Sources of Continuum Spectra: Molecular Species produce broad absorption bands due to the close “spacing” of vibration and rotation states associated with excited states and ground states.

    These broad continuum spectra can interfere (overlap) with a number of discrete atomic transitions.

    Formation of molecular species can also reduce the effective gaseous atomic concentration of analytes, also resulting in reduced analyte signal.

    The adverse effects from characteristic molecular species can be reduced by selecting appropriate characteristic signals, altering flame chemistry and in some cases by background subtraction.

    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 46

    Topic 2: Atomic Spectroscopy Sample Introduction Techniques in Atomic spectroscopy: • The goal of the sample introduction technique is to transfer a reproducible and representative sample into the atomizer. • This step limits the accuracy, precision and the detection limits of the technique. • This is strongly dependant on the physical and chemical state of the analyte and sample matrix. For solid samples, sample introduction is a major problem. For liquids and gases, this is relatively simple. Liquids are converted to a fine mist or aerosol and then introduced or vaporized.

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    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 47

    Topic 2: Atomic Spectroscopy Pneumatic Nebulizers:

    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 48

    Topic 2: Atomic Spectroscopy Other Atomic Absorption Introduction Techniques: •  Ultrasonic Nebulizers: Similar aerosol effect to pneumatic, however, quartz crystal used to produce a dense, homogenous mist. •  Electrothermal Vaporizers (Liquid and Solid): Sample is vaporized by rapid resistive heating of a graphite or tantalum material. Material is entrained in a flow of inert gas. A transient peak is observed. Peak Height or Peak area used to quantify. •  Hydride Generation Techniques: Volatile Hydrides are generated by a chemical reaction (see below). Higher transfer efficiency of specific analyte types; arsenic, antimony, tin, selenium, bismuth and lead. Results in an increase in sensitivity of 10 to100.

    •  Direct Sample Insertion (Solid): Sample is physical placed into atomizer by a probe or similar and transient signal is produced.

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    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 49

    Other Atomic Absorption Introduction Techniques:

    Arc and Spark Ablation (Solid): Electrical discharges involving the surface of a sample can lead to the ablation of surface material to form a plume of particulate and vapors which is then swept into atomizer by inert gas.

    Laser Ablation: Material ablated from surface by an intense laser beam. Versatile because the material can be of almost any type.

    Glow discharge Techniques: A glow discharge takes place in a low- pressure atmosphere (1 to 10 torr) of argon gas between a pair of electrodes maintained at a dc potential of 250 to 1000 V. The argon gas to break down into positively charged argon ions and electrons. The electric field accelerates the argon ions to the cathode surface that contains the sample. Neutral sample atoms are sputtered from surface.

    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 50

    Topic 2: Atomic Spectroscopy Sample Atomization Techniques:

    •  Flame Atomization: The sample is nebulized into a gaseous oxidant flow, mixed with a fuel and then transported into a flame region for atomization.

    •  Electrothermal Atomization: A small aliquot of sample (µL) is injected into a graphite furnace where it is atomized via resistively heating to ~3000°C

    •  Glow Discharge Atomization: Small amounts of solid conductive samples are sputtered from a surface acting as an electrical cathode. An electrically accelerated stream of ionized argon is provides the energy for the sputtering and atomization process.

    •  Hydride Atomization: Volatile metal hydrides are produced from a chemical reaction and then atomized by heating in a quartz tube.

    •  Cold-Vapor Atomization: Mercury metal has sufficient volatility to be atomized by vaporization and thus the atomic absorption (@253.7 nm) can be obtained cold. Can produce low ppb detection limits.

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    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 51

    Topic 2: Atomic Spectroscopy

    Solution
 Nebulization
 Desolvation
Spray
 Aerosol


    Molecules
 Atoms
 Atomic
Ions
Dissociation
 Ionization


    Excited
 Molecules


    Excited
 Atoms


    Excited
 Atomic
Ions


    Temperature
Dependant
 Population
excitation


    Temperature
Dependant
 Population
excitation


    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 52

    Topic 2: Atomic Spectroscopy Flame Atomization Techniques: The temperature and chemistry of the flame will have a significant impact on the relative populations of molecules, atoms, ions and there electronically excited states. This will subsequently have a significant impact on the analytical signal.

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    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 53

    Topic 2: Atomic Spectroscopy Flame Structure: Primary combustion zone: In hydrocarbon, blue luminescence zone (C2, CH etc.). Thermal equilibrium not yet reached. Interzonal region: Common zone to use for spectroscopy. rich in free atoms and the hottest part of the flame Secondary combustion zone: Atoms and other reagents converted to more stable species, such as oxides.

    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 54

    Topic 2: Atomic Spectroscopy Electrothermal Atomizers (Graphite Furnace): A small aliquot (µL) of sample (or entire sample) is introduced into a graphite furnace that is heated electrically to 2000°C to 3000°C.

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    Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

    Winter 2009 Page 55

    Topic 2: Atomic Spectroscopy Electrothermal Atomizers (Graphite Furnace):

    Advantages: Sample size 0.5 µL to 10 µL, with detection limits of 10-10 to 10-13 g of analyte.

    Disadvantages: Reproducibility (RSD) 5% t