Chapter 9 - Chapter 9 Atomic Absorption and Atomic...

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Unformatted text preview: Chapter 9 Atomic Absorption and Atomic Fluorescence Spectrometry Flame Atomization: In a flame atomizer, a solution of the sample is nebulized by a flow of gaseous oxidant, mixed with a gaseous fuel, and carried into a flame where atomization occurs. The following processes then occur in the flame. Desolvation (produce a solid molecular aerosol) Dissociation (leads to an atomic gas) Ionization (to give cations and electrons) Excitation (giving atomic, ionic, and molecular emission) Types of Flames: Several common fuels and oxidants can be employed in flame spectroscopy depending on temperature needed. Temperatures of 1700 o C to 2400 o C are obtained with the various fuels when air serves as the oxidant. At these temperature, only easily decomposed samples are atomized. For more refractory samples, oxygen or nitrous oxide must be employed as the oxidant. With the common fuels these oxidants produce temperatures of 2500 o C to 3100 o C. Burning Velocity : The burning velocities are of considerable importance because flames are stable in certain ranges of gas flow rates only. If the gas flow rate does not exceed the burning velocity, the flame propagates itself back in to the burner, giving flashback. As the flow rate increases, the flame rises until it reaches a point above the burner where the flow velocity and the burning velocity are equal. This region is where the flame is stable. At higher flow rates, the flame rises and eventually reaches a point where it blows off of the burner. Flame Structure: Important regions of a flame include: 1. primary combustion zone 2. interzonal region 3. secondary combustion zone 1. Primary combustion zone: Thermal equilibrium is ordinarily not reached in this region, and it is, therefore, seldom used for flame spectroscopy. 2. Interzonal region: This area is relatively narrow in stoichiometric hydrocarbon flames, is often rich in free atoms and is the most widely used part of the flame for spectroscopy. 3. Secondary combustion zone: In the secondary reaction zone, the products of the inner core are converted to stable molecular oxides that are then dispersed into the surroundings. Temperature Profiles: A temperature profile of a typical flame for atomic spectroscopy is shown in Fig. 9-3. The maximum temperature is located in the flame about 1 cm above the primary combustion zone. It is important particularly for emission methods to focus the same part of the flame on the entrance slit for all calibrations and analytical measurements. Flame absorbance Profiles: Fig. 9-4 shows typical absorption profiles for three elements. Magnesium exhibits a maximum in absorbance at the middle of the flame. The behavior of silver, which is not readily oxidized, is quite different, a continuous increase in the number of atoms, and thus the absorbance, is observed from the base to the periphery of the flame. Chromium, which forms very stable oxides, shows a continuous decrease in absorbance beginning close to the burner tip. Flame Atomizers:...
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This note was uploaded on 05/06/2010 for the course CHEM 4414 at Arkansas Tech.

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Chapter 9 - Chapter 9 Atomic Absorption and Atomic...

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