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Atomic Absorption Spectroscopy

Lecture 13

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Flame Atomizers (Continuous Atomizers)

There are several types of flame atomizers available. The simplest is a turbulent flow burner that is very similar to conventional Bunsen burner. This type of burner suffers from fluctuations in temperature since there is no good mechanism for homogeneous mixing of fuel and oxidant. The drop size of nebulized sample is also inhomogeneous which adds to fluctuations in signal. The path length of radiation through the flame is small which suggests a lower sensitivity of the technique.

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Turbulent flow burners are also susceptible to flashback. These drawbacks were overcome using the most widely used laminar flow burner (also called premix burner) where quite flames and long path length are obtained. Flashback is avoided and very homogeneous mixing between fuel, oxidant, and droplets take place. Larger droplets are excluded and directed to a waste container. A schematic representation of the burner is shown below:

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Fuel and Oxidant Regulators

The adjustment of the fuel to oxidant ratio and flow rate is undoubtedly very crucial. Although stoichiometric ratios are usually required, optimization is necessary in order to get highest signal. However, in the determination of metals that form stable oxides, a flame with excess fuel is preferred in order to decrease oxide formation.

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Performance Characteristics of Flame Atomizers

Reproducibility of flame methods are usually superior to other atomization techniques. However, the residence time of an atom in a flame is in the order of 10-4 s which is very short. This is reflected in a lower sensitivity of flame methods as compared to other methods. Also, conventional flames with reasonable burning velocities can produce relatively low temperatures which make them susceptible to interference from molecular species.

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2 .Electrothermal AtomizationThese have better sensitivities than flame

methods. The increased sensitivity can be explained on the basis that a longer atom residence time is achieved (can be more than 1 s) as well as atomization of the whole sample in a very short time. As the name implies, a few mL of the sample are injected into the atomization chamber (a cylinder of graphite coated with a film of pyrolytic carbon) where the following processes take place:

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a. Evaporation: the solvent associated with the sample is evaporated in a low temperature (~120 oC) slow process (seconds)

b. Ashing: sample is ashed to burn organics associated with the sample at moderate temperatures (~600 oC, seconds)

c. Atomization: The current is rapidly increased after ashing so that a temperature in the range from 2000-3000 oC is obtained in less than1 second.

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Electrothermal Atomizers (Discrete Atomizers)

The heart of the atomizer, beside efficient heating elements and electronics, is a cylindrical graphite tube opened from both ends and has a central hole for sample introduction. It was found that porous graphite results in poor reproducibility since some of the analyzed materials will diffuse through porous graphite and will thus lead to a history effect.

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Therefore, the cylindrical graphite is made from a special type of nonporous high quality graphite called pyrolytic graphite. The length of the cylinder is 2-5 cm and it has less than 1 cm diameter. When the tube is fixed in place electrical contacts are achieved which are water cooled. Two inert gas streams (argon) flow at the external surface and through the internal space of the tube to prevent oxidation and clean the tube after each measurement. Usually, samples are analyzed in triplicates where three consecutive reproducible signals are required for each sample..

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