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be small enough to allow the velocity of gases flowing into the flame to be greater than the

burning velocity.

If the flame burns faster than the flow velocity through the slot, the flame can enter the
premix chamber and explode. With fast burning gases, slot burners cannot be used because the

burning rate is too high.
Flame Profile

Different substances require different amounts of energy to be converted from species

that are found in solutions to atoms. If too little energy is used, the substance cannot be
converted into atoms: and if too much energy is used, the substance can be converted into ions

rather than atoms. The energy for the conversion to atoms is supplied by the heat of the flame.
Ionization occurs whenever the energy that is available from the flame exceeds the ionization

energy of the sample. The amount of energy that can be supplied by the flame is directly
proportional to the temperature of the flame. Because different substances require different

amounts of energy to be converted to atoms, it is necessary to have control over the flame

temperature.
Small changes in flame temperature can be achieved by varying the ratio of the oxidant

gas to the fuel gas supplying the flame.
Lean flame: A flame in which sufficient oxidant is used to efficiently react with all of the

fuel is a lean flame.

Fuel-Rich flame: A flame in which an excess of fuel is used is a fuel-rich flame.
Lean flames are hotter than fuel-rich flames. Greater temperature changes are brought

about by changing the oxidant, the fuel, or both the oxidant and the fuel. Table lists the oxidant,
the fuel, and the approximate maximum temperature of several flames which can be used for

atomic spectroscopy.

For AAS it is generally best to choose the lowest temperature flame that will efficiently
atomize the assayed element. A higher temperature could result in loss of signal owing to

ionization. For most assays an air-acetylene flame is used.

In addition to the fuel and oxidant, sometimes an inert gas is also used in the mixture of

flame gases to help control the temperature. The two most popular examples are the argon-
oxygen-acetylene flame and the helium-oxygen-acetylene flame. The flame temperature is

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