Excitation and Ionization Interferences

Excitation and ionization interferences are nonspectral interferences. When a sample is aspirated into a flame, the elements in the sample may form neutral atoms, excited atoms, and ions. These species exist in a state of dynamic equilibrium that gives rise to a steady emission signal. If the samples contain different amounts of elements, the position of equilibrium may be shifted for each sample. This may affect the intensity of atomic emission. For example, if sodium is being determined in a sample that contains a large amount of potassium, the potassium atoms may collide with unexcited sodium atoms in the flame, transferring energy in the collision and exciting 

A related problem is ionization interference. If the analyte atoms are ionized in the flame, they cannot emit atomic emission wavelengths, and a reduction in atomic emission intensity will occur. For example, if potassium is ionized in the flame, it cannot emit at its atomic emission line at 766.5 nm and the sensitivity of the analysis will decrease. If a large amount of a more easily ionized element (EIE), such as cesium, is added to the solution, the cesium will ionize preferentially and suppress the ionization of potassium. The potassium ions will capture the electrons released by the cesium, reverting to neutral potassium atoms. The intensity of emission at 766.5 nm will increase for a given amount of potassium in the presence of an excess of cesium. The added cesium is called an ionization suppressant. Ionization interference is a problem with the EIEs of groups 1 and 2. The use of ionization suppressants is recommended for the best sensitivity and accnracy when determining these elements. Of course, as ionization increases, ion emission line intensity increases. It may be possible to use an ionic emission line instead of an atomic emission line for measnrements. 

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