Atomic Spectroscopy Interferences

Introduction

Atomic-absorption spectroscopy (AAS) and atomic-emission spectroscopy (AES) both rely on the analyte existing as free atoms in the gas phase. There are two common types of interferences that reduce the concentration of free gas-phase atoms: ionization and the formation of molecular species.

Note that the distribution of gas-phase atoms between the ground and excited states is a physical property that depends on the temperature of the environment. This distribution will affect the analytical signal, but it is not a chemical interference.

Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration. To prevent any bias due to differences between the standards and the samples, any reagents that are added to reduce chemical interferences should be added to the standards as well as the sample solution.

Preventing Ionization

Since samples are usually liquids or solids, the sample must be vaporized and atomized in a high-temperature source such as a flame, graphite furnace, or plasma. This high-temperature environment can also lead to ionization of the analyte atoms. Analyte ionization can be suppressed by adding a source of electrons, which shifts the equilibrium of the analyte from the ionic to the atomic form:

Analyte Analyte⁺ + e^-

Cesium and potassium are common ionization suppressors that are added to analyte solutions. These atoms are easily ionized and produce a high concentration of free electrons in the flame or plasma.

Preventing Refractory Formation

Some elements can form refractory compounds that are not atomized in flames or plasmas. An example is the presence of phosphates, which interferes with calcium measurements due to formation of refractory calcium phosphate:

3 CaCl₂ (aq) + 2 PO₄^3- (aq) Ca₃(PO₄)₂ (s) + 6 Cl^- (aq)

Formation of refractory compounds can be prevented or reduced by adding a releasing agent. For calcium measurements, adding lanthanum to the sample (and standard) solutions binds the phosphate as LaPO₄. LaPO₄ has a very high formation constant K_f and effectively ties up the phosphate interferent.