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Interference leads to systematic error in atomic absorption (AA) measurements by enhancing or diminishing the analytical signal or the background. These interferences can be grouped into three main categories: spectral interference, chemical interference, and physical interference.
Spectral interference occurs when signals from other elements or molecules overlap with the analyte signal, falsely elevating or masking the analyte's absorbance. This interference can be corrected using Zeeman, Smith–Hieftje, or deuterium background correction methods.
The Zeeman correction method uses a magnetic field to split the absorption line into three polarized components: two σ (shifted) and one π (unshifted). The analyte and background absorbances are measured separately by alternating the magnetic field, enhancing accuracy in complex matrices.
The Smith-Hieftje correction method involves pulsing a hollow cathode lamp (HCL) at high currents, causing the emission line to broaden and undergo self-reversal, where the central analytical line diminishes. This leads to strong emission on both sides of the line, absorbed by the background. Absorbance is measured under normal and high-current conditions, allowing differentiation between analyte and background signals. Though it requires only a single light source, the method's sensitivity decreases, especially when self-reversal is insufficient or recovery is too slow.
The deuterium (D2) background correction method uses a D2 lamp as a broad-spectrum light source to correct background absorption in atomic absorption spectroscopy (AAS). A rotating mirror alternates between the narrow-band hollow cathode lamp (HCL) and the broad-band D2 lamp. The D2 lamp measures background absorbance across a wide wavelength range, while the HCL measures analyte and background absorbance at a specific wavelength. The difference between the two signals isolates the analyte absorbance. Though inexpensive, it lacks precision in high-accuracy measurements.
Additionally, high-resolution spectrometers can minimize spectral interference from overlapping spectral lines. Sometimes, the analyte can be repeatedly extracted with a solvent before analysis.
Chemical interferences occur when the unwanted matrix components interact with the analyte, reducing atomization efficiency. A chemical modifier, such as a releasing agent or complexing agent, can be added to the sample to enhance atomization or prevent the formation of interfering compounds. Common chemical interferences include interferences due to ionization and refractory compound formation.
Elements or compounds that ionize at the same temperature as the analyte can alter its ionization. Ionization can be suppressed by adding an excess of a solution containing an element that ionizes more easily, which represses the analyte's ionization.
Furthermore, chemical reactions between the analyte and other species in the sample matrix can form nonvolatile compounds that do not readily atomize. This hinders the formation of free atoms for absorption. Such interference is avoided by the addition of a chemical competitor or the use of very high temperatures.
Calibration standards can be prepared with a sample matrix similar to the real samples, which helps compensate for chemical interferences arising from the matrix.
Physical interferences arising from non-chemical factors, such as gas flow rate variations or flame temperature changes, affect the nebulization or atomization process. These interferences can be resolved by using internal standards or diluting the sample. Modifying sample matrices and preparing calibration standards with a similar matrix can further reduce physical interferences.
