Methods for eliminating interference factors in atomic absorption spectroscopy?

Repost: Interferences in Atomic Absorption Spectroscopy and Their Suppression. Abstract: Although atomic absorption spectroscopic analysis suffers from few interferences, such effects still occur frequently in practical applications, and sometimes they can be quite severe. It mainly introduces the nature of physical interferences, chemical interferences, ionization interferences, and spectral interferences, as well as methods for suppressing these interferences. 1. Physical interference: Physical interference refers to the effects on absorbance measurements that occur due to changes in any physical factors during the processing, transfer, evaporation, and atomization of the sample. Its physical factors include the viscosity, density, total salinity of the solution, surface tension, vapor pressure of the solvent, as well as the pressure and flow rate of the atomizing gas. These factors affect the injection speed of the test solution, the amount of extraction, the atomization efficiency, the distribution of droplet sizes, the evaporation of solvents and solid particles, and the average residence time of atoms in the absorption zone, thereby causing changes in absorption intensity. Physical interference is non-selective in nature. 　　 The method for elimination is to prepare a standard solution with the same or similar composition as the sample under test and use the standard addition method. If the concentration of the sample solution is too high, dilution can also be used. 2. Chemical interference: Chemical interference refers to the chemical reaction between atoms of the element being analyzed and co-existing components, resulting in the formation of more thermodynamically stable compounds that affect the atomization of the element under analysis. The presence of Al inhibits the atomization of Ca and Mg, as it forms compounds with high thermal stability ； The presence of PO43- leads to the formation of Ca3(PO4)2, which affects the atomization of Ca; similarly, F- and SO42- also affect the atomization of Ca. Chemical interferences are selective; to eliminate their effects, appropriate methods must be chosen based on their different properties. 3. Ionization interference: Ionization interference refers to the phenomenon where, under high-temperature conditions, atoms are ionized into ions, resulting in a decrease in the number of atoms in the ground state and thus a reduction in the absorbance value. Ionization interference is related to the atomization temperature, as well as the ionization potential and concentration of the element being measured. The ionization of an element increases as the temperature rises, and decreases as the ionization voltage and concentration of the element increase. Alkali metals have low ionization potentials, so ionization interference is significant. Method of elimination: An effective way to eliminate ionization interference is by adding an antionizing agent (also known as an ionization inhibitor). A deionizing agent is generally an element with a lower ionization potential than the element being measured. Under the same conditions, the deionizing agent is ionized first, producing a large number of electrons that suppress the ionization of the element being measured. (Sometimes the ionization potential of the deionizing agent is not necessarily lower than that of the element being measured; however, due to the large amount of deionizing agent used, even if its ionization potential is slightly higher, it still exerts a suppressive effect owing to the principles of ionization equilibrium.) 4. Spectral interference. Spectral interference usually occurs in two forms: overlapping absorption lines and interference from non-absorption lines. Absorption line overlap occurs when the absorption line of an element present in the sample is very close in wavelength to the analytical line of the element being measured; this results in the two spectral lines overlapping or partially overlapping, leading to an overestimated value of absorbance. When the theoretical value of spectral line overlap is Δλ 0.03 nm, the interference becomes severe. And when the interference line is also a sensitive line, interference will be noticeably apparent over a Δλ of 0.1–0.2 nm. The interference that is typically encountered is from non-sensitive lines, so the interference is not noticeable. The method to eliminate interference from absorption lines is to choose another analysis line; if the interference still cannot be eliminated, then sample separation is necessary. Non-absorbing line interference refers to multiple emissions within the spectral passband; in other words, the light source emits not only the resonance lines of the element being measured but also other spectral lines in the vicinity of those resonance lines. These interfering lines can be non-measured lines emitted by multi-line elements such as Co, Ni, Fe, etc., or they can be spectral lines emitted by impurities within the light source’s lamp (metallic impurities, gaseous impurities, metal oxides). For these multiple emissions, if the atoms of the element under test do not absorb them, they are detected by the detector, resulting in a constant background signal that reduces the absorbance and lowers sensitivity. It is also possible that the atoms of the element being measured cause multiple absorptions of these emissions; however, since the absorption coefficient for these absorptions is smaller than that for the resonance lines, the absorbance does not decrease, which in turn reduces sensitivity. 　 Method to eliminate it: The slit width can be reduced, making the spectral passband narrow enough to suppress the spectral lines from multiple emissions ； If the wavelength difference is small, an analytical line should be selected ； Reducing the lamp current can also decrease multiple emissions ； If the lamp has been in use for a long time and impurities such as oxides form inside it, reverse current can be applied to carry out a purification process.